Materials for Energy Efficiency and Thermal Comfort in Buildings (Woodhead Publishing Series in Energy)

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Materials for energy efficiency and thermal comfort in buildings

i © Woodhead Publishing Limited, 2010

Related titles: Sustainability of construction materials (ISBN 978-1-84569-349-7) Major concerns have been raised on the sustainability of construction activities, in particular the sustainability of construction materials. A great deal of research has been carried out to quantify the ecological and environmental impact caused during the life cycle of construction materials. This book is the first of its kind to compile all the information on the sustainability of construction materials together into one complete volume. Sustainability of construction materials presents current research from a leading team of international contributors and will be an invaluable resource for all those professions involved in infrastructure development, construction and design. Durability of composites for civil structural applications (ISBN 978-1-84569-035-9) This comprehensive book on the durability of FRP composites will make it easier for the practising civil engineer and designer to use such materials on a routine basis. It addresses the current lack – or inaccessibility – of data related to the durability of the materials which is proving to be one of the major challenges to the widespread acceptance and implementation of FRP composites in civil infrastructure. The book should help further the acceptance of composites for civil structural applications and will be essential reading for practising engineers, decision makers and students involved in architectural engineering, construction and materials, disaster reduction, environmental engineering, maritime structural technology, transportation engineering and urban planning. Mineral wool: production and properties (ISBN 978-1-84569-406-7) Mineral wool has a unique range of properties combining high thermal resistance with long-term stability. It is made from molten glass, stone or slag that is spun into a fibrelike structure which creates a combination of properties that no other insulation material can match. It has the ability to save energy, minimise pollution, combat noise, reduce the risk of fire and protect life and property in the event of fire. Mineral wool: production and properties is suitable for engineers, researchers and for graduate and postgraduate students who want to broaden their knowledge of experimental methods in this field. Details of these and other Woodhead Publishing materials books can be obtained by: ∑ visiting our web site at www.woodheadpublishing.com ∑ contacting Customer Services (e-mail: [email protected]; fax: +44 (0) 1223 893694; tel.: +44 (0) 1223 891358 ext. 130; address: Woodhead Publishing Limited, Abington Hall, Granta Park, Great Abington, Cambridge CB21 6AH, UK) If you would like to receive information on forthcoming titles, please send your address details to: Francis Dodds (address, tel. and fax as above; e-mail: francis. [email protected]). Please confirm which subject areas you are interested in.

ii © Woodhead Publishing Limited, 2010

Woodhead Publishing Series in Energy: Number 14

Materials for energy efficiency and thermal comfort in buildings Edited by Matthew R. Hall

CRC Press Boca Raton Boston New York Washington, DC

Woodhead

publishing limited

Oxford Cambridge New Delhi iii

© Woodhead Publishing Limited, 2010

Published by Woodhead Publishing Limited, Abington Hall, Granta Park, Great Abington, Cambridge CB21 6AH, UK www.woodheadpublishing.com Woodhead Publishing India Private Limited, G-2, Vardaan House, 7/28 Ansari Road, Daryaganj, New Delhi – 110002, India www.woodheadpublishingindia.com Published in North America by CRC Press LLC, 6000 Broken Sound Parkway, NW, Suite 300, Boca Raton, FL 33487, USA First published 2010, Woodhead Publishing Limited and CRC Press LLC © Woodhead Publishing Limited, 2010 The authors have asserted their moral rights. This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. Reasonable efforts have been made to publish reliable data and information, but the authors and the publishers cannot assume responsibility for the validity of all materials. Neither the authors nor the publishers, nor anyone else associated with this publication, shall be liable for any loss, damage or liability directly or indirectly caused or alleged to be caused by this book. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming and recording, or by any information storage or retrieval system, without permission in writing from Woodhead Publishing Limited. The consent of Woodhead Publishing Limited does not extend to copying for general distribution, for promotion, for creating new works, or for resale. Specific permission must be obtained in writing from Woodhead Publishing Limited for such copying. Trademark notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation, without intent to infringe. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library. Library of Congress Cataloging in Publication Data A catalog record for this book is available from the Library of Congress. Woodhead Publishing ISBN 978-1-84569-526-2 (book) Woodhead Publishing ISBN 978-1-84569-927-7 (e-book) CRC Press ISBN 978-1-4398-2970-7 CRC Press order number N10167 The publishers’ policy is to use permanent paper from mills that operate a sustainable forestry policy, and which has been manufactured from pulp which is processed using acid-free and elemental chlorine-free practices. Furthermore, the publishers ensure that the text paper and cover board used have met acceptable environmental accreditation standards. Typeset by Replika Press Pvt Ltd, India Printed by TJ International Limited, Padstow, Cornwall, UK

iv © Woodhead Publishing Limited, 2010

Contents

Contributor contact details

xv

Woodhead Publishing Series in Energy

xix

Preface

xxi

Part I Fundamental issues and building physics: understanding energy efficiency and thermal comfort in the built environment 1

Heat and mass transport processes in building materials

M. R. Hall, University of Nottingham, UK and D. Allinson, Loughborough University, UK

1.1 1.2 1.3 1.4 1.5 1.6 1.7

Introduction Heat transfer: the transport of energy Mass transfer: the transport of matter Summary Sources of further information and advice References Appendix: Nomenclature

3 7 26 46 47 48 50

2

Hygrothermal behaviour and simulation in buildings

54

H. M. Künzel, Fraunhofer Institute for Building Physics IBP, Germany and A. Karagiozis, Oak Ridge National Laboratory, USA

2.1 2.2 2.3

Introduction Hygrothermal loads Modelling simultaneous heat and moisture transfer processes Input data for hygrothermal calculations Hygrothermal calculation results Model validation and practical applications

2.4 2.5 2.6

3

54 55 57 59 63 65 v


vi

Contents

2.7 2.8 2.9

Limitations of current hygrothermal models Conclusions and future trends References

73 73 75

3

Ventilation, air quality and airtightness in buildings

77

D. W. Etheridge, University of Nottingham, UK

3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9

Introduction Ventilation systems Physical mechanisms Feasibility of natural ventilation Natural ventilation design Issues concerning materials Future trends Sources of further information and advice References and further reading

4

Heat energy storage and cooling in buildings

S. Wu, University of Nottingham, UK

4.1 4.2 4.3 4.4 4.5

101 102 106 118

4.6 4.7 4.8

Introduction Psychrometrics Fundamentals of thermal energy storage Materials for thermal energy storage Thermal storage applications for building heating and cooling Sources of further information and advice References Appendix: Nomenclature

5

Thermal comfort in buildings

127

K. Parsons, Loughborough University, UK

5.1 5.2 5.3 5.4 5.5 5.6 5.7

Introduction Thermal comfort Measurement of thermal comfort The thermal index: an assessment technique Thermal comfort indices International Standards and thermal comfort Behavioural thermoregulation, thermal comfort and the adaptive model Equivalent clothing index (IEQUIV) Equivalent clothing index, the comfort temperature range and temperature limits in offices Sustainable thermal comfort References

5.8 5.9 5.10 5.11


77 77 81 86 90 92 97 98 99 101

122 125 125 126

127 127 129 132 132 137 141 142 144 144 146

Contents

6

Environmental health and safety in buildings

P. Brimblecombe, University of East Anglia, UK

6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8

Introduction Safety issues in occupied spaces Combustion, fire and combustible materials Infiltration of outdoor pollutants Indoor emissions and outgassing Occupant activity Transformations within the interior Particles in buildings that impact on environmental health and safety Materials and toxicity Advanced material requirements Future trends References

6.9 6.10 6.11 6.12

vii

148 148 150 151 155 157 161 163 164 165 166 167 168

Part II Materials and sustainable technologies: improving energy efficiency and thermal comfort in the built environment 7

Life cycle assessment and environmental profiling of building materials

K. Steele, Arup: Façades and Materials, UK

7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9

Materials sustainability A life cycle approach to selecting building materials A brief history of life cycle assessment (LCA) Environmental labelling Life cycle assessment (LCA) of building materials Life cycle assessment (LCA) standardisation UK context Other issues References

175 176 177 178 179 182 187 189 191

8

Inorganic mineral materials for insulation in buildings

193

R. Gellert, FIW München, Germany

8.1 8.2 8.3 8.4 8.5 8.6 8.7

Introduction Regulatory requirements Building-related properties Ecological and health aspects Individual product profiles Summary References


175

193 197 201 207 211 227 228

viii

Contents

9

Natural fibre and fibre composite materials for insulation in buildings

R. Gellert, FIW München, Germany

9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8

Introduction Regulatory requirements Building-related properties Ecological and health aspects Individual product profiles Reference buildings Summary References

229 231 235 237 240 249 255 255

10

Polymeric foam materials for insulation in buildings

257

D. Feldman, Concordia University, Canada

10.1 10.2 10.3 10.4 10.5 10.6 10.7 10.8

Introduction Foams classification, materials and foaming mechanism Processing technologies Thermoplastic and thermosetting foams Future trends Sources of further information and advice References Appendix: Abbreviations

257 258 262 265 269 269 269 272 274

11

Thermal insulation materials for building equipment

M. Zeitler, FIW München, Germany

11.1 11.2 11.3 11.4 11.5 11.6 11.7 11.8

Introduction Insulation materials Form pieces and form parts Support and spacer ring constructions Vapour retarder materials Claddings Insulation system Heat loss of the operational installation in the technical building equipment References

11.9 12

Reflective materials and radiant barriers for insulation in buildings

D. W. Yarbrough, R&D Services Inc., USA

12.1 12.2 12.3

Background and definitions Applications and assemblies Basis for thermal performance


229

274 275 296 297 298 298 298 301 303 305 305 307 309

Contents

12.4 12.5 12.6 12.7

ix

Measurement of thermal performance Codes and standards Sources of further information and advice References

313 315 316 318 319

13

Aerogel materials for insulation in buildings

C.-H. Yu, Q.J. Fu and S. C. E. Tsang, University of Oxford, UK

13.1 13.2 13.3 13.4 13.5 13.6 13.7 13.8

Introduction Processing material and properties Aerogel formation Physical properties of aerogels Applications of aerogels Future trends Conclusions References

319 321 325 332 336 336 339 339

14

Hygrothermal materials for heat and moisture control in buildings

345

M. R. Hall, University of Nottingham, UK

14.1 14.2 14.3 14.4 14.5 14.6 14.7 14.8 14.9

Introduction Characterization of hygrothermal functional properties Material classes Applications in buildings and occupied spaces Future trends Source of further information and advice Acknowledgements References Appendix: Nomenclature

345 349 351 358 360 361 361 362 363

15

Desiccant materials for moisture control in buildings

365

B. Warwicker, UK

15.1 15.2 15.3 15.4 15.5 15.6 15.7 15.8

Introduction Desiccant cycle Desiccant applications Health and comfort Air quality Natural and commercial desiccants: typical materials Practical applications of commercial desiccants Practical applications of natural desiccants for modifying building humidity 15.9 Bibliography 15.10 Appendix: Energy efficiency ratio (EER) and coefficient of performance (COP) © Woodhead Publishing Limited, 2010

365 366 367 367 369 370 373 378 382 382

x

Contents

16

Phase change materials for energy storage and thermal comfort in buildings

M. M. Farid and A. Sherrif, University of Auckland, New Zealand

16.1 16.2 16.3

Introduction Background Selection of phase change materials (PCM) and fabrication of PCM-gypsum wallboards (PCMGW) Full-scale testing facilities Benefits of applying thermal energy storage Computer simulation Conclusions References

16.4 16.5 16.6 16.7 16.8 17

Porous materials for direct and indirect evaporative cooling in buildings

X. Zhao, De Montfort University, UK

17.1 17.2

Introduction Assessing the capacities of evaporative cooling systems and the associated requirements in materials Comparative analyses of potential materials for evaporative cooling Potential applications of porous materials in buildings Conclusions References Appendix: Nomenclature

17.3 17.4 17.5 17.6 17.7 18

Prefabricated building units and modern methods of construction (MMC)

M. Mapston, MMConsult and one42morrow, UK and C. Westbrook, Mtech Consult Ltd, UK

18.1 18.2 18.3 18.4 18.5 18.6

Materials led building design Offsite construction Standardisation in construction Types of offsite construction Comfort factors in lightweight buildings Design led materials for addressing lightweight performance issues 18.7 Delivering sustainable comfort: a question of balance 18.8 Thin solutions (insulation and mass) 18.9 Thermal mass in offsite potential 18.10 Phase change materials (PCMs) 18.11 Advancements in phase change materials for buildings


384 384 385 386 389 390 394 396 397 399 399 404 409 417 422 423 426 427

427 428 431 431 440 441 442 444 445 445 446

Contents

18.12 New membrane developments 18.13 Composite design 18.14 References

xi

448 449 451

19

Roofing materials for thermal performance and environmental integration of buildings

J. Jones, Virginia Tech, USA

19.1 19.2 19.3 19.4 19.5 19.6 19.7 19.8 19.9 19.10

A question of design Roof construction Thermal performance of roofing materials Vegetated roofing systems Solar integrated roofing systems Natural ventilation Rainwater harvesting The role of roofs in building rating systems Conclusions References

455 458 464 466 474 476 478 480 481 482

20

Assessing and benchmarking the performance of advanced building façades

484

J. A. Clarke and C. M. Johnstone, University of Strathclyde, UK

20.1 20.2 20.3 20.4 20.5 20.6 20.7 20.8

Advanced façades Façade component options Performance models and model parameter measurement Integrated performance rating Application in practice Benchmarking façade performance Sources of further information and advice References

455

484 485 486 494 495 499 501 502

Part III Application of advanced building materials and design: improving energy efficiency and thermal comfort in the built environment 21

Advanced building materials and eco-building design

B. Watts, Max Fordham LLP, UK

21.1 21.2 21.3 21.4 21.5 21.6

Introduction Issues in eco-building design Challenges of sustainable development for a designer The eco-building design process Heelis: brief case history Future trends


505 505 506 513 520 523 526

xii

Contents

21.7 21.8

Sources of further information and advice References

531 531

22

Materials for energy efficiency and thermal comfort in domestic buildings

533

A. Peacock, Heriot Watt University, UK

22.1 22.2 22.3 22.4 22.5 22.6 22.7 22.8 22.9 22.10 22.11 22.12

Introduction EU domestic housing stock UK domestic housing Building fabric heat loss and CO2 emissions Architectural vernacular Intervention strategies Internal wall insulation External wall insulation Carbon payback time Summary Sources of further information and advice References

533 534 534 537 540 541 543 545 546 558 559 559

23

Materials for energy efficiency and thermal comfort in commercial buildings

562

D. H. C. Chow, University of Nottingham Ningbo, China

23.1 23.2 23.3 23.4

Introduction Energy efficiency and thermal comfort in offices Energy efficiency and thermal comfort in retail spaces Energy efficiency and thermal comfort in factories and warehouses 23.5 Embodied energy 23.6 Material choice 23.7 Modelling and monitoring thermal performance and comfort 23.8 Future trends in design and refurbishment 23.9 Sources of further information and advice 23.10 References 24

Materials for energy efficiency and thermal comfort in high performance buildings

P. C. J. Davda, G. Sex and J. Broomfield, The Austin Company of UK Ltd, UK

24.1 24.2 24.3 24.4

Introduction User considerations External considerations Internal considerations


562 563 570 574 577 578 580 583 586 586 589

589 591 595 598

Contents

24.5 24.6 24.7 24.8 24.9

Process areas People areas Environmental design and computer modelling Environmental and energy considerations Building Research Establishment Environmental Assessment Method (BREEAM®) 24.10 Future trends 24.11 Sources of further information and advice 24.12 Notes and references

xiii

598 612 612 613 622 623 626 626

25

Materials for energy efficiency and thermal comfort in new buildings

L. Shao, De Montfort University, UK

25.1 25.2 25.3 25.4 25.5 25.6 25.7 25.8 25.9

Introduction Challenges facing the new build construction sector The role, shape and trend of legislation Thermal mass Phase change materials (PCMs) Vegetation and reflective materials Future trends Sources of further information and advice References

631 632 634 637 640 643 645 647 647

26

Materials for energy efficiency and thermal comfort in the refurbishment of existing buildings

649

M. Gillott and C. Spataru, University of Nottingham, UK

26.1 26.2 26.3 26.4 26.5 26.6 26.7

Introduction Change of use in buildings Approaches to low carbon retrofit systems/technologies Post-occupancy evaluation (POE) Sources of further information and advice Acknowledgements References and further reading

649 656 661 668 677 679 679

27

Application of design and passive technologies for thermal comfort in buildings in hot and tropical climates

681

M. B. Gadi, University of Nottingham, UK

27.1 27.2

Thermal comfort in different climates Climate impact on urban pattern and building form and fabric Climate impact on building fabric

27.3


631

681 685 688

xiv

Contents

27.4

27.6 27.7 27.8 27.9

Approaches and lessons learned from traditional hotclimate architecture Applications of design and passive technologies in modern buildings Thermal performance of passive solar systems Conclusions Sources of further information and advice References and further reading

Index

27.5

689 696 699 704 705 706 709



(* = main contact)

Chapters 1 and 14 Dr Matthew R. Hall* Department of the Built Environment Faculty of Engineering University of Nottingham University Park Nottingham NG7 2RD UK E-mail: [email protected]

Dr David Allinson Department of Civil & Building Engineering Loughborough University Leicestershire LE11 3TU UK

Chapter 2 Dr Hartwig M. Künzel* Fraunhofer Institute for Building Physics IBP Fraunhofer Str. 10 83636 Valley Germany E-mail: [email protected]

Dr Achilles Karagiozis Oak Ridge National Laboratory, BTC PO Box 2008, Building 3147 Oak Ridge, TN 37831 USA

Chapter 3 Dr David W. Etheridge Department of the Built Environment Faculty of Engineering University of Nottingham University Park Nottingham NG7 2RD UK E-mail: david.etheridge@nottingham. ac.uk

Chapter 4 Dr Shenyi Wu Department of the Built Environment Faculty of Engineering University of Nottingham University Park Nottingham NG7 2RD UK E-mail: [email protected]

xv © Woodhead Publishing Limited, 2010

xvi


Chapter 5

Chapter 10

Professor Ken Parsons Department of Human Sciences Loughborough University Loughborough LE11 3TU UK

Professor Dorel Feldman Department of Building, Civil and Environmental Engineering Concordia University 1455 De Maisonneuve Blvd.W Montreal Quebec Canada H3G 1M8

E-mail: [email protected]

Chapter 6


Professor Peter Brimblecombe University of East Anglia Norwich NR4 7TJ UK E-mail: [email protected]

Chapter 7 Dr Kristian Steele Arup: Façades and Materials 13 Fitzroy Street London W1T 4BQ UK E-mail: [email protected]

Chapters 8 and 9 Dr Roland Gellert Forschungsinstitut für Wärmeschutz e. V. München (FIW München) Lochhamer Schlag 4 D-82166 Gräfelfing Germany

Chapter 11 Dr-Ing. Martin Zeitler Head of Department Industrial Insulation Forschungsinstitut für Wärmeschutz e. V. München (FIW München) Lochhamer Schlag 4 82166 Gräfelfing Germany E-mail: [email protected]

Chapter 12 David W. Yarbrough R&D Services, Inc. 102 mill Drive Cookeville, TN 38501 USA E-mail: [email protected]




Chapter 13

Chapter 18

Dr Chih-Hao Yu, Dr Qijia J. Fu and Professor S. C. Edman Tsang* Inorganic Chemistry Laboratory University of Oxford Oxford UK

Mike Mapston* MMConsult and one42morrow


Chapter 15 Professor Brian Warwicker The Mill Station Road Ardleign Co7 7RS UK E-mail: [email protected]

Chapter 16 Mohammed M. Farid* and Adam Sherrif Department of Chemical and Materials Engineering School of Engineering University of Auckland Auckland New Zealand E-mail: [email protected]

Chapter 17 Professor Xudong Zhao Institute of Energy and Sustainable Development De Montfort University The Gateway Leicester LE1 9BH UK

xvii


Charles Westbrook Mtech Consult Ltd Maple House Sitka Drive Shrewsbury Business Park Shrewsbury SY2 6LG UK E-mail: [email protected]

Chapter 19 James Jones College of Architecture and Urban Studies Virginia Tech Blacksburg, VA 24061 USA E-mail: [email protected]

Chapter 20 Professor Joe Clarke* and Cameron Johnstone Energy Systems Research Unit (ESRU) University of Strathclyde James Weir Building 75 Montrose Street Glasgow G1 1XJ UK E-mail: [email protected]



xviii


Chapter 21

Chapter 25

Bill Watts Max Fordham LLP 42–3 Gloucester Crescent London NW1 7PE UK

Professor L. Shao Institute of Energy & Sustainable Development De Montfort University The Gateway Leicester LE1 9BH UK



Chapter 22 Dr Andrew Peacock Heriot Watt University The Energy Academy Riccarton Edinburgh EH15 2BA UK E-mail: [email protected]

Chapter 23 Dr David H. C. Chow University of Nottingham Ningbo 199 Taikang East Road Ningbo, 315100 China E-mail: [email protected]

Chapter 24 Prakash C. J. Davda*, Graham Sex and John Broomfield The Austin Company of UK Ltd Cardinal Point Park Road Rickmansworth Herts WD3 1RE UK

Chapter 26 Dr Mark Gillott* and Dr Catalina Spataru Department of the Built Environment Faculty of Engineering University of Nottingham University Park Nottingham NG7 2RD UK E-mail: [email protected]

Chapter 27 Mohamed B. Gadi Department of the Built Environment Faculty of Engineering University of Nottingham University Park Nottingham NG7 2RD UK E-mail: mohamed.gadi@nottingham. ac.uk




1 Generating power at high efficiency: Combined cycle technology for sustainable energy production Eric Jeffs 2 Advanced separation techniques for nuclear fuel reprocessing and radioactive waste treatment Edited by Kenneth L. Nash and Gregg J. Lumetta 3 Bioalcohol production: Biochemical conversion of lignocellulosic biomass Edited by K.W. Waldron 4 Understanding and mitigating ageing in nuclear power plants: Materials and operational aspects of plant life management (PLiM) Edited by Philip G. Tipping 5 Advanced power plant materials, design and technology Edited by Dermot Roddy 6 Stand-alone and hybrid wind energy systems: Technology, energy storage and applications Edited by J.K. Kaldellis 7 Biodiesel science and technology: From soil to oil Jan C.J. Bart, Natale Palmeri and. Stefano Cavallaro 8 Developments and innovation in carbon dioxide (CO2) capture and storage technology Volume 1: Carbon dioxide (CO2) capture, transport and industrial applications Edited by M. Mercedes Maroto-Valer 9 Geologic repository systems for safe disposal of spent nuclear fuels and radioactive waste Edited by Joonhong Ahn and Michael J. Apted xix © Woodhead Publishing Limited, 2010

xx


10 Wind energy systems: Optimising design and construction for safe and reliable operation Edited by John D. Sørensen and Jens N. Sørensen 11 Solid oxide fuel cell technology: Principles, performance and operations Kevin Huang and John Bannister Goodenough 12 Handbook of advanced radioactive waste conditioning technologies Edited by Michael I. Ojovan 13 Nuclear reactor safety systems Edited by Dan Gabriel Cacuci 14 Materials for energy efficiency and thermal comfort in buildings Edited by Matthew R. Hall 15 Handbook of biofuels production: Processes and technology Edited by Rafael Luque, Juan Campelo and James Clark 16 Developments and innovation in carbon dioxide (CO2) capture and storage technology Volume 2: Carbon dioxide (CO2) storage and utilisation Edited by M. Mercedes Maroto-Valer 17 Oxy-fuel combustion for fossil-fuel power plants: Developments and applications for advanced CO2 capture Edited by Ligang Zheng


Preface

The creation of this book was stimulated by the vibrant international research activity coupling energy and buildings, along with the future driving potentials and demands in these fields that will no doubt ensue. The behaviour of our buildings, both in terms of the energy they consume and the thermal comfort they provide, can be described as a function of the materials from which they are made using the underpinning principles of building physics. In terms of the outdoor environment in which these buildings are placed, we face many challenges not least of which include the predictions for significant climatic changes and greater frequency of extreme weather events. We also face future shortages of energy and clean water supply, and an expanding (and ageing) population with ever-increasing demands for the right to a comfortable, healthy living environment. It is therefore vital that all those who can contribute to tackling these issues should have access to a comprehensive summary of the current knowledge that the many experts who have contributed to this book possess. Coupled with this is the intention to help identify gaps in the current levels of understanding, and to inspire further work where it is most needed along with a broader awareness and appreciation for the wealth of inter-disciplinary fields that can make many valid contributions to one another. There is a strong need to reduce our dependence on mechanical systems for controlling the indoor environment, especially against a background of forecasted climatic changes where cooling loads will increase whilst at the same time energy companies are less able to meet end user demands. Of course, it is vital that the exploration of better ways of meeting our energy demands (e.g. wind, tidal, geothermal, and solar technologies, etc.) be allowed to continue. However, ultimately the goal must be to achieve drastic demand-side reductions in energy, when our built environment currently accounts for around 50% of all carbon-related emissions contributing to global warming. We also have an expanding population whose average age is increasing, and who have higher expectations of health and comfort from their living environments. Any solutions to cut energy consumption will not be acceptable if they involve a backwards step in the quality of xxi © Woodhead Publishing Limited, 2010

xxii

Preface

life; rather they should coincide with significant advancements. Materials and allied technologies play a vital role in tackling some of the key issues relating to operational energy efficiency and thermal comfort in buildings using measures such as: ∑

reduction in fabric heat losses by increasing thermal insulation and eliminating cold bridging ∑ reduction in heat losses from air infiltration through improved air tightness ∑ reduction in indoor cooling loads by optimised air temperature and humidity buffering of the indoor environment and controlling solar gains. The structure and approach of this book is to provide an integrated and comprehensive package with parts designed to support and complement each other. The first part reviews the fundamental scientific and engineering principles associated with the various aspects of building physics that relate to materials and energy efficiency. The second part focuses entirely on both conventional and advanced materials, plus material technologies whose principle applications are to improve energy efficiency and thermal comfort in buildings and other occupied spaces. The final part covers the application of advanced building materials and design across a comprehensive range of building types and classifications. The reader can, for example, read about a particular material and/or technology in one part, whilst cross-referencing against the fundamentals that underpin its physical properties and the reasons for its behaviour, e.g. aerogel insulation materials and heat transfer in porous materials. Similarly, the reader can also read about best practice applications or a particular case study on a type of building, and cross refer this against detailed explanations of the specific materials and technologies that are employed for that application. Naturally, each chapter can also be used in isolation as a stand-alone contribution in its particular field. Part I: ‘Fundamental issues and building physics’ begins with a large, detailed chapter by Hall and Allison covering all key aspects of fundamental heat and mass (water) transport processes in building fabrics with particular emphasis on porous materials. This is then supported by a detailed, technical chapter by Künzel and Karagiozis on hygrothermal behaviour in buildings which deals with the coupled, inter-dependent nature of heat and moisture both in terms of storage and transport within materials. The mechanisms and systems of ventilation are explained in the next chapter (by Etheridge) along with issues relating to air quality and air tightness in buildings, along with the principles and design of natural ventilation. The fundamentals of thermal storage are addressed in Chapter 4 by Wu, including the principles of psychrometrics and sensible, latent and chemical storage, along with


Preface

xxiii

discussion of heat storage materials and the principles of cooling. In Chapter 5, Parsons explains the principles behind the measurement and quantification of thermal comfort, including adaptive comfort responses and thermal pleasure. Finally in this part, Brimblecombe explains in detail the principles of indoor air quality and the chemistry behind indoor pollutants from off-gassing, particulates, occupant behaviour and outdoor infiltration along with environmental health and safety aspects, including indoor fire and combustion risks/hazards and toxicity. It is widely recognised that the operational energy consumption of most building types currently outweighs their embodied energy by some margin. However, as we make dramatic increases in energy efficiency the embodied energy of the materials and components that we use will become proportionally larger and may account for a large proportion of the energy associated with buildings in the future. Along with this point is the fact that, in general terms, it is important to understand and consider the origins and manufacturing processes of the materials, as well as the energy that is required to manufacture them. For these reasons, Part II: ‘Materials and sustainable technologies’ begins with a chapter by steele that explains the principles of life cycle analysis and the approach of environmental profiling of building materials including summaries of the various energy-consuming processes upon which we rely and the environmental impacts that they can have. The following two chapters, both by Gellert, each consider insulation materials for use in buildings. The first discusses the composition, types and physical properties of various organic and inorganic mineral-based materials, whilst the second compares and explains the various types of natural fibre and fibre composite insulation materials and their performance. In Chapter 10, Feldman gives a detailed explanation of the chemical composition, manufacture and physical properties of polymeric foam insulation materials with descriptions of performance evaluation and new innovations. The following chapter by Zeitler covers thermal insulation for pipes and other building equipment and discusses the range of appropriate material types, supply forms and installations, comparison of material properties and details of regulatory performance assessment. The next topic to be covered, by Yarbrough in Chapter 12, is that of reflective materials and radiant barriers for insulation in buildings, which includes detailed coverage of the basis and approach for thermal performance assessment, regulatory codes and standards, and discussion of relevant applications and assemblies. Aerogel materials for insulation in buildings are covered in Chapter 13 by Yu, Fu and Tsang, which gives a detailed account of the processing and formation of aerogels, an explanation and discussion of their physical (including thermal) properties, and a summary of applications and future material developments. The application of hygrothermal materials for temperature and humidity control in buildings is discussed in Chapter 14 by Hall, including details of


xxiv

Preface

characterisation and testing of material functional properties and detailed descriptions of the various material classes and their physical properties. Chapter 15 by Warwicker follows on from this topic providing a detailed evaluation of dessicant materials, including desiccant cycles, materials class and properties, and explanations of their application for cooling and for humidity control in buildings. Phase change materials and their applications for thermal energy storage and thermal comfort in buildings are covered in Chapter 16 by Farid and Sherrif, along with case study evaluations and numerical model predictive studies. Next is a technical chapter by Zhao covering the use of porous materials for direct and indirect evaporative cooling systems and technologies in buildings with particular emphasis on novel façade technologies as well as explanations of the principles of evaporative cooling that are employed. To meet the modern demand for rapid assembly and low tolerances a resurgence of interest in pre-fabricated techniques has evolved into Modern Methods of Construction (MMC). A thorough overview of this topic is provided in Chapter 18 by Mapston and Westbrook evaluating new and emerging techniques and components and their role in improving energy efficiency and thermal comfort. Roofing materials and roof design is discussed in Chapter 19 by Jones from the perspective of improving the environmental performance and energy efficiency of buildings, whilst also evaluating material selection and thermal performance, green roofs and renewables integration. The final chapter in Part II, by Clarke and Johnstone, provides an excellent discussion on approaches to the design and environmental performance benchmarking of building fabrics by using advanced building façades as an example, and making use of relevant case studies. Part III: ‘Application of advanced building materials and design’ begins with an intriguing and insightful overview by Watts of the major issues in balancing comfort and health with sustainability in the eco-design of buildings. The next two chapters, by Peacock and Chow, respectively, both address materials and design applications for addressing energy efficiency and thermal comfort, firstly in domestic buildings and then in commercial buildings including offices, retail spaces and warehouses. Chapter 24 by Davda, Sex and Broomfield covers in detail specific issues in balancing energy efficiency with the requirements of high performance and specialist buildings, along with strategies and approaches for achieving this in laboratories and R&D facilities, medical research facilities, as well as clean rooms and high tech industry and assembly facilities. Chapter 25 by Shao provides an informative discussion of best practice approaches and considerations in relation to energy efficiency and thermal comfort design for new buildings, since it is vital that we make significant improvements in both affordable as well as state-of-the-art housing, for which there is great and increasing demand. Of course there are many additional challenges brought about by


Preface

xxv

the slow rate of renewal in our global building stock and the need to find acceptable retrofit solutions to reducing energy consumption whilst improving thermal comfort. Chapter 26 by Gillott and Spataru addresses the many issues associated with existing buildings and presents detailed discussion as to how energy efficiency and thermal comfort can be enhanced in line with modern best practice standards using materials and allied technologies for building retrofit. Finally, due to the wide variety in climatic regions across the globe, the last chapter, by Gadi, addresses how material selection and its incorporation within building design can be used to work with, rather than against, the ambient conditions found in different climatic zones in order to provide both energy efficiency and thermal comfort. It is hoped that this book will give great pleasure to many readers around the world and will prove to be a useful and informative resource for learning and instruction to students within the field. In addition, it is intended that this book will also act as a comprehensive and useful point of reference for researchers, academics, architects and engineers, and the industry, to maintain an awareness of the current state of knowledge and understanding, to give ideas and inspiration for new work in need of attention, and to develop a mutual appreciation for the many disciplines that can work together in the name of progress. Dr Matthew R. Hall University of Nottingham


Thispublicationwassponsoredby Thi bli ti db TheAustinCompanyofUKLimited Establishedin1938,Austinishometoacomprehensivesetofconsulting, design,engineering,managementandconstructionservices. Austinservicesareusedindividually,incombination,orasacomplete packageoncomplexandhighspecificationconstructionprojects,for creatingorupgradingindustrial,commercialandpublicbuildings. OurinͲhousetechnical,designandengineeringteamshavetheknowledge andexperiencerequiredtoundertakeprojectsfromconcepttocompletion. WorkthroughouttheUKandinEuropeisundertakenfromourofficesin NorthLondonandtheNorthWestofEngland.Internationalprojectscanbe handledthroughanetworkofcompanyandassociatedoffices. FacilitiesdesignedandconstructedbyAustingreatlyenhancecorporate future. AustinareleadingServiceProvidersforNewandRefurbishedFacility Austin are leading Service Providers for New and Refurbished Facility projects.

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xxvi © Woodhead Publishing Limited, 2010

1

Heat and mass transport processes in building materials M. R. H a l l, University of Nottingham, UK and D. A l l i n s o n, Loughborough University, UK

Abstract: This chapter introduces heat and mass transport in terms of the fundamentals and their application in the field of materials and building physics. It is the scientific topic that underpins all aspects of energy efficiency and thermal comfort in terms of the materials that make up our buildings and occupied spaces. An overview of thermodynamics and the conservation laws are provided to serve as a refresher for some readers and as a basic introduction for others. The chapter then deals with heat transfer by providing explanations of the fundamental science and then applying this to topics that are relevant to material properties and their application in buildings. The introduction of mass to these materials (e.g. water) adjusts the thermal properties, which in turn can alter the driving potentials for mass transport, which affects the thermal properties, etc., hence the true situation in materials is fully transient and highly time dependent. It is essential to consider this for accurate analysis and understanding of fabric behaviour, or of the indoor environment behaviour in response to the fabric it is made of. It is also an essential approach for studying phenomena such as surface and interstitial condensation, mould growth, as well as implications of changes to fabric (e.g. retrofit upgrades) and for thermal comfort. Therefore the next section in the chapter introduces mass transport where the approach is to, again, provide explanations of the fundamental science and then apply this to topics that are relevant to material properties and their application in buildings. Clearly mass transport is a subject in its own right, as is heat transfer. However, the chapter concludes by making the important point that in reality the two occur simultaneously and are inter-dependent, which leads on to the subject of hygrothermal behaviour. Key words: heat and mass transfer, porous materials, thermal properties, hygric properties.

1.1

Introduction

This chapter aims to explain and discuss the fundamental laws and processes that apply to the transport of heat energy and mass within the context of the fabric of buildings, i.e. the materials from which they are made. The chapter does not attempt to replace the multitude of excellent key texts already available on the general subject of heat and mass transfer and instead points to references and sources of further information where the reader may wish 3 © Woodhead Publishing Limited, 2010

4


to deepen their knowledge. Its objective is to serve as a learning tool for keen students as well as a ‘one stop’ revision/reference tool for experienced researchers.

1.1.1 Laws of thermodynamics As far as we understand, all matter possesses a quantity of mass and a quantity of energy. These two values are inextricably linked and we can assume that their total quantities never change – referred to as the laws of conservation of mass and conservation of energy, respectively. The reasons for this are explained in the following section. The further implications are that energy and mass are constantly moving from one place to another. Energy is formally described as the capacity of a system to do work and can occur either as potential energy (i.e. that stored in a body such as nuclear, electrical, etc.) or kinetic energy (i.e. energy of motion). The internal energy, U, of a body is the sum of potential and kinetic energies between component atoms and molecules, the total quantity of which is measured in Joules, and manifesting itself as the physical property of temperature. One can express this as a thermodynamic temperature, T in Kelvin (K) or as a Celsius temperature, q in degrees Celsius (°C). Note that a temperature of zero on the Kelvin scale (–273.15 °C) is called absolute zero because in theory it is the coldest possible temperature when internal kinetic energy is zero. When the temperature of matter in one region is higher than that in another region, transport of energy will attempt to occur from the hotter body to the cooler body until temperature equilibrium is restored. When energy is in transport from a higher temperature body to a lower one, it is described as heat energy. Mass is formally described in terms of its inertia (i.e. its resistance to acceleration), although it can also be measured in terms of the gravitational force it exerts on other objects, or (in practice) as the force by which it is gravitationally attracted to the Earth’s mass, i.e. its weight. Mass transfer is when transport of matter occurs from a higher concentration of mass in one region to a lower concentration in another. One can immediately appreciate how and why heat and mass transport occurs and the importance that this has within the context of this book. A popular phrase that is used to describe the first law of thermodynamics is that ‘heat is work and work is heat’. Work, W is the fundamental physical property in thermodynamics and simply describes motion against an opposing force, i.e. 1 Joule (J) is equal to the energy needed for 1 Newton of force to push an object over a distance of 1 metre. The energy of a system can be changed either by the system doing work, doing work on the system, or by transferring energy to or from another system in the form of heat. A ‘system’ in this sense can include a control volume of a material, e.g. a collection of molecules. An ‘open system’ is one where both matter and energy are



5

free to enter or leave. In a ‘closed system’ only energy is able to enter or leave, whilst in an ‘isolated system’ neither mass nor energy can enter or leave. In a non-isolated system, the internal energy (U), can be changed by the transport of mass, transport of heat energy (Q; unit = Watts), or by the system doing work. An adiabatic system is one where, theoretically, there is no transfer of heat. The first law of thermodynamics can now be written such that for an adiabatic system Q = 0 and so DU = W, whereas in a nonadiabatic system DU = Q + W, i.e. the conservation of energy law applies. We can expand this to include the concept of enthalpy, H, which describes the thermodynamic potential of a system, summing internal energy and the product of pressure and volume so that H = U + PV. The second law of thermodynamics can be explained by the assertion that heat energy cannot of itself pass from one body to a hotter body. This suggests that the natural process of heat transfer is irreversible since heat cannot pass between two systems in thermal equilibrium with one another, nor can it pass from a cooler system to a hotter one. From this observation we create the concept of entropy, S which is a measure of the unavailability of energy to do work. The second law can be used to relate temperature and entropy to heat input/output, Q from a non-isolated system. This enables us to re-write the first law equation as DU = TDS – W. Following on from the previous explanation of the first law of thermodynamics, we can combine this with the second law under Clausius’ Dictum, which states that the total amount of heat energy in an isolated system is constant and the entropy of the system tends to increase (over time) towards a maximum. This results in a prediction known as the heat death of the universe, assuming that it is an isolated system, where entropy would tend to maximum and no more work could be done. Finally, there is the third law of thermodynamics and the zeroth law of thermodynamics. The third law states that for a perfect crystalline solid at absolute zero (0 K) DS = 0, enabling absolute values for entropy to be stated with reference to this point. The zeroth law logically concludes that if two non-isolated systems are in equilibrium with a third system, then all systems are in equilibrium with each other. Heat capacity is an important material property that can have significant effects on heat transfer. The specific heat capacity at constant pressure, cp (W/kg K) is the energy required to raise the temperature of 1 kg of a material by 1 K. If the temperature difference across a material changes, the heat capacity (along with the thermal conductivity) will largely determine how long it takes for the temperature gradient to form within that material. This is an important concept and is recurring throughout this chapter and the rest of the book. For further detail on the laws of thermodynamics and their implications for the properties of matter, see Atkins’ Physical Chemistry (Atkins & De Paula, 2006).


6


1.1.2 Conservation laws The conservation laws are an essential prerequisite for understanding how and why the transport of energy (in the form of heat) and mass actually occur and how it involves matter. These principles can be accurately described using classical physics, which is perfectly accurate for engineering-scale situations without resorting to a quantum mechanics approach and is more than enough for materials and building physics in this context. The classical physics approach to conservation in energy (heat) and mass transport is the foundation of the subject of transport phenomena and is dealt with very comprehensively by Bird et al. (2001) in their landmark textbook Transport Phenomena. This section provides an overview of the concepts as a prelude to heat and mass transport in materials. To begin, we must consider a material at the molecular level and we can use the simple example of one oxygen molecule (in air) colliding with another oxygen molecule (adapted from Bird et al., 2001). According to the law of conservation of mass, the sum of masses entering and leaving the collision must be equal:

mA + mB = m¢A + m¢B

1.1

where mA = m¢A and mB = m¢B. According to the law of conservation of momentum, the sum of momenta entering and leaving the collision must be equal: m r· + m r· + m r· + m r· A1 A1

A2 A2

B1 B1

B2 B2

= m¢A1r·¢A1 + m¢A2r·¢A2 + m¢B1r·¢B1 + m¢B2r·¢B2

1.2 · Note that rA1 is the position vector of atom 1 in molecule A, whilst rA1 is the velocity of that atom and hence the linear momentum, p = mxr·A1. The centre of mass of molecule A is at position vector rA, and the distances of each atom from this point are RA1 and RA2, respectively. Note that rA1 = rA + RA1 and that RA1 = – RA2, and that the same rule applies to velocity such · that r·A1 = r·A + rA1. This means that the conservation of momentum equation can be reduced to: m r· + m r· = m¢ r·¢ + m¢ r·¢ 1.3 A A

B B

A A

B B

Finally, the law of conservation of energy states that the sum of the energies of the colliding molecules must be equal before and after the collision. Following on from the second law of thermodynamics, if one of the molecules has more energy (higher temperature) than the other before the collision, energy will transfer in the form of heat from the hotter molecule to the colder molecule. The interatomic potential energy, fA is the force of the chemical bond joining the atoms. The law of conservation of energy gives us: © Woodhead Publishing Limited, 2010


7

1 1 ˆ Ê1 ˆ Ê1 2 2 2 2 ÁË 2 mA1rA1 + 2 mA 2 rA2 + f A ˜¯ + ÁË 2 mB1rB1 + 2 mB 2 rB2 + f B ˜¯

1 1 ˆ Ê1 ˆ Ê1 = Á mA¢ 1rA1 ¢ 2 + mA¢ 2 rA2 ¢ 2 + mB¢ 2 rB2 ¢ 2 + f A¢ ˜ + Á mB¢ 1rB1 ¢ 2 + f B¢ ˜ 1.4 2 2 ¯ Ë2 ¯ Ë2

The total energy within a molecule is the sum of kinetic energies of the atoms plus the interatomic potential energy and is given by: 2 2 1.5 u A = 1 mA1rA1 + 1 mA 2 rA2 + fA 2 2 · · · From the r·A12 term recall that r·A = r·A + rA, and that rA1 = – rA2, therefore the law of conservation of energy reduces to:

ˆ ˆ Ê1 Ê1 2 2 ÁË 2 mA rA + u A ˜¯ + ÁË 2 mB rB + u B ˜¯

Ê = Á mA¢ A¢ 2 Ë2

ˆ u A¢ ˜ ¯

Ê 2 ÁË 2 mB¢ B¢

ˆ u B¢ ˜ ¯

1.6

Note that the energy of the molecule (including vibration, rotation and potential energy), and the kinetic energy of the molecule are distinct. However, in collision with another molecule kinetic energy can be converted into internal energy and vice 1 versa. 1 = + r + we are dealing with heat transfer To summarise, inr the+following sections which is the transport of energy as heat, and mass transfer which is the transport of mass. Clearly both involve all of the conservation laws and it is a useful starting point towards appreciating how and why the proceeding behaviours occur.

1.2

Heat transfer: the transport of energy

Heat transfer is the transport of energy that results from temperature difference. For the occupied space, this heat transfer may be undesirable, as in the heat loss through the walls of a heated building in winter or the heat gained through the windows of a car on a hot summer’s day. It may also be desirable, such as the use of the winter sun to warm the interior of a building or cooling by natural ventilation. In order to control the temperature within a space we must understand and try to quantify the mechanisms by which heat is gained and lost. This section offers an introduction to heat transfer by conduction, convection and radiation, as well as the effects of heat capacity on heat transfer. Heat may also be transferred by the movement of moisture (especially in vapour form) and this mass transfer is covered in Section 1.3.


8


Ventilation can be extremely significant in terms of heat gain and loss from a building and is considered separately in Chapter 3.

1.2.1 Dry state thermal conduction Thermal conduction is the transfer of internal energy (as heat), which occurs between neighbouring molecules of a solid, liquid or gas and between different materials in close contact with each other without the need for any bulk movement of that material. The rate of conduction heat transfer is proportional to the temperature gradient and the coefficient of proportionality is termed the thermal conductivity, l (W/m K). As heat flows from a location of higher temperature, T (K), to one of lower temperature, heat flux q (W/ m2) by conduction can be described, in one dimension, y (m), by Fourier’s Law of Thermal Conduction (see Fig. 1.1): The temperature can vary in all three dimensions simultaneously such that the x and z axes can also be written:

qx = – l

∂T ∂T and qz = – l ∂x ∂z

1.7

The simultaneous heat fluxes in each dimension can of course be written as a single flux with temperature divergence, if the x, y, z vectors are known, to give the more general expression:

q = – l—T

1.8

For a thermally homogeneous material of known thickness, d (mm) with surface temperatures T1 at x = 0 and T2 at x = d, under steady state conditions we get a steady state temperature gradient as shown in Fig. 1.2. T1

y d

where t is small qy = – l ∂T ∂y

T(y, t)

x

T2

1.1 Temperature gradient evolution in a material as a function of time. T1

y d x

T(y)

where t is large and l(T2 – T1) qy = can be neglected d

T2

1.2 Steady state temperature gradient across a material.



9

The thermal conductivity of a material is typically measured by a steady state method such as heat flow meter apparatus that complies with ISO 8301 Thermal Insulation – Determination of Steady State Thermal Resistance and Related Properties, or guarded hot box apparatus (BS EN ISO 8990). In the field, thermal conductivity of materials in situ can be measured by heat probe methods (Pilkington et al., 2008; Bristow, 1998), or heat flux meters (Doran, 2000). The ‘as built’ conductivity of materials is often found to be higher than the design value and this can be due to a number of factors including degradation with age, weathering, dirt and moisture accumulation, thermal bridging and construction defects. Doran found ‘as built’ U-values were on average 20% higher than design values for wall and ceiling constructions in the UK (Doran, 2000). ISO 10456 (1999) gives methodologies for the calculation of the change in thermal conductivity when influenced by variations in specimen moisture content, temperature and age based on tables of factors that are given for a number of common materials. Thermal conductivity of porous building materials can be greatly affected by their moisture content and it is important to be aware of the moisture content of the material being tested. Relationships between moisture content and thermal conductivity can be determined experimentally (ISO 10051, 1996). For building fabric, the overall thermal transmittance, U is often termed the ‘U-value’ (W/m2 K) which is simply the reciprocal of the total thermal resistance, RT for a fabric of known thermal conductivity, l and thickness, d. The thermal resistance or ‘R-value’ (m2 K/W) of a material or layer is equal to d/l and multiple resistances are commutative, e.g. in the case of a wall RT = Rbrick + Rair cavity + Rconcrete + Rplasterboard. Indoor and outdoor surface resistances (Rsi, Rso) can be included as discussed below. This is analogous to electrical resistance since q = DT/R for resistances in series. Note, in the equation above, that a thermal resistance has been attributed to the air cavity. Clearly heat transfer across a fluid-filled space in this sense would occur by natural convection although fluid conduction also occurs. The use of a single resistance term is to treat this heat transfer as a conductive term, for convenience, by expressing the equivalent resistance offered as a result of the convection coefficient in the cavity. This can be calculated in each case but standard tabulated values are normally used depending upon flow direction and whether the cavity is ventilated or unventilated. For further information and for typical values, refer to CIBSE Guide A (CIBSE, 2006). In a similar manner, the complex fluid–surface interactions that occur at the boundary when air meets the wall surface, for example, can be reduced to a single ‘equivalent resistance’ term. These values take into account the convection coefficient for any solid-fluid heat transfer, as well as short wave radiation gains by the solid, and long wave radiation gains/losses by the surface. Again, standard values for internal surface resistance (Rsi) and


10


outside surface resistance (Rso) have been produced for convenience (CIBSE, 2006).

1.2.2 Moisture-dependent thermal conduction From the previous section it can be seen that if T1 and T2 are known for a material of thickness d, then l can be calculated simply by measuring the heat flux through the material under steady state conditions. For porous materials, they absorb and store a certain amount of moisture from the air (depending upon the relative humidity) and can absorb liquid water, e.g. by rainfall or by capillarity from the ground (see Section 1.3 for more details). This has an effect on the thermal conductivity of the material that, for some materials, is very significant and for others is less so. Heat flow meter apparatus can be used to test materials in their fully dry state, when partly moist (referred to as unsaturated), or when fully saturated (Hall & Allinson, 2008a). Under steady state conditions, the thermal conductivity (l) of the specimen is calculated using: ls ·[(k1 + (k2 · T )) + ((k3 + (k4 · T )) · HFM )

l=

+ ( k5 k6 T HFM 2 (( + ( dT · )) · )]

1.9 The values for the calibration constants of the apparatus k1 – k6 inclusive are determined separately, and the Heat Flowmeter Output (HFM) is measured in mV. Steady state conditions are deemed to occur when the percentage variation in heat flux throughout the sample is ≤ 3%. The sample interval of the heat flow meter is given by the greater of 300 seconds or:

rCslsR

1.10

Prior to testing for moisture-dependent thermal conductivity, specimens (normally in the form of a thin slab) are oven dried at 105 °C to constant mass to give a moisture content of zero (Hall & Allinson, 2008a). They are then wrapped in a thin vapour-tight membrane and sealed using a small amount of adhesive around the sides (not in contact with hot/cold plates) of the specimen. They can later be unsealed and sprayed with a pre-measured quantity of distilled water before being re-sealed inside the membrane and stored under ambient conditions for 48 hours to allow the moisture to distribute within the material. This process can be repeated each time increasing the water content until the specimens are submerged in a bath of distilled water to achieve full saturation. The thermal conductivity of the membrane must be pre-determined in order to factor it out of the final calculation. The moisture content-dependant thermal conductivity of a material, l* is defined under steady state conditions by ISO 10051 (1996):



qm = – l* · dT dx

11

1.11

where qm is the measured density of heat flow rate at the hot and cold sides of the specimen (W/m2). The effect of moisture content on relative humidity was determined based on ISO 10051 (1996) Determination of Thermal Transmissivity of a Moist Material. Since the enthalpies of water vapour and liquid water are different from one another, the total flux, gt is equal to gv + gl (ISO 10051, 1996). When a temperature difference (DT) is applied across the specimen, the water begins to move around due to factors such as natural convection, diffusion and changes in capillary potential (see Section 1.3). In the early stages (when t is small), water close to the hot side can vaporise and migrate towards the cool side by diffusion. This has a significant effect in increasing q over that of a dry specimen since heat energy is transported sensibly by conduction through the solid and convection in the vapour, but also latently by the vapour which can condense near the cold side. As t becomes larger the accumulation of condensed water near the cold side can result in a gradient in capillary potential that induces liquid water to transport towards the warm side. Eventually the mass flux can become counterbalanced with vapour migrating from hot plate to cold plate and condensed liquid migrating from cold towards hot. Assuming that the sealed specimen contains a constant total amount of water, the counterbalancing situation is reached when:

g t = 0 ¤ g v = – g l

1.12

ISO 10456 (1999) Procedures for Determining Declared and Design Thermal Values provides methodologies for the theoretical calculation of the change in thermal conductivity when influenced by, for example, variations in specimen moisture content, temperature and age. l2 (the corrected thermal conductivity) is calculated simply by multiplying l1 (the measured thermal conductivity) by the moisture conversion factor Fm, where (ISO 10456, 1999):

Fm = efu(u2–u1)

1.13

The mass of moisture content in the second (wetter) state, u2, and the first (dryer) state, u1, is expressed in mass per mass units (kg/kg), as is the moisture content conversion coefficient, fu. A series of tables for standard values of fu can be obtained from ISO 10456.

1.2.3 Transient thermal conduction The heat capacity is significant when the dynamic (transient or time varying) heat transfer is considered. Two commonly used material characteristics that incorporate heat capacity are diffusivity and effusivity. Thermal diffusivity


12


is calculated from the ratio of the thermal conductivity to the volumetric heat capacity:

a= l rc p

1.14

Materials with a higher thermal diffusivity will reach thermal equilibrium with their surroundings more quickly. Thermal effusivity is calculated from the square root of the product of thermal conductivity and volumetric heat capacity:

b = lrc p

1.15

It can be used to infer the magnitude of heat transfer on contact with a material. For example, if one were to touch both timber and steel at the same room temperature, the steel would feel colder as it has a higher effusivity and heat is transferred from one’s hand into the material at a higher rate. The general form of the heat diffusion equation relies upon the conservation of energy principle (described in Section 1.1.2). It can be used as the basis to measure temperature distribution as a function of time in each of the three dimensions, T(x, y, z), and can be written as follows (Incropera et al., 2007):

∂ Ê l ∂T ˆ + ∂ Ê l ∂T ˆ + ∂ Ê l ∂T ˆ +  = r ∂x ÁË ∂x ˜¯ ∂y ÁË ∂y ˜¯ ∂z ÁË ∂z ˜¯

p

∂ ∂

1.16

Note that q· is a heat energy source term associated with the rate of heat T q per c unit generation from the material measured as Watts volume of the z t material in question (W/m3). In a case of known dimensions, the effective length, L equals the distance between maximum and minimum points on a single temperature gradient. The quantity hL/l is a dimensionless parameter called the Biot number (Bi) that is used in transient conduction problems involving convection exchanges. It is effectively the measure of a temperature drop in the solid relative to the temperature difference between the surface of that solid and a fluid (liquid, gas or a gas/vapour mixture such as air) (Incropera et al., 2007).

R (L /l A) Ts,1 – Ts,2 Bi ∫ hL = cond = = l Rconv (1/hA) T ,2 – Ts• s

1.17

Figure 1.3 shows the effect of Biot number on the steady state temperature distribution established in a wall with surface convection at the wall–air boundary. The so-called lumped capacitance method assumes that T is uniform at any instant, even if decreasing as a function of time. This would



13

qconv

qcond Bi > 1

x

1.3 Effect of the Biot number on a steady state temperature gradient in a wall Biot in wall (taken from Incropera et al., 2007).

T(x, 0) = Ti

T(x, 0) = Ti T•, h

T •, h

T• –L

L x

t

T•

T•

T• –L L Bi > 1 T = T (x, t)

1.4 Effect of the Biot number on temperature gradient evolution in different walls (taken from Incropera et al., 2007).

suggest l = • , or we can simply assume that temperature gradient exists but is negligible and can be ignored. When Bi 1 the majority of temperature difference evolves within the solid as opposed to near the solid–fluid boundary. With reference to the thermal diffusivity, and in addition to the Biot number, the second key dimensionless term that is essential for transient conduction is the Fourier number, Fo where:

Fo = a2t L

1.18

and

Bi · Fo =

hAs t rVc

1.19

From this we can calculate the temperature change of the solid over a period of time or the solid temperature at a known time, t using:

DT = Ti – T• = exp È– Ê hAs ˆ t ˘ = exp (– Bi · Fo) Í ÁË rVc˜¯ ˙ DTi Ti – T• ˚ Î

1.20

In cases where the simple lumped capacitance method is invalid (e.g. where Bi >> 1), temperature gradients can no longer be assumed to be negligible and so more sophisticated numerical methods must be used. A full review of these are beyond the scope of this chapter. Further details can be obtained by referring to Incropera et al. (2007). Some specific models and techniques are discussed in further detail later in this section. In all of the approaches to thermal conduction in building fabrics that have been discussed so far, the heat transfer has largely been assumed to be in one dimension. This is perfectly acceptable where energy efficiency and fabric heat loss/gain are the primary concerns. In situations where distribution of heat by conduction needs to be understood (e.g. due to thermal bridging around a column or a joint), then a two-dimensional or even three-dimensional approach may be required. One of the most common approaches is to use the Control Volume Method (CVM) which involves discretising the fabric in the form of a mesh, where thermal conduction between nodal points is considered. The finer the mesh, the closer the numerical result becomes to the true value; hence the complexity and scale of the mesh gives more accurate results at the expense of processing time. Many commercial software packages use an approach such as this, including FLUENT by ANSYS.

1.2.4 Thermal convection Convection occurs when there is conduction of heat into the molecules of a fluid and the bulk motion of that fluid that carries those molecules away



15

from the heat source. Typically it is used to describe the heat exchange at a surface due to the movement of air across that surface. The air movement may be the result of natural convection due to the buoyancy of warmed (less dense) air immediately above a flat roof for example, or through forced convection caused by wind or vehicle motion and may be a combination of the two. It can be described in a form similar to conduction but introducing a convection coefficient, hc (W/m2 K):

qconv = hc(T2 – T1)

1.21

The value of the convection coefficient will depend on geometry (shape and orientation of surface), material properties (surface roughness), fluid properties (viscosity), fluid free stream velocity (speed and direction) and the temperature of the bulk fluid and the surface. These will determine whether the flow over the surface is buoyancy driven or forced, laminar or turbulent or, in fact, a combination of these. The complexity of the fluid dynamics has lead to the use of simplified empirical calculations. A couple of examples are given below and more can be found in recommended papers such as Cole and Sturrock (1977), McClellan and Pedersen (1997) and Liesen and Pedersen (1997). The CIBSE Guide C (2007) proposes a simple method of calculating the convection coefficient for external surfaces at any wind speed, cs (m/s) based on empirical wind tunnel experiments:

hc = 5.8 + 4.1 · cs

1.22

Standard BS EN 15026 (2007) gives a similar correlation (hc = 4 + 4 cs) and includes a note that cs has to be measured near the building’s surface, which is often not rigorously pursued in practice. It is important to point out at this stage that determining a correlation between the unobstructed wind speed at 10 m height above ground (unobstructed wind) and the surface resistance is very useful but that if this is not possible then constant values that have been validated for building applications should be used instead. An empirical method for calculating surface wind speed from the unobstructed wind speed is given in the ASHRAE Handbook (ASHRAE, 2005). The ASHRAE detailed algorithm (Energy Plus Engineering Reference Online, 2009) considers that the exterior convection coefficient comprises components due to natural convection, hn, and forced convection, hf, such that hc = hn + hf. The natural convection coefficient is described by Eq. 1.23 for upward heat flow and Eq. 1.24 for downward heat flow:

Ê 3 Tsurface – Tair ˆ hn,up = 9.482 Á ˜ Ë 7.238 – cos(f ) ¯

1.23


16


Ê 3 Tsurface – Tair ˆ hn,down = 1.810 Á ˜ Ë1.382 – cos(f ) ¯

1.24

Note that f is the angle of the surface from the horizontal. The forced convection coefficient is calculated from surface geometry, surface roughness and wind speed:

h f = 2.537 W f R f

Pvz A

1.25

where Wf relates to surface orientation (1 for windward surfaces), Rf is a roughness index (ranging from 2.17 for very rough to 1.00 for very smooth), P is the perimeter of the exposed surface (m), vz the local wind speed (m/s) and A the surface area of the fabric (m2). The choice of calculation method can make significant differences to the convection coefficient and ultimately the heat transfer and therefore care must be taken to validate models wherever possible. It should, however, be remembered that knowledge of the air speed at the surface may be equally important to the accuracy of any simulation and this can be difficult to determine, especially for the microclimate around vehicles and structures. Convection heat transfer also occurs inside porous materials, and it is the reduction in this term that gives insulating materials their low heat transfer properties. Solid insulation materials normally have a low density and contain a large proportion of pores (or voids) that are inter-connected such that they offer a very high level of resistance to any fluids attempting to pass through them, i.e. tortuosity (see Section 1.3 for further details). The purpose of so-called ‘insulating materials’ is to offer a very high level of thermal resistance (R, m2 K/W) and to be as thin as possible, i.e. l must be very low to ensure d does not have to increase. As with all materials, we know that the total thermal conductivity of an insulation material, ltotal is quantified with respect to the heat flux under steady state conditions at a known mean temperature. Consequently, the primary modes of heat transfer through insulating materials can be considered to have a cumulative effect in determining ltotal. The heat transfer modes in a typical porous insulation material can be described as: ∑ ∑ ∑ ∑

conduction through the solid material, lsolid conduction through fluid in the pores, lfluid radiation between the inside surfaces of pores, lrad natural convection through fluid in the pores, lconv.

Note that convective and radiative terms have been lumped as effective conductions, where



ltotal = lfluid + lsolid + lrad + lconv

17

1.26

Clearly, in the case of building physics, the fluid can be air, air–water mixture or water, in which case lfluid can vary significantly and heat energy can also pass in latent form due to vaporisation/condensation of water as discussed in Section 1.2.2. This can have serious implications for the performance of insulation materials and highlights the obvious need for them to remain dry if they are to be effective. In conventional insulation materials with pore diameters greater than ~1 mm, the convective heat transfer component can represent a significant proportion of ltotal at standard temperature and pressure (STP). As pore diameters become smaller and the inter-connected paths between them become more tortuous, the movement of fluid becomes restricted and so convection becomes less dominant and the thermal conductivity of the quiescent fluid phase becomes the limiting factor. If we assume that the pores are filled with air, then quiescent air has a thermal conductivity of approximately 0.026 W/m K. If the pore diameters are sufficiently small then we can assume lconv = 0. In order for ltotal < 0.026 (W/m K) to occur, lfluid must be significantly reduced. The molecules in the fluid transfer kinetic energy to one another in the form of heat when they collide. The mean molecular free path is the average distance one of the molecules must travel before it collides with another of the same type. Therefore if this distance is reduced then lfluid must decrease because more molecular collisions need to happen in order to transfer the same quantity of heat energy across a distance d than before, which of course is statistically much less likely. The mean molecular free path in porous materials is reduced when the pore diameter becomes sufficiently small that molecule–pore wall collisions are statistically more likely than molecule–molecule collisions (Bird et al., 2001). The ratio of the mean molecular free path to a characteristic length (e.g. the mean pore radius in a porous material) is referred to as the Knudsen number which uses the symbol Kn. Note that in very permeable materials when Kn >> 1 it may be necessary to use the Monte Carlo method (or similar approach) to estimate the molecular trajectories inside the pore and hence predict the number of collisions. The net result is that when Kn ≥ 1, lfluid can be written as:

l fluid =

l 0fluid 1 + a · Kn

1.27

where l0fluid is the thermal conductivity of quiescent air, and a is a constant specific to the gas in the pores that is usually considered to be about 2 for air. Some solid porous insulation materials have extremely small pore diameters resulting in Knudsen numbers of about 1 to 2. This would give a lfluid term


18


of between about 0.0052 and 0.0087 W/m K. It is important to understand these phenomena because lately there has been increased attention towards ‘nano insulation materials’, such as aerogels (see Chapter 13), offering a high performance alternative to other thin insulation technologies such as vacuum insulation panels (VIPs, see Chapter 8). This means that because of the low lfluid in reality, a ltotal value of 0.016 W/m K or less can easily be achieved.

1.2.5 Thermal radiation Thermal radiation is electromagnetic radiation that is emitted by a body as a result of its temperature. All objects with a temperature above absolute zero emit thermal radiation in a spectrum of wavelengths. The amount of radiation emitted by a black body at any one wavelength is described by the spectral black body emissive power distribution or Planck’s Law, which may be written as:

E bl =

2p hc 2 10 l (exp(hc/lKT ) – 1)

1.28

6 5

Plotting the spectral emissive power for a black body against wavelength for a number of temperatures produces a series of curves, known as Planck’s curves, as shown in Fig. 1.5. The total emissive power can be found by integrating Planck’s law from

Spectral emmisive power, W/m2 mm

100000000

5762 K

10000000 1000000 100000 10000 1000 100

V i s i b l e

1250 K 800 K

300 K 250 K

10 1 0.1 0.01 0

1 2 3 4

5 6

7 8 9 10 11 12 13 14 15 16 17 18 19 20 Wavelength, mm

1.5 Planck’s isothermal curves for spectral emissive power vs. wavelength.



19

l = 0 to l = ∞ (which gives us the area under the curve for a particular temperature) and is known as the Stefan–Boltzmann law: •

Ú0 E bl dl = E b = sT 4

1.29

where s is the Stefan–Boltzmann constant (s = 5.669 ¥ 10–8 W/m2 K4). The ratio of the emissive power, E, of a surface to the emissive power of a blackbody, Eb, at the same temperature is known as the emissivity, e (i.e. e = E/Eb), therefore:

E = esT 4

1.30

A black body is defined as a body that absorbs all incident radiation at any given temperature and wavelength. Real surfaces, however, absorb and reflect thermal radiation and may also transmit thermal radiation, as shown in Fig. 1.6, and this behaviour can vary with temperature and wavelength. Kirchoff’s law tells us that the amount of radiative energy emitted by a surface must equal the amount of radiative energy absorbed by that surface. The material properties of interest are therefore the absorbtivity (a) or emissivity (e), reflectivity (r) and transmissivity (t), which describe the fractions of the incident radiation that are absorbed, reflected and transmitted such that a + r + t = 1 and e = a. The net radiant heat transfer between two surfaces is dependent on their temperatures, sizes and view factors. Radiation view factors describe the fraction of the surface area of the hemispherical view from a surface that comprises the other surface. Techniques for determining view factors include mathematical, reference tables, ray tracing and fish eye lens photography and other methods that can be applied to urban areas (Grimmond et al., 2001). Incident radiation

Reflected

Absorbed

Transmitted

1.6 Absorption, reflection and transmission of incident thermal radiation by a material.


20


An example of their use would be for the long wave radiative heat exchange between a surface and the external environment. This could be divided into that between the surface and the ground, Fgnd, the surface and background objects, Fbg, and the surface and the sky, Fsky. Assuming temperature could be assigned to each of these: Tgnd, Tbg and Tsky, according to Mcclellan and Pedersen (1997) the radiative heat exchange at the surface can be simplified to: 4 q = es(Fgnd(T 4gnd – T surface ) + Fbg(T 4bg – T 4surface) 4 4 + Fsky(T sky – T surface))

1.31

where Fgnd + Fbg + Fsky = 1. The determination of the ground, background and sky temperatures must also be considered. For inside surfaces such as the walls, floor and ceiling of a room, other methods have been developed such as the one described by Liesen and Pedersen (1997). Radiation that originates from the sun, which has a black body temperature of around 6000 K, has a much shorter wavelength than that from objects at typical terrestrial temperature and for this reason is termed short wave (SW) radiation. This is a useful distinction as a material’s wavelength dependent behaviour to thermal radiation can be split into LW and SW for convenience. A commonly used example is snow, which has a very low emissivity in the short wave (highly reflective) and a very high emissivity in the long wave (highly absorbent). The gas molecules and solid particulates that make up the earth’s atmosphere absorb, reflect and scatter solar radiation; the intensity at the surface is therefore dependent on the sun’s relative position and the atmospheric conditions. Cloud cover is an obvious example of this dependence. The solar radiation that is incident on a surface can be divided into that direct from the sun, diffuse radiation from the sky and reflected radiation from the ground and other surfaces. When the surface is transparent, such as glass, a portion of this radiation will be transmitted into the room, a portion reflected away from the surface and a portion absorbed by the glass. Different methods for calculating these effects have been developed (see, for example, CIBSE, 2006 and McClellan and Pedersen, 1997).

1.2.6 Steady state environmental parameters The ratio of temperature change across a boundary is proportional to the ratio of thermal resistances such that (McMullan, 1992):

DT = R TT RT

1.32

where ΔT = temperature difference across a layer, TT = total temperature difference across the element, R = resistance of that layer, and RT = total



21

resistance of the element. The formula can simply be rearranged to give the temperature difference (DT) across each boundary layer within a multilayer fabric, e.g. a wall. This technique is referred to as the Glaser Method and is illustrated in Fig. 1.7. Surface resistances are included to make surface temperatures more realistic; these are usually taken as tabulated values from the CIBSE Guide A (2006) or similar. The choice of values depends upon level of exposure and air velocity near the surface, e.g. typical Rsi = 0.123 m2 K/W. A similar approach is taken for air pockets, for example in a cavity wall where standard values are assumed for ventilated or unventilated cavities. The main weakness of this model is that the two temperature set points (T1 and T2) are taken from indoor/outdoor air temperatures which ignores additional gains or losses through the fabric as a result of radiation or convection. This is important because the temperature profile will be less accurate and surface temperatures can be very inaccurate, e.g. on a hot sunny day surface temperature can

Inside air temp.

Temperature (°C)

20

10

Outside air temp. 0 Inner wall (brick)

Air cavity

Outer wall (brick)

EPS insulation

1.7 Glaser’s method for steady state temperature gradients in a multilayer composite wall.


22


exceed air temperature due to solar gain but a standard Glaser model would show it as being lower. These problems can be overcome by using a more complex Glaser method which uses nodal points as opposed to fixed air temperatures. These nodal points are hypothetical ‘equivalent’ temperatures that lump together a more complete variety of heat energy inputs. Instead of an outdoor air temperature point, a sol-air temperature node is used; a hypothetical temperature that includes the outside air temperature, SW radiation gains (from the sun), LW radiation gains from surrounding objects, and convective exchanges. The indoor air temperature is replaced by two nodal points: environmental temperature node and air temperature node. The environmental node includes convection from the room air, and radiation both from surrounding objects and direct sources, e.g. heaters/plant. Heat transfer occurs between the environmental node and the indoor air which is limited by the air convection transfer coefficient. Heat transfer from the sol-air node occurs directly to the indoor air node only in the case of infiltration (leakage), and to the environmental node in the form of conduction or conduction equivalents, i.e. it is regulated by fabric resistance RT. The nodal network diagram in Fig. 1.8 illustrates this model.

1.2.7 Transient environmental parameters The nodal diagram shown in Fig. 1.8 is used under steady state environmental conditions, where nodal temperatures are fixed values, e.g. using only the mean value for a day, month or season. However, it can also be extended for use with dynamic environmental conditions where the sol–air and environmental node temperatures fluctuate between maxima and minima about a mean. This principle is employed in the cyclic admittance method (see CIBSE Guide A, 2006) where the periodic fluctuation about mean values is taken to be a sinusoidal frequency pulse, normally over a duration

Indoors Qie QSe QaU QPa Qia QSa

Wall 1 S AU

qei

Outdoors

qeo

1 hc S A qai

1 pC pV

1.8 Nodal network diagram for the admittance model under steady state conditions (adapted from Rees et al., 2000).



23

of 24 hours. This approach has the disadvantage that the resistances (and hence the associated material properties) are assumed to be constant, which if rigorously pursued is invalid except under steady state conditions since l is temperature dependent. However, the accuracy is normally perfectly acceptable across the temperatures ranges used in building physics. Obviously the sinusoidal temperature fluctuations are also an assumption because 24 hour cycle temperature changes do not follow this precise pattern. However, this assumption does offer the advantage of being able to compare the behaviour of one cross-sectional fabric with another under identical timedependent environmental conditions. In order to adapt the nodal diagram for dynamic environmental conditions the terminology is changed slightly, as shown in Fig. 1.9. Note that the Celsius temperatures and nodal heat inputs/outputs are shown with a tilde symbol to denote fluctuation which occurs simply as a function of time. The other significant change is the replacement of steady state transmittance, U, with thermal admittance, Y, for fabric conduction, where U is the rate of heat transfer across a fabric, and Y is the rate of transfer to the fabric, with identical units (W/m2 K). Unlike U, which is simply the reciprocal of RT, Y is calculated using a set of matrices which employ density, heat capacity, thickness and thermal conductivity (see CIBSE Guide A, 2006). As a result, heat transfer from the environmental node to the fabric (and subsequent storage of heat energy) can be approximated using the admittance method which is useful for determining any reductions in peak cooling loads inside a building. High thermal admittance is one of the two most important features of fabrics that are referred to as having ‘thermal mass’. This is also an important concept when considering passive cooling as a result of fabric energy storage, and is discussed in more detail in Chapter 4. The second important feature of thermal mass fabrics is that of thermal decrement factor, f. This is used to represent the reduction in temperature

Indoors

Wall

Outdoors

~

Q ie ~

Q se

1 SAY

~

q ei

~

~

Q aU ~

Q Pa ~

Q ia ~

Q Sa

q eo

1 hc S A ~

q ai

1 pC pV

1.9 Nodal network diagram for the admittance model under transient environmental conditions (adapted from Rees et al., 2000).


24


gradient formation (as a function of time) across a fabric as a result of heat storage within that fabric, and hence the rate of heat transfer is attenuated by the factor f. This forms an additional component to calculating peak cooling load. It can also be used in conjunction with the nodal diagram for a sol–air temperature fluctuating as a function of time, t, to approximate the response of the environmental node temperature which of course now fluctuates as a function of f. In this situation, decrement has an associated time lag, f (hours), which causes the maxima and minima to move out of phase with the sol–air node temperature as shown in Fig. 1.10. The consequences of this behaviour are that low thermal mass walls tend to exhibit less attenuation of environmental temperatures (dashed lines) in response to sol–air fluctuations (solid lines) and with a shorter decrement time lag than do high thermal mass walls. This translates to reduced potential for heat transfer from the environmental node to the indoor air node since the temperature gradients between the two will be lower. Since thermostatic controls for HVAC systems typically use indoor air temperatures, high decrement fabrics can (in theory) be used to reduce the heating cooling loads considerably. One final point is that since decrement applies to heat transfer across the fabric, it is used to attenuate U as opposed to Y for heat loss/gain to the environmental node. This is very important for building fabric design because it is possible to optimise material type, thickness and positioning in relation to heat gain/loss (U), passive cooling (Y), and suppression of temperature fluctuation (f, f). Examples are given in Fig. 1.11 using different

P

f

Inside

max qeo

Outside

Wall

qeo(t)

Aeo

Aei

qeimax qi

qei(f )

t

max qei

qeimin

x=0

x=L

t

min qeo max qeo

1.10 A diagrammatic illustration of thermal responses in the admittance model (adapted from Asan, 1998).


Temp (deg C)

25

x = L

x = 0 x = L

Solid wall

Insulation 25

qeo(t)

20

5 10 15 20 t (hours) Y = 4.8 U = 1.9

5 10 15 20 t (hours) Y = 0.29 U = 0.29

x = 0

5 10 15 20 t (hours) Y = 0.29 U = 0.26

x = 0 L 1 L 2 x = L 3

25

qeo(t)

20 qei(f )

15

5 10 15 20 t (hours) Y = 4.7 U = 0.26

x = 0 L 1

L 2 x = L 3

Composite walls 25

qeo(t)

20 qei(f )

15

L 1 x = L 2

Externally insulated 25

qeo(t)

20 qei(f )

15

x = L2

Internally insulated 25

qeo(t)

20 qei(f )

15

x = 0 L 1

20 qei(f )

15

qeo(t)

5 10 15 20 t (hours) Y = 5.1 U = 0.26

qei(f )

15

5 10 15 20 t (hours) Y = 0.3 U = 0.26



x = 0

1.11 Comparison between Y, U and f for different fabric design configurations (taken from Hall & Allinson, 2008b).

25

26


configurations of a 300 mm dense concrete wall with 100 mm of polystyrene insulation. The U value is essentially unaffected by positioning of the insulation, but admittance and decrement both are. Internal insulation gives very low admittance, opposite for external insulation. It is possible to have too much mass in one place; if it is too thick it is harder for heat energy to pass deeply enough from the environmental node in sufficient time and it starts to ‘leak’ towards the sol–air node. Optimum thicknesses of the high thermal mass material (in this case concrete) exist; they must be positioned on the inside in contact with environment and air. The exact thickness varies with material properties but for conventional mass materials (e.g. dense brick, concrete, compressed earth) this value is between 100 and 200 mm. Commercial software packages that use this approach for modelling heat transfer with transient environmental conditions include Ecotect and HEVACOMP, and it is the approved method for CIBSE. An alternative approach to the admittance method is to use a ‘heat balance method’ approach. This looks at the cross-sectional fabric as above by considering the resistive/capacitive thermal properties, but more realistically simulates heat energy fluxes to and from the fabric. For further details of this method refer to the ASHRAE Fundamentals set of handbooks. For 2D and 3D problems the usual approach is to use a CVM for numerical modelling or a commercial package such as FLUENT. The various approaches and techniques to modelling heat transfer are beyond the scope of this chapter. The admittance and heat balance methods are discussed in more detail in Chapter 4 with particular regard to their application for thermal storage and cooling loads.

1.3

Mass transfer: the transport of matter

Mass transfer is inextricably linked with heat transfer and is the study of the mechanisms responsible for the transport of matter (normally as a fluid) from one place to another. The following section explains the fundamental concepts of mass transfer and combines this with the subject of porous materials and in the context of building physics.

1.3.1

Psychrometrics

Psychrometrics (as distinct from psychometrics) is the study of the thermodynamic properties of air–vapour mixtures, typically focusing on the interrelation among temperature, partial pressures and enthalpy. Under normal atmospheric conditions the air in our atmosphere (mainly N2, O2, CO2 plus others) contains varying amounts of water molecules in vapour state. The moisture content of air is defined as the mass of water vapour molecules



27

per unit mass of air molecules within a finite volume and is expressed in kg/kg. The pressure of a gas (air) or vapour (e.g. water) is the result of the mean kinetic energy associated with the Brownian motion of its molecules. We know from Dalton’s Law that when two gases are mixed, in this case air and water vapour, that the total pressure of the mixture is equal to the sum of the two pressures which are now called partial pressures. We also know from this law that partial pressures are commutative such that if the partial vapour pressure (per unit volume) increases, the air pressure stays the same and the total pressure increases by Dpv. The relative humidity, j, of the air–vapour mixture is defined as the partial vapour pressure as a proportion of the saturation vapour pressure within a finite volume at known temperature. It can be given either as the decimal value of the fraction pv/ pvsat or as a percentage. Note that the dew point temperature is the point at which water vapour in a known volume of air will condense, since it occurs at the point where j Æ 1 (or 100% RH). If a bulb thermometer is placed in the air–vapour mixture, the temperature recorded is a consequence of the internal energies of the gas and vapour, and is referred to as the dry bulb temperature. If the thermometer bulb is then surrounded by a wick saturated in liquid water, evaporation from the wick occurs, the rate of which is controlled by the temperature and moisture content of the surrounding air. Evaporative cooling of the surface results in a reduction of the measured temperature (called the wet bulb temperature), and the depression between this and dry bulb directly corresponds to the relative humidity of the air. On this basis, a psychrometric chart (see Chapter 4 for further details) can be plotted linking dry bulb and wet bulb temperatures to corresponding relative humidities, which in turn have corresponding partial vapour pressures (and therefore moisture contents) and dew point temperatures. Note that the enthalpy of the air corresponds to the sum of the internal energies of the gas molecules and water vapour molecules.

1.3.2 Porous materials Porous materials contain voids (or pores), either in isolation or interconnected to form complex networks of channels, which are filled with fluid under normal atmospheric conditions, e.g. air, liquid water, or air and water vapour. Depending upon the permeability of the material, fluids are able to enter or leave the voids and to move along within continuous voids. Therefore, all permeable materials are porous but the degree to which porous materials are permeable varies considerably. It can even tend towards zero for materials with isolated or closed voids. The permeability of a porous material can be quantified and expressed as a coefficient to describe the restrictions imposed on fluid flow. These restrictions can be attributed to a wide range of parameters such as the total volume of void space (i.e. bulk porosity, n), the void size


28


distribution, the extent of interconnectivity between voids, as well as minor factors such as the roughness of void interior surfaces. One other important factor is tortuosity, because we may have implicitly assumed up until this point that the interconnected voids are all straight with no corners or bends. When fluid is travelling from one point to another inside a continuous void the resistance to flow increases if the path becomes more ‘tortuous’ or challenging. This is due to the cumulative effect of the required number of changes in flow direction, the extent to which direction must be changed, variations in void diameter, and other possible examples. The permeability of porous materials is dealt with in more detail in Section 1.3.5. If the specimen of a porous material has a total volume, VT, which consists of solid state matter, Vs, and fluid-filled void, Vv, then VT ∫ Vs + Vv (see Fig. 1.12). Under atmospheric conditions, when the material is dry the voids are filled with air (Vv = Va) and when saturated the voids are filled with water (Vv = Vw). An interesting observation is that, when water enters a dry or partly dry (known as ‘unsaturated’) porous material, it must displace an equal volume of air from the moment it crosses the boundaries defined by VT; assuming the air to be incompressible. What can actually happen is that air is not always able to be displaced, which can prevent saturation occurring, and can even become trapped by advancing water at the ends of sealed voids with no escape. Its only option is to slowly dissolve in the water but this can take several months to occur. For these reasons full saturation rarely occurs except where the air in a porous material is first fully evacuated by placing in a vacuum chamber, and then introducing water to the sealed chamber and immersing the specimen; a process called vacuum saturation. Of course, one

Volumes

Voids Vv Total volume

Porous material

Masses

Air Va

AIR

Air ma

Water Vw

Water

Water mw

ma = 0

Total mass mt

Vt Solids Vs

Solids

Solids ms

1.12 The three phase model for porous materials.



29

or more isolated pores can remain inaccessible to water even under these conditions since they are not connected to the rest of the void network. In this sense, the only true way to assess bulk porosity is to determine the specific gravity of the solid material, and gravimetrically determine the specimen’s mass as a fraction of maximum theoretical mass, where n = 0. Therefore, determining the porosity by gravimetrically measuring the maximum mass, of fluid that the voids can hold gives us the apparent density of the material or, conversely, the apparent percentage water absorption. When water enters the finite volume that we have described as VT (by entering the voids), we describe the process as absorption. Absorbed water molecules may also be ‘adsorbed’ to the internal surfaces of voids by van der Waals forces (since water molecules are polar). Absorbed water can be classified into one of three domains known as hygroscopic water, capillary water and gravitational water (Hall & Allinson, 2009a). The classification of the water is dependent upon (i) its phase (vapour or liquid) when it enters VT, and (ii) the extent to which it is electrostatically attracted to the inside surfaces of the void. Hygroscopic water is absorbed as a vapour phase, capillary water is absorbed as a liquid phase, and gravitational water is liquid that is absorbed when the capillary potential in the pore network is zero (i.e. super saturation). This latter point can be better understood following an appreciation of the phenomenon known as capillarity, as described in detail in Section 1.3.4. Suffice to say that the threshold between capillary and gravitational water is the point where electrostatic attraction between the water molecules and the void surfaces is insufficient to oppose the force due to gravity. It is helpful to describe the water content of a porous material in terms of relative water content, q. This is in fact a dimensionless value for the water content and can be calculated using (Hall, 1977):

q=

(w – wa ) (wb – wa )

1.33

where 0 ≤ q ≤ 1. Note that w = gravimetrically determined water content (kg/m3), and that wa = minimum moisture content and wb = maximum moisture content. In the following sections we will see how several critical values for q occur, and that their numerical values are (i) characteristic of particular materials, and (ii) correspond to the three domains of absorbed water described above.

1.3.3 Kelvin’s equation and sorption isotherms Since water molecules are dipolar they can attract and bond to one another through a type of van der Waals force known as hydrogen bonding. The


30


internal molecular attractive forces in water may be termed cohesion, whilst the forces of attraction that exist between water molecules and those of dissimilar materials (e.g. the negative charged surface of a void) may be termed adhesion. If the adhesion forces between water and a dissimilar solid (e.g. glass) are greater than the intermolecular forces within the water, then the surface of the glass becomes ‘wet’ (Bowles, 1984). The formation of droplets occurs because the internal intermolecular forces of a finite amount of water are in equilibrium with the atmospheric pressure of the surrounding air (see Fig. 1.13). Since the thickness of the surface tension ‘skin’ is molecular, it follows that the unit for surface tension is force/length (Bowles, 1984). The internal/external pressure difference, known as p (where p = pi – po), inside the curved surface of the droplet is directly proportional to the surface tension g, and therefore inversely proportional to the radius of curvature r (see Fig. 1.16). This is shown by the equation below where:

p=

2g r

1.34

Although the classification of absorbed water (determined by its phase upon entry) cannot change, its phase once inside VT can change, for example the condensing of hygroscopic water from vapour to liquid phase (Hall & Allinson, 2009a). This phase changing can occur depending upon factors such as geometry of the voids and electrostatic surface charge within the voids. The reason for this is the simple fact that the saturation vapour pressure above a curved surface of water is different from that of a completely flat surface. The saturation vapour pressure above a flat surface of liquid water, psat* (when r = •), is dependent upon the pressure applied to that liquid by the surrounding air. For a given air pressure, the saturation vapour pressure of the water becomes psat = psat *eVm DP /RoT (Atkins & De Paula, 2006). When g = surface tension R = radius of curvature

q

g

g

Atmosphere (air)

po

q

Water droplet

pi R qq

p = pi – po

where: p = total pressure pi = pressure inside droplet po = atmospheric pressure (outside droplet)

Glass surface

1.13 The formation of a water droplet (adapted from Bowles, 1984).



31

the surface of the water is curved to a known radius, r, a pressure change (negative for water) occurs where DP = –2g/r (see above for droplet). Note that DP is referred to as the capillary potential, Y, which is used to quantify the motivation for capillarity and is often termed ‘suction’ because it is a negative pressure differential measured in Pascals. We can see that the saturation vapour pressure of water held inside a pore of radius r (and having a contact angle a) is less than that of a flat surface, i.e. psat < psat*. This of –2 V /rR T course leads to Kelvin’s equation, psat = psat *e g m 0 , where Vm = molar volume of water (~ 18 ml at STP), and R0 = the gas constant (Atkins & De Paula, 2006). In the case of hygroscopic moisture (vapour phase, of known partial pressure) that has been absorbed by a void, Kelvin’s equation can simply be rearranged to find the ‘critical radius’ of that void (rcrit), assuming pv = psat. At this point moisture vapour condenses inside the pore to restore thermodynamic equilibrium. Obviously, where radii vary within complex pore structures, it follows that vapour condenses to fill the pore with liquid when r < rcrit, and vice versa. This can lead to an interesting phenomenon known as the ‘ink bottle effect’, where a larger void has only narrow entry points. If r < rcrit at the entry points, but r > rcrit within the large void itself, then vapour can condense in the entry points effectively blocking the void from absorption or desorption. Kelvin’s equation can also be re-written so that if void radius is known, then the relative humidity, j, of air (where j = pv/psat) at which condensation will occur is given by:

j=

Ê 2g pv 1 ˆ = exp Á psat Ë r Rvap rwT ˜¯

1.35

where Rvap is the gas constant for water vapour (J/kg K) and rw the density of liquid water (kg/m3). It follows that in a porous material there are a wide range of voids and void sizes. As relative humidity increases the hygroscopic water increases, and so does the potential for some of this to condense to liquid inside the material because rcrit increases. Therefore at constant temperature, there is an equilibrium moisture content (EMC) for every value of j between 0 and 1 (some of which will condense if r < rcrit) that is characteristic of each porous material. Furthermore, some of the absorbed water molecules can be adsorbed by the surfaces of the void which raises their enthalpy of vaporisation correspondingly. This results in hysteresis between the EMC when Dj is positive (progressive wetting), and when Dj is negative (progressive drying). By plotting moisture content against j, a pair of sorption isotherms (or ‘moisture storage functions’) can be plotted for a particular material: a wetting isotherm and a drying isotherm (see Fig. 1.14). The gradient of an isotherm (below q80, where capillary condensation makes the isotherm slope increase rapidly) gives us a single value known as the sorption capacity, x. Once a material’s sorption isotherms have been


dw ∂ RH

Drying isotherm Wetting isotherm

0

10

20

30

40

50 RH (%)

60

70

80

90

100

1.14 A typical sorption isotherm with wetting and drying curves (taken from Hall & Allinson, 2009a).

Decreases

Increasingly wet Porous media

Suction pressure Increases

x=

Film of hygroscopic water

q80

Increasingly dry

qr

Hygroscopic water

Capillary condensation

Hygroscopic water

Multi molecular

Hygroscopic domain


Mono molecular

Capillary domain

Water molecule sorption

Pore spaces filled with air

qaev qc


qs

All pore space filled with water

10–7

Capillary water

10–8

Soil Pore spaces particles filled by air

5

Condensation

Critical pore radius (m) 10–9 2

5

Gravitational domain

3

32


33

characterised in this way, the EMC can be interpolated for any value of j. Note that if j periodically fluctuates as a function of time, the EMC corresponds to the values on the wetting isotherm for the periods of time when Dj is positive, and then jumps across to the drying isotherm for the periods of time when Dj is negative. It is important to distinguish that the sorption isotherm enables us to predict how much time is needed for the moisture content to reach EMC once Dj has occurred. The response time of the material to Dj is measured separately using the moisture buffer value (MBV) test (DTU, 2005). Some materials have relatively high moisture storage functions (i.e. steep isotherm gradients) but low MBVs, which could be interpreted as having a larger capacity to store moisture as a function of j, but a slower response time to changes in j as a function of time. Other materials exhibit the opposite sort of behaviour, i.e. quick response times but lower storage capacity. The behaviour of materials in this respect is discussed in more detail in Chapter 14. The sorption isotherm in Fig. 1.14 also shows that at very low relative humidity, single layer adsorption followed by multi-layer adsorption of water vapour molecules occurs within the pore structure of the material. Metastable groups of adsorbed water vapour molecules can spontaneously nucleate into a liquid water meniscus that is in equilibrium with the relative humidity for a given pore radius (Hall & Allinson, 2009a). It is unclear how valid the assumptions of Kelvin’s equation become when the theoretical values for rcrit (which are constants at known T) reach this scale. It is logical to assume that the validity applies to the portion of a sorption isotherm where capillary condensation will occur in pore radii sufficient to permit thicknesses greater than multi-molecular layers (i.e. droplets) which typically occur at j ≈ 0.5. Despite this, the occurrence of significant liquid water flow (induced by dY/ dx) is typically not considered until j > 0.8 (Künzel, 1995). The reference moisture content for this value is referred to as q80 as shown in Fig. 1.14. Another point of interest on the sorption isotherm is the transition from hygroscopic domain to the capillary domain, which occurs at the hypothetical point when j Æ 1 (Hall & Allinson, 2009a). This corresponds to the value known as the residual moisture content, qr, which has to be interpolated as opposed to measured experimentally. For numerical modelling, the maximum hygroscopic moisture content that can be readily determined experimentally is normally taken to be 95 or 98% (Künzel, 1995; Valen, 1998). When q is in the capillary domain, water transport is dominated by capillary potential, Y, or ‘suction’ (Pa) and the microstructure is referred to as ‘unsaturated’.

1.3.4 Capillarity Capillarity (or capillary suction) is the natural phenomenon that many of us have experienced at an early age by placing one end of a narrow tube in a


34


tray of water and observing the rise of water inside the tube. We may also notice, by using a range of different width tubes, that as the radius decreases the height to which the water rises will increase. Referring to the portion of the sorption isotherm (see Fig. 1.14) above the residual water content, qr, we can see the so-called capillary domain. During wetting, absorption will continue until the relative water content is at capillary saturation, qc, at which point the total water pressure Pw = Pa – (2g/r + rwgh), for the supported mass of water inside the pore structure (where g is the gravitational constant 9.81 m/s2) (Atkins & De Paula, 2006). We already know that saturation vapour pressure above a curved water surface is different from that above a flat surface by the Kelvin equation. Water rises inside the capillary simply to offset the pressure differential between the water and the surrounding air (which includes air + water vapour). The maximum theoretical height that the column of water can achieve in a fully wet capillary is then given by:

h=

2g rrw g

1.36

As previously mentioned, gravitational water is liquid that is absorbed when the capillary potential in the voids is close to zero. We now know that this is additional water absorbed when the relative moisture content is > qc. When q >> qc we refer to this as the gravitational domain although it is often known as ‘super saturation’, because the water theoretically cannot enter by capillarity. It is perhaps obvious that, if qc qAEV) the force due to gravity on the additional mass of the non-capillary (or gravitational) water is greater than the attraction of the remaining net surface charge inside the pore structure. Vos and Tammes (1968) performed various experiments on the capillary movement of water and concluded that, amongst other things, water moving through a porous material by capillarity could travel twice as far in a horizontal direction as in an upward direction. This is because, in the latter case, the direction of flow is perpendicular to the force of gravity and so its effect in opposing the flow is at a minimum. If a temperature gradient exists along a capillary vessel, the flow of water within that capillary will occur in the direction of the lower temperature (Vos & Tammes, 1968). This is because the electrostatic potential of the void internal surfaces is a monotonously decreasing function of temperature. This occurs because of the principles surrounding the conservation of energy in a system. If a pocket of water is held in a cylindrical void of uniform diameter, and one end of the void is



35

heated, the water moves towards the colder end. In addition, water that is held within a void of uniform temperature, but that has a tapering diameter, will move towards the narrower end of the capillary through inducement by the higher capillary potential. The actual electrostatic charge of the void’s internal surfaces is assumed to be the same, and so when the void radius decreases towards the tapered end, the term 2g/r becomes larger which increases Y. Consequently, if two porous materials are in contact with one another, water transport will occur from the larger diameter voids into the smaller diameter voids. This process occurs until the net gradient of pressure differentials has equilibrated (Vos & Tammes, 1969). Capillary conduction describes the flow of liquid water through the pore network. If the water content is sufficiently high (i.e. capillary saturation), then Y = 0 and the rate of flow of the water is governed by (i) fluid pressure differential and (ii) the resistance to flow offered by the material’s pore network, e.g. due to tortuosity. This saturated flow is governed by Darcy’s law and is explained in more detail in Section 1.3.5. When the water content is between qr and qAEV, the material is ‘unsaturated’. The gradient of capillary potential, Y (or ‘suction’) provides the motivation force for liquid transport in unsaturated porous materials. The liquid conductivity, K (s/m) is therefore dependent on capillary potential, and so the liquid water flux can be written as:

gl = K (Y ) ∂Y ∂x

1.37

We can see from Fig. 1.15 that Y can become extremely high in the hygroscopic domain and devices for measuring suction pressures (potentiometers) are typically designed for the capillary domain, making them incapable of measuring suction pressures of this magnitude. However, we know that Y = DP = 2g/r and so we now have the relationship:

Y = RvaprwT ln (j)

1.38

In this way, capillary potential can be approximated from the EMC corresponding to a given relative humidity, j. In an applied sense, when capillary domain water is absorbed into porous building materials a number of complexities can arise at various stages of the process. Hall (1977) used the classic example of a dry brick placed in a shallow tray containing clean water to illustrate this. From the moment this is done water is absorbed into the brick by capillarity and then attempts to distribute itself throughout the pore network. The absorbed water partially displaces the air that previously occupied the dry pores. At the surfaces of the material that are not immersed, the process of evaporation has begun to occur which results in a cooling of the surface. The resulting thermal gradient assists transport of water to the


36

Materials for energy efficiency and thermal comfort in buildings Residual air content

AEV

qs

Gravitational domain

Drying

qr 0 10

Capillarity only

Wetting

Capillarity + other

Relative moisture content

qaev qC

Capillary domain

Hygroscopic domain

100

1000 10 000 100 000 1 000 000 Capillary potential (kPa)

1.15 Capillary potential vs. relative water content with corresponding domains (Hall & Allinson, 2009a).

evaporating boundary, at which point equilibrium can be achieved between capillary water absorption and evaporative drying. Soluble salts in the absorbed water can be deposited in crystalline form at the surfaces where water evaporates; a process known as ‘efflorescence’.

1.3.5 Liquid flow In a fully saturated porous material the mean flow velocity, ux, of liquid water is proportional to the pressure gradient in the direction of flow, as defined by Darcy’s law. The pressure gradient is hydrostatic, the value of which is the difference in height (or static head) h1 – h2. Darcy’s law can therefore be written for flow velocity where:

u x = – k ∂h ∂x

1.39

The Darcy coefficient of permeability, k (m/s), is a constant within the formula and its value depends mainly upon (i) the pore structure of the material (e.g. porosity, tortuosity), and (ii) the properties of the permeating liquid such as viscosity and surface tension. Measurement of k is normally determined experimentally by measuring the steady-state flow rate of water through a saturated specimen under a static pressure differential. The apparatus is referred to as a permeability cell, and the test specimen is normally a circular



37

disc, the sides of which are impermeably sealed to ensure uni-axial flow of the liquid. It is reasonable to consider the fluid (water) in this case to be incompressible and so the pressure gradient is linear, under steady state conditions. The intrinsic permeability, k, of a material is given by:

k=

kh rw

1.40

Note that this is distinct from the hydraulic conductivity of a material, K, which is measured in s/m. As with saturated specimens, liquid transport inside an unsaturated porous material is still governed by pressure differentials. The difference is that the mean pressure difference is negative, and hence is described as a ‘suction’ or more accurately as a capillary potential, Y. For unsaturated porous media the mean flow velocity, ux, can instead be written with a moisture-dependent coefficient of permeability (Hall & Yau, 1987):

u x = – K (q ) ∂Y ∂x

1.41

As previously discussed, a fully dried material can only exist after drying to constant mass at 105 °C, and a fully saturated material can normally only occur using vacuum saturation. We know that capillary domain water (entering VT in the liquid phase) is absorbed by the phenomenon known as capillarity. We also know that the transport of liquid water by capillarity occurs due to the pressure differential created by curvature of the absorbed water as a result of its dipole attraction to the void interior surfaces. However, by considering continuum-level absorption of liquid water into an entire complex network of interconnected voids (as in a real porous material), it is often helpful to consider the absorbed water as a bulk quantity. This means that the capillary potential of the pore network is reduced to a volume-averaged mean property. The volume-averaged Y, for example, results from the net contribution by every pore in the network; some will have smaller radii and higher Y values, others will have larger radii and lower Y values. Hence, when liquid water is absorbed by a porous material, the ‘sharp wet front’ (SWF) approximation is that the advancing wet front of the liquid can be represented by a rectangular cross-sectional profile, as shown by Fig. 1.16. SWF theory, in this sense, assumes that the porous material is semi infinite and that (if edge effects are ignored) the transport of water is one-dimensional (see Fig. 1.16). This is indeed appropriate in many cases where a material’s physical properties are being assessed, as in other similar cases such as the permeability cell described above. The SWF approximation also assumes that in the wetted region q = qc, and that this water content is uniform and constant. The wetting front itself has a hypothetical constant capillary


38

Materials for energy efficiency and thermal comfort in buildings P P0

P0

Yf x

0 xf

L

x f¢

A

1.16 Diagrammatic illustration of the sharp wet front analogy for absorption (adapted from Hall & Hoff, 2002).

potential, Yf, which differs from the capillary potential of the unsaturated region ahead by the amount Y – Yf. It can be appreciated from Fig. 1.16 that the one-dimensional absorption of water into the dry porous material (of length L) begins at the interface between a static water source and the material itself, at a time t = 0 and where x = 0 (Hall & Hoff, 2002). Note that the advancing wet front is located at xf = l(t) throughout the absorption process. The porous medium has an effective permeability coefficient, Ke and so the expression for mean flow velocity using Darcy’s law can be expressed as (Hall & Hoff, 2002):

Yf u = – K e dF = – K e dx L

1.42

Note that at x = 0 the total potential F = P0, where P is the hydrostatic fluid pressure. Also, at x = xf the total potential F = P0 + Yf. This gives a new expression for the cumulative volume of absorbed water per unit inflow surface area, i (mm3/mm2, or simply mm) as a function of elapsed time, t (min) (Hall & Hoff, 2002):

i = (2feKe |Y f |)0.5t0.5

1.43

The cumulative volume of absorbed water per unit inflow surface area (i) is first calculated thus:

i = Dw/Ar = St0.5

1.44

Since the sorptivity, S, is the linear regression slope of the straight line produced from i/t0.5, it follows that in the one-dimensional case S can also be expressed as:

i = (2f K Y )0.5 S = 0.5 e e f t © Woodhead Publishing Limited, 2010

1.45


39

Note that many people use the term ‘water absorption coefficient’ in place of the sorptivity, as defined in EN ISO 15148, and it has the symbol Aw. In the case of absorption by capillarity, i can be determined using the relationship i = Dmw/Arw (Hall & Hoff, 2002; Hall & Kam-Ming, 1986). This can be measured (and the mathematical relationship validated) experimentally by the partial immersion test for porous materials. The test method is essentially a gravimetric determination of absorbed water, as a function of time, in a partially immersed porous material. The cumulative mass of absorbed water per unit inflow surface area increases linearly against the square root of elapsed time, referred to as the ‘linearity rule’. The sample rests on small stands in a tray of water, and is periodically weighed on a balance at certain elapsed time intervals (e.g. every minute) which involves interrupting the test for a brief period, normally less than 30 seconds. The wetted sample surface has to be wiped with a damp cloth to remove excess water that can present a degree of operator error and a potential drying effect depending upon the type, and moisture content, of the cloth. The testing procedure can be enhanced by maintaining a constant temperature of 20 °C (± 1°) in order to ensure more constant density and viscosity of the water. The depth to which the sample is partially immersed is 5 mm ± 1, which is neglected in the calculation for inflow surface area following the assumption of onedimensional liquid transport. For experimental control, however, the partial immersion depth can be adequately maintained by equipping the water-filled tray with a weir with constant overflow and a re-circulating pump (see Fig. 1.17). Viscosity, h, and surface tension, g, are inversely proportional when the linearity rule for partially-saturated flow applies such that (s/h)0.5 is proportional to S, and hence the chief motivational force in the sharp wet front model is the gradient of capillary potential.

Test specimen Constant head

Deionised water

Weir Tank 5 mm

Circulating pump

1.17 Apparatus for partial immersion test for water absorption.


40


1.3.6 Vapour diffusion In broad terms, diffusion is the redistribution of a quantity from an area of higher concentration to an area (or areas) of lower concentration. In terms of mass, which in the context of this chapter is water, diffusion applies to the redistribution of water vapour. The mean rate of water vapour diffusion is proportional to the concentration gradient in the direction of mass transport, as stated by Fick’s law of diffusion. This can be written as:

gv,air = – D

∂Cv ∂x

1.46

The coefficient of diffusivity (D) is a constant within the formula and its value depends mainly upon the pore structure of the material (e.g. porosity, tortuosity). Note the initial conditions at t = 0 when the mass concentration of water vapour, Cv (kg/m3), decreases across a distance x1 – x2. Since the water vapour concentration is essentially vapour density, it can be expressed in terms of its partial pressure (because it is mixed with air) using the ideal gas law to give:

Cv = rv =

pv M w RoT

1.47

where rv is density of water vapour (kg/m3), pv is partial pressure of water vapour (Pa), Mw is the molar mass of water (kg/kg mol), and Ro is the universal gas constant (J/kg mol K). We can transpose this term into the Fick’s law equation above to give:

gv,air = – D

M w ∂pv RoT ∂x

1.48

As the water vapour permeability of still air, da (kg/m s Pa), can be defined as:

da = D

Mw RoT

1.49

it follows that our equation for Fick’s law of diffusion, now specific to water vapour moving in still air, can be re-written as the partial differential:

gv,air = – d a

∂pv ∂x

1.50

Water vapour is, of course, absorbed by porous materials and so (from the moment it passes the threshold to enter VT) becomes hygroscopic water. It enters VT through pore openings at the surface of the material and is then



41

able to migrate throughout the accessible pore network. Importantly, the fluid transport mechanism in porous materials is not defined simply by its domain classification. Once fluid has entered VT (i.e. been absorbed), the intrinsic conditions of the pore network will determine (i) fluid phase, and (ii) its associated transport mechanism(s). To elaborate on this, hygroscopic moisture (absorbed as vapour) will subsequently be adsorbed by internal surfaces, condense or diffuse. Capillary moisture (absorbed as liquid) will either flow or vaporise. The intrinsic properties that determine these conditions include pore radius, pore geometry, surface charge, and surface temperature. Therefore, absorbed water can change phase inside the pore network regardless of its domain. Note that the domain thresholds are always constant for a particular material. The complex spatial connectivity of the pore spaces increases path lengths, turbulence and flow restrictions (collectively known as ‘tortuosity’) and so providing additional resistance to vapour diffusion. The water vapour diffusion resistance factor, m, is used to quantify these effects and is calculated simply as the ratio between the vapour permeability of still air (see Eq. 1.49 above) and the vapour permeability of the material, dp (kg/m s Pa):

m=

da dp

1.51

The total rate of water vapour transfer through a porous material can therefore be calculated in one of two ways using:

gv = – d p

∂pv d ∂p =– a v ∂x m ∂x

1.52

The actual transport of water vapour, inside the complex pore network of a porous material, occurs by some or all of the following mechanisms: ∑ Fickian vapour diffusion through larger air-filled pores ∑ Knudsen diffusion through very small pores, and ∑ surface diffusion along the inner surface of all pores. For pores with very small radii, the mean molecular free path of the water vapour molecules can approach or even exceed the pore diameter itself. The ‘mean molecular free path’ is simply a mean distance over which a gas/ vapour-phase molecule will travel before (statistically) it has a collision with another gas/vapour-phase molecule of the same type (Bird et al., 2001). We know from the conservation laws that the total mass, energy and momentum before and after such a collision are the same. We can also recall that this is the fundamental mechanism by which molecular kinetic energy is transferred (in the form of heat) from the molecules at a higher energy state to those of a lower state, e.g. natural convection in a fluid. In the case of water vapour,


42


when the mean molecular free path is less than the diameter of the vessel in which it is contained (i.e. the pore), the molecules are statistically more likely to impact the pore walls than one another. The resulting collisions between pore walls and water molecules (dipolar) means that a particular process known as Knudsen diffusion dominates transport of water vapour within pores of this size (typically nanoscale). In addition, water vapour molecules (dipolar) that are adsorbed by pore walls, either in a single layer or as multiple layers, can migrate along the surface of the pore by a process called surface diffusion. The water vapour flow velocity through a porous material can be determined experimentally using the wet cup/dry cup method (BS EN ISO 12572, 2001), which must be conducted inside a temperature and relative humidity controlled environmental chamber (see Fig. 1.18). The base of the cup is filled with a small quantity of saturated salt solution. The test specimen is sealed in at the top of the cup suspended above the salt solution. The species of salt selected determines the saturation vapour pressure of the air above the solution, at known temperature, and hence controls the relative humidity, j, of that air. The constant values for j for a wide variety of different salts can be found in Kaye and Laby’s Tables of Physical and Chemical Constants (1995). The environmental chamber in which the test takes place may typically operate at 23 °C air temperature and 50% relative humidity. If the vapour pressure inside the cup produces a relative humidity higher than this, it is a ‘wet cup’ test (e.g. potassium nitrate solution, KNO3 94.0% RH at 23 °C) and the driving potential for diffusive vapour transport will be from the cup to the environmental chamber (through the specimen). Clearly under these Environmental chamber Desiccator

Fan

Continuous air flow Specimen

Seal Support

Specimen

Cup

Saturated salt solution (control)

Saturated salt solution (test)

1.18 Apparatus for water vapour permeability testing (taken from Hall & Allinson, 2009a).



43

circumstances the moisture content of the specimen is greater than in the reverse situation of a ‘dry cup’ test. In this case the partial pressure inside the cup would be lower than ambient and water vapour transport is from the environment to the cup, hence the cup gains mass as opposed to losing mass in the wet cup test. The sides of test specimens must be sealed with epoxy resin to ensure uni-directional vapour transport through the specimen and to maintain constant inflow/outflow surface area. The absorbed mass of water (Dmw), in the case of a dry cup test, should increase linearly against the elapsed time (t). The slope of the straight line produced by Dmw/t gives the water vapour flow rate, G, through the specimen in kg/s. The water vapour permeance can then be calculated from: G 1.53 A · Dpv where A = specimen inflow surface area (m2), and Dpv = water vapour pressure difference across the specimen (Pa) which was calculated from the mean of the measured temperature and relative humidity over the course of the test. W =

1.3.7 Evaporation and drying In saturated or unsaturated porous materials, the process of drying occurs when the liquid water lost (by evaporation) at or near the surface is greater than the mass of water being stored or absorbed, i.e. when Dmw(t) is negative (Hall & Allinson, 2010). By referring to the sharp wet front (SWF) approximation discussed in Section 1.3.5, we can see that in the case of drying liquid water that is lost at (or near) the surface of the 1D semi-infinite material may be replaced by water moving from the interior flowing due to capillarity (Hall & Hoff, 2002). Obviously the phase change process results in evaporative cooling at or near the surface of the material which serves only to increase the gradient of capillary potential in this direction. The interface at which evaporative drying occurs can occur where x = xevap > 0 (i.e. still within the pore network). If this occurs, the water is transported from x = L to x = xevap by capillarity, and then from x = xevap to x = 0 by vapour diffusion, before finally leaving VT (i.e. desorption) (Hall & Hoff, 2002; Schaffer, 1932). The transport mechanism for desorption to the surrounding atmosphere is vapour diffusion augmented by natural convection. As water is being desorbed it is reasonable to assume that the partial pressure equals the saturation vapour pressure in the thin layer of air immediately adjacent to the surface, therefore we can say that j = 1 at the surface–fluid boundary. The volume (or quantity) of water being vaporised per unit surface area can be described as the evaporation flux, er, which can be quantified with reference to Fick’s law of diffusion (Hall & Hoff, 2002):


44


er = – rw Dw

∂p ∂cw = – rw Dv v ∂x ∂x

1.54

For the semi-infinite 1D porous material in Fig. 1.16, the sharp wet front (SWF) approximation was used in Section 1.3.5 to mathematically describe the advancing wet front of water as it was absorbed through the interface at x = 0 (Hall & Hoff, 2002; Hall & Allinson, 2009b). We assumed that as the wet front advanced towards x = L, the wetted region had an ‘effective’ relative moisture content that was uniformly ≤ qc. The cumulative volume of water per unit inflow surface area (i, mm3/mm2) increases linearly with the square root of elapsed time and can therefore be quantified as i = St0.5 (Hall and Hoff, 2002; Hall and Allinson, 2009b). If the source of water at the interface x = 0 is removed, evaporative drying immediately begins to occur as described by the processes above. Since this will result in q decreasing as a function of time, it is appropriate for us to use a term for the cumulative desorption per unit outflow surface area, id (mm3/mm2). Crucially, for absorption we know that Di is proportional to Dt0.5, and for desorption Did is also proportional to Dt0.5. The latter relationship allows us to create an opposite single term to sorptivity, and that is ‘desorptivity’, Sd (mm min–0.5) (Hall & Hoff, 2002) where:

id = t 0.5 Ú

q0

q1

f (q )dq = Sd t 0.5

1.55

As mentioned previously, the process of drying is essentially moisture lost from a porous material, where Dmw(t) < 0, which occurs by evaporation at or near the surface of VT. The mechanisms of drying can change as a function of time and so are normally characterised as being in one of two stages as discussed in the following sections. Stage 1 evaporative drying For Stage 1 drying to occur the material must have pore diameters that are sufficiently wide and/or continuous to allow gravitational water to exist. The rate of evaporative drying for gravitational pore water is the same as that for the surface of free (or unbound) water, e.g. from a Petri dish, under the same environmental conditions (Hall et al., 1984). Stage 1 is commonly referred to as the ‘constant drying rate’ process which exists where the evaporation flux, er @ constant. This is true but it may be even more accurate to state that the rate of Stage 1 drying, r1 is controlled by the ambient environmental conditions of partial vapour pressure and surface air flow velocity (Hall & Allinson, 2010). The reason for this distinction is because only if these environmental conditions are constant will the drying rate itself be constant, hence the need for the approximation symbol for evaporative flux above. The surface–fluid © Woodhead Publishing Limited, 2010


45

boundary layer at x @ 0 is assumed constant and at saturation vapour pressure, which means that Fick’s law of diffusion can be used to quantify r1 where vapour diffusion across that boundary layer is considered:

M D Ê dp ˆ r1 = – rw Dv Á v ˜ = – rw w w dx RT Ë ¯

Ê dpv ˆ ÁË dx ˜¯

1.56

If we look more closely at what is happening, the thin layer of air in contact with the surface that we have termed ‘surface–fluid boundary layer’ can in fact be quantified as having finite thickness d. Within this layer, we already know that the partial pressure pv = psat at x = 0; however, we can add that pv decreases linearly with distance towards x = d which is known as the ‘far stream’ or outer edge of the surface–fluid boundary layer. This linear partial pressure gradient that occurs within the boundary layer can be accounted for by modifying Eq. 1.56 to give (Hall & Hoff, 2002):

r1 = rw

M w Dw Dpv RT d

1.57

Stage 2 evaporative drying Unlike Stage 1 drying which, as detailed in the previous section, requires the presence of gravitational water, Stage 2 drying only requires the presence of capillary water. That is to say, the porous material is ‘unsaturated and so its relative moisture content is between qr and qAEV. The Stage 2 drying rate is often referred to as the ‘falling drying rate’ because the evaporation flux, er, exceeds the mean flow velocity, u, of the water inside the porous material as it is transported towards xevap. Since the material is unsaturated, internal flow velocity of the liquid water is determined by the gradient of capillary potential, which is of course dependent on water content. As the material continues to dry, the internal flow velocity progressively decreases due to the falling gradient of capillary potential and so reducing the quantity of water being transported to xevap as a function of time (Hall & Allinson, 2010). We can therefore write an expression that allows us to quantify the Stage 2 rate of evaporative drying:

r2 =

did 1 –0.5 = t dt 2

q0

Úq

1

f (q )dq

1.58

Both the internal and surface capillary potentials are at zero when drying begins (at t = 0) because we have assumed uniform distribution of water. As evaporation proceeds there is a corresponding decrease in q due to evaporative loss thus creating a differential between the surface and internal capillary potentials, respectively. In experimental practice, establishing a


46


capillary potential (or vapour pressure) gradient is not instantaneous and so a short period of time (of varying length) may be anticipated until the system equilibrates and a constant flow velocity is approached. The transition from Stage 1 to Stage 2 drying occurs at the point when the water content equals the air entry value (AEV), qAEV. Above this critical point gravitational water can exist within the pores, and below this point the capillary potential is always sufficient to prevent free draining. On the pore water characterisation curve in Fig. 1.15, we can see hysteresis between the respective absorption and desorption values for the moisture-dependent capillary potential (or suction). This hysteresis occurs due to the combined effects of: ∑ ∑

differences between advancing and receding fluid contact angles, and the ‘ink bottle’ effect (continuous-pore radii variations).

We can therefore conclude that both the type (Stage 1 or 2) and rate of drying from a porous material can be determined from classification of the absorbed water. This can be illustrated using a hypothetical capillary tube of length l, where the maximum height of capillary rise hc is less than l. We can immediately see that when hc = l the relative water content equals qc. Since by definition qc ≤ qAEV, and evaporative loss occurs at the boundary x = hc, reduction in q occurs as a function of time through Stage 2 drying. If the hypothetical capillary tube is filled by capillarity until q = qc, and then filled from the top until q = qs, then Stage 1 evaporative drying begins at the boundary x = l (see Fig. 1.19). We can recall that this will mean the drying rate is constant during Stage 1. The transition to Stage 2 drying occurs when the reduced water content equals qAEV at which point the rate of drying begins to fall as a function of the gradient in capillary potential inside the capillary tube (Hall & Allinson, 2010).

1.4

Summary

To summarise, this chapter considers materials within the context of building physics that should enable the reader to appreciate how heat energy and mass (water vapour and liquid) can be absorbed, stored and released in relation to the building fabric. The fundamentals and driving potentials behind these processes have been discussed at length, which should enable an appreciation of the following facts regarding heat and mass transport phenomena: ∑ They occur simultaneously and are strongly inter-dependent. ∑ They cannot be treated in isolation from one another. ∑ They are essential in relation to the performance of the fabric of our buildings (energy efficiency, durability, weather tightness, etc.). ∑ They are essential for understanding the interaction between building fabric and occupied spaces, i.e. indoor environments.


Heat and mass transport processes in building materials 1D isothermal stage 1 drying

P Y S, P s

YI

0 Far stream

47

L

x=L

x=0

d

A 1D isothermal stage 2 drying

P

YI YS, P s L

Far stream

0

x=L

x=0

d

x = xevap A

1.19 Illustration of the sharp wet front analogy applied to Stage 1 and Stage 2 evaporative drying in porous materials (adapted from Hall & Hoff, 2002).

∑

They are essential for understanding conditions of indoor comfort and health.

1.5

Sources of further information

There is a vast array of information sources, databases, organisations and academic research communities working in different areas of this field. The following section provides directions and links for keen readers to find out more. The following offer membership, conferences, meetings and publications plus involvement in knowledge transfer networks and student sponsorship:


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∑

The Chartered Institute of Building Services Engineers (CIBSE), 222 Balham High Road, London SW12 9BS. Tel +44 (0)20 8675 5211, Fax +44 (0)20 8675 5449. ∑ American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE), 1791 Tullie Circle, N.E., Atlanta, GA 30329, USA. ∑ Institute of Materials, Minerals and Mining (IOM3), 1 Carlton House Terrace, London SW1Y 5DB. Tel: +44 (0)20 7451 7300, Fax: +44 (0)20 7839 1702.

Many leading international journals support the dissemination of high quality original research and are an excellent source for state-of-the-art knowledge, understanding and applications plus an awareness of active researchers and their particular specialism within the field. Recommended journals include: Building and Environment, Applied Thermal Engineering, Construction and Building Materials, Energy and Buildings, Journal of Building Physics and International Journal of Heat and Mass Transfer.

1.6

References

Asan H, 1998, ‘Effects of Wall’s Insulation Thickness and Position on Time Lag and Decrement Factor’, Energy and Buildings, 28, 299–305. ASHRAE, 2005, ASHRAE Handbook – Fundamentals, ASHRAE, Atlanta GA, USA. Atkins P & De Paula J, 2006, Atkins’ Physical Chemistry, 8th edn, Oxford University Press. Bird RB, Stewart WE & Lightfoot EN, 2001, Transport Phenomena, 2nd edn, Wiley. Bowles JE, 1984, Physical and Geotechnical Properties of Soils, 2nd edn, McGraw-Hill International Book Company. Bristow KL, 1998, ‘Measurement of thermal properties and water content of unsaturated sandy soil using dual-probe heat-pulse probes’, Agricultural and Forest Meteorology, 89, 75–84. BS EN ISO 8990, 1996, Thermal insulation – Determination of Steady-state Thermal Transmission Properties – Calibrated and guarded hot box, British Standards Institute, London. BS EN ISO 12572, 2001, Hygrothermal performance of building materials and products – Determination of water vapour transmission properties, British Standards Institute, London. BS EN 15026, 2007, Hygrothermal performance of building components and building elements. Assessment of moisture transfer by numerical simulation, British Standards Institute, London. CIBSE, 2006, Guide A – Environmental Design, 7th edn, Chartered Institute of Building Services Engineers, London. CIBSE, 2007, Guide C – Reference Data, Chartered Institute of Building Services Engineers, London. Cole RJ & Sturrock NS, 1977, ‘The convective heat exchange at the external surface of buildings’, Building and Environment, 12, 207–214.



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Doran S, 2000, ‘Field investigations of the thermal performance of construction elements as built’, Building Research Establishment Report #78132 – client report for DETR, Building Research Establishment, Garston. DTU, 2005, ‘Moisture buffering of building materials’, Report BYG-DTU R-126, Denmark Technical University. Energy Plus Engineering Reference Online, accessed March 2009, http://apps1.eere. energy.gov/buildings/energyplus/pdfs/engineeringreference.pdf Grimmond CSB, Potter SK, Zutter HN & Souch C, 2001, ‘Rapid methods to estimate sky-view factors applied to urban areas’, International Journal of Climatology, 21, 903–913. Hall C, 1977, ‘Water Movement in Porous Building Materials – I: Unsaturated Flow Theory and its Applications’, Building and Environment, 12, 117–125. Hall C & Hoff WD, 2002, Water Transport in Brick, Stone and Concrete, Taylor & Francis, London. Hall C & Kam-Ming T, 1986, ‘Water Movement in Porous Building Materials – VII: The Sorptivity of Mortars’, Building and Environment, 21, 113–118. Hall C & Yau MHR, 1987, ‘Water Movement in Porous Building Materials – IX: The Water Absorption and Sorptivity of Concretes’, Building and Environment, 22, 77–82. Hall C, Hoff WD & Nixon MR, 1984, ‘Water Movement in Porous Building Materials – VI: Evaporation and Drying in Brick and Block Materials’, Building and Environment, 19, 13–20. Hall M & Allinson D, 2008a, ‘Assessing the Effects of Soil Grading on the Moisture Content-Dependent Thermal Conductivity of Stabilised Rammed Earth Materials’, Applied Thermal Engineering, 29, 740– 747. Hall M & Allinson D, 2008b, ‘Assessing the Moisture Content-Dependent Parameters of Stabilised Earth Materials Using the Cyclic-Response Admittance Method’, Energy and Buildings, 40, 2044–2051. Hall M & Allinson D, 2009a, ‘Analysis of the Hygrothermal Functional Properties of Stabilised Rammed Earth Materials’, Building and Environment, 44, 1935–1942. Hall M & Allinson D, 2009b, ‘Influence of Cementicious Binder Content on Moisture Transport in Stabilised Earth Materials Analysed using 1-D Sharp Wet Front Theory’, Building and Environment, 44, 688–693. Hall M & Allinson D, 2010, ‘Analysis of evaporative drying in stabilised compressed earth materials using unsaturated flow theory’, Building and Environment, 45, 3, 509–518. Incropera FP, DeWitt DP, Bergman TL & Lavine AS, 2007, Fundamentals of heat and mass transfer, 6th edn, John Wiley & Sons. ISO 8301, 1991, Thermal insulation – Determination of Steady state thermal resistance and related properties – Heat flow meter apparatus, International Organization for Standardization, Geneva, Switzerland. ISO 10051, 1996, Thermal Insulation – Moisture Effects on Heat Transfer – Determination of Thermal Transmissivity of a Moist Material, International Organization for Standardization, Geneva, Switzerland. ISO 10456, 1999, Building materials and products: Procedures for determining declared and design thermal values, International Organization for Standardization, Geneva, Switzerland. Kaye GWC & Laby TH, 1995, Tables of Physical and Chemical Constants, 16th edn, available National Physics Laboratory Online. Künzel H, 1995, ‘Simultaneous heat and moisture transport in building components’, Report based on PhD thesis, Fraunhofer Institute of Building Physics, Germany. © Woodhead Publishing Limited, 2010

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Liesen RJ & Pedersen CO, 1997, ‘An evaluation of inside surface heat balance models for use in a heat balance cooling load calculation procedure’, ASHRAE Transaction, 103, 485–502. McClellan TM & Pedersen CO, 1997, ‘Investigation of outside heat balance models for use in a heat balance cooling load calculation procedure’, ASHRAE Transaction, 103, 469–484. McMullan R, 1992, Environmental Science in buildings, 3rd edn, Macmillan Press, Hampshire. Pilkington B, Griffiths R, Goodhew S & de Wilde P, 2008, ‘Thermal Probe Technology for Buildings: Transition from Laboratory to Field Measurements’, J. Arch. Engrg, 14, 111–118. Rees SJ, Spitler JD, Davies MG & Haves PH, 2000, ‘Qualitative Comparison of North American and UK Cooling Load Calculation Methods’, International Journal of Heating, Ventilation, Air Conditioning and Refrigeration Research, 6, 75–99. Schaffer RJ, 1932, The Weathering of Natural Building Stones, HMSO, London. Valen MS, 1998, Moisture Transfer in Organic Coatings on Porous Materials – The Influence of Varying Environmental Conditions, PhD Thesis, Norwegian University of Science and Technology, Trondheim. Vos BH & Tammes E, 1968, Flow of water in the liquid phase, [report] Report no. B 1-68-38, Inst. TNO for Building Materials and building structures, Delft, Holland. Vos BH & Tammes E, 1969, Moisture and moisture transfer in porous materials, [report] Report no. BI-69-96, Organisation for industrial research TNO, Institute TNO for building materials and building structures, Delft, Holland.

1.7 A A s Bi C Cm c c cp c p* cs Cv D D w D v D s d e e r E

Appendix: Nomenclature area area of surface Biot number heat capacity molar heat capacity velocity of light (3.0 ¥ 108) specific heat capacity constant pressure specific heat capacity moisture content-dependent specific heat capacity wind speed mass concentration of water vapour diffusion coefficient for water vapour in still air molecular diffusivity of water molecular diffusivity of vapour hydraulic diffusivity thickness void ratio evaporation rate emissive power of a surface


m2 m2 – J/K J/mol K m/s J/kg K J/kg K J/kg K m/s kg/m3 m2/s m2/s m2/s m2/s m m3/m3 mm/s W/m2


E b Ebl Fo f F g G gl g v gv,air H h h c h n h f he i i d K K k Kn L Lp m mf m w Mw n P P 0 Pa p v psat psat* Pw Q q qlat qsens

emissive power of a blackbody spectral emissive power of a blackbody fourier’s number decrement factor radiation view factor total rate of mass transfer water vapour flow rate rate of liquid water transfer rate of water vapour transfer rate of water vapour diffusion in still air enthalpy planck’s constant (6.63 ¥ 10–34) total (or mean) convection coefficient natural convection coefficient forced convection coefficient specific latent enthalpy of evaporation (or condensation) cumulative absorbed volume of water per unit inflow surface area cumulative desorbed volume of water per unit outflow surface area boltzmann’s constant (1.38 ¥ 10–23) hydraulic conductivity intrinsic (or specific) permeability Knudsen number effective length perimeter length mass moisture factor for thermal conductivity mass of water molar mass of water bulk porosity fluid pressure standard atmospheric pressure (barometric) total air pressure partial pressure of water vapour saturation vapour pressure saturation vapour pressure above a flat water surface total water pressure heat flow total heat flux latent heat flux sensible heat flux


51

W/m2 W/m2mm – – – kg/s m2 kg/s kg/s m2 kg/s m2 kg/s m2 J Js W/m2 K W/m2 K W/m2 K J/kg mm3/mm2 mm3/mm2 J/K s/m m2 – m m kg – kg kg/kg mol m3/m3 Pa Pa Pa Pa Pa Pa Pa W W/m2 W/m2 W/m2

52


r rs r1 r2 R R T R0 R v Rvap R f S S d S r t T u x U V V a V s V T V v v w w W Y – a a b g da dp e h q q80 qAEV qc qr qs qsurf

radius rate of evaporative drying of a flat water surface rate of stage 1 (constant rate) evaporative drying rate of stage 2 (falling rate) evaporative drying thermal resistance total thermal resistance universal gas constant resistance to water vapour transfer individual gas constant of water vapour roughness index sorptivity desorptivity degree of saturation (or saturation ratio) time thermodynamic temperature mass vector flow velocity thermal transmittance volume volume of air volume of solid state matter total volume of a specimen volume of void wind velocity specific moisture content water vapour permeance Thermal admittance angle absorptivity thermal diffusivity thermal effusivity surface tension water vapour permeability of still air water vapour permeability emissivity viscosity relative moisture content relative moisture content in equilibrium with 80% humidity air entry value relative moisture content capillary saturation relative moisture content hygroscopic limit of relative moisture content saturated relative moisture content relative moisture content at x = 0


m g/m2 h g/m2 h g/m2 h m2 K/W m2 K/W J/kg mol K m2 s Pa/kg J/kg K – mm/min0.5 mm/min0.5 m3/m3 s K m/s W/m2 K m3 m3 m3 m3 m3 m/s kg/m3 kg/m2 s Pa W/m2 K degrees – m2/s Ws0.5/m2 K N/m kg/m s Pa kg/m s Pa – Pa s – – – – – – –


k l l l * lsolid lfluid l0fluid lrad lconv m x a x d r r rd rs rw r v r T rsi s t f j Y w v

Darcy’s coefficient of permeability wavelength dry state thermal conductivity moisture-dependant thermal conductivity thermal conductivity of a solid thermal conductivity of a fluid thermal conductivity of quiescent air effective conductivity due to radiation effective conductivity due to natural convection through a fluid water vapour diffusion resistance factor moisture capacity during absorption (from wetting curve) moisture capacity during desorption (from drying curve) reflectivity density dry density solid density density of water (assumed 1000 kg/m3 at 4 °C) density of water vapour total density solid density of each constituent in a composite Stefan–Boltzmann constant (5.669 ¥ 10–8) transmissivity thermal admittance time lag relative humidity capillary potential decrement factor time lag mass fraction of a constituent


m/s m W/m W/m W/m W/m W/m W/m W/m

53

K K K K K K K

– kg/kg kg/kg – kg/m3 kg/m3 kg/m3 kg/m3 kg/m3 kg/m3 kg/m3 W/m2 K4 – – – Pa – kg/kg

2


H. M. K ü n z e l, Fraunhofer Institute for Building Physics IBP, Germany and A. K a r agi o z i s, Oak Ridge National Laboratory, USA

Abstract: Energy efficiency and comfort are two new critical expectations of the modern building owner. With the recent technological advancement in all fields of engineering, buildings are also expected to be durable. Premature or accelerated ageing with the associated damage due to inappropriate design or operation is simply no longer acceptable. To prevent moisture-induced damage in buildings, hygrothermal simulation tools should be used to design envelope parts with good performance. The building envelope responds to the dynamic exterior and interior environmental loads. Temperature and humidity conditions in building structures must not exceed certain thresholds for effective moisture control. This has been recognized recently by a number of standards organizations in Europe and North America that deal with hygrothermal analysis. This chapter presents an overview of the fundamentals, the required material and construction input data, the required exterior and interior environmental loading, and the description of the output results from the heat and moisture transfer models. A few example cases are also presented that demonstrate how moisture problems may be investigated and how better design alternatives can become possible using hygrothermal analysis. Finally, the limits of the current state-of-the-art simulation models are described and futures trends and developments are discussed. Key words: hygrothermal analysis, moisture control, environmental loading, building envelope.

2.1

Introduction

Moisture control has become a worldwide issue because building operation and construction practices have been changing. The need to save energy has resulted in better insulated and airtight envelope systems which can become more sensitive to moisture problems than the traditional, poorly insulated constructions. Consumer demand for higher thermal comfort has increased the number of heating and air-conditioning systems installed in buildings. However, if the building envelope has not been designed to handle the higher thermal comfort requirements, under the recently imposed temperature and vapour pressure gradients, mould growth or condensation may occur. Appropriate moisture control design is a prerequisite for energy efficient and 54 © Woodhead Publishing Limited, 2010


55

damage-free new constructions as well as for the restoration or retrofit of traditional architecture. The right choice of moisture control measures for a particular building depends on a variety of parameters such as local climate conditions and construction traditions which differ from country to country, as well as constraints imposed by the architectural design and last but not least the available budget. It is almost impossible to establish general rules that apply everywhere. For this reason considerable scientific effort has been committed to the development of hygrothermal models that help to predict the transient temperature and humidity conditions in building envelope systems (walls, roofs and basements). Assessing the hygrothermal behaviour of traditional and modern buildings by applying hygrothermal models has become current standard practice (Karagiozis, 2001). This chapter deals with the exterior as well as the interior hygrothermal loads acting on constructions and the mathematical models that use these loads to simulate the response of the construction. Required input data such as material properties and boundary conditions are also described. The performance of hygrothermal simulation tools is demonstrated in comparison with experimental investigations. International standards and guidelines for moisture control are summarized and limitations of current models are discussed. Finally, future developments concerning moisture design with a safety factor are highlighted.

2.2

Hygrothermal loads

The main function of a building envelope is the protection of an enclosed space from the natural exterior environment. In Fig. 2.1 the hygrothermal loads acting on building envelope components are represented schematically for an external wall. Generally, temperature and humidity show considerable diurnal variations at the envelope’s exterior surface which are propagated only to a minor extent to the interior surface. During the daytime, the exterior surface heats up due to solar irradiation which leads to the evaporation of moisture from the surface layer. Around sunset when the short-wave solar radiation ceases, the long-wave (infrared) emission may lead to an overcooling (cooling down below ambient air temperature) of highly insulated wall systems and condensate may appear on the façade. The building envelope may also be exposed to wind-driven rain, which generally represents a more significant water load than exterior condensation. Groundwater is another frequent moisture load which may result in rising damp when water-proofing of the foundation is insufficient. Ambient temperature, relative humidity and partial vapour pressure form the permanent boundary conditions on both sides of the building envelope. The exterior conditions depend on the climate and the local environment. The indoor climate condition depends on the purpose and operation of the


56


Exterior

Solar radiation (direct & diffuse) Heat exchange with outdoor environment Wind-driven rain

Heat exchange with indoor environment

Vapour exchange with indoor air

Interior

Air pressure difference

Vapour exchange with outdoor air

Groundwater

2.1 Hygrothermal loads and their alternating diurnal or seasonal directions acting on the building envelope according to ASHRAE (2009).

building. Most buildings are controlled to keep the interior space comfortable for human beings and/or suitable for furnishings or artefacts. The resulting interior conditions represent an important hygrothermal load for the envelope that can be more severe than the exterior load especially when indoor moisture generation is high. For some specialized commercial constructions, libraries and museums, the temperature and humidity is controlled by HVAC (heating, ventilating and air conditioning) systems whose set-points are usually well defined. In many commercial and residential buildings the interior conditions are influenced by the behaviour of the occupants. In an average household up to 10 litres of water are evaporated every day. This moisture must be removed by ventilation or air-conditioning in order to assure comfortable and hygienic conditions. Often the operation of mechanical equipment (e.g. exhaust fans, fresh intake fans, recovery heat ventilators), or even leaky ducts due to improper workmanship, cause air pressure differentials which, in combination with stack and wind pressures, can significantly contribute to the moisture load, often more so than vapour diffusion. Another important moisture load, the so-called ‘initial construction moisture’ is often forgotten. In Europe building damage as a result of migrating construction moisture has become more frequent because tight construction schedules leave little time for building materials to dry out.



57

Construction moisture is either delivered with the building products or it is absorbed during the construction process. Cast-in-place concrete, autoclaved aerated concrete (AAC), and calcium silicate brick (CSB) are examples of materials that contain a lot of moisture when delivered. Unprotected wood kiln dried or even ‘green’ wood used predominately in North America and Scandinavian countries for framing, along with wood sheathing boards, can contain excessive amounts of moisture that if enclosed prematurely can do damage to the building construction (Karagiozis, 1998). Stucco, mortar, clay brick and concrete blocks are examples of materials that are either mixed or brought into contact with water at the construction site. All other porous building materials may take up precipitation or groundwater when left unprotected before the enclosure of the building. Vapour pressure differences within the envelope may cause moisture accumulation and drying at different locations. Air pressure difference along the face of the building and across the building envelope may assist in transport of moisture, which leads to either an accumulation or removal of moisture. Usually several hygrothermal load cycles are overlapping such as summer/winter, day/night and rain/sun. Therefore, a precise analysis of the expected loads should be carried out before designing a new building or changing the envelope of a traditional construction.

2.3

Modelling simultaneous heat and moisture transfer processes

In civil and architectural engineering, as well as for the purpose of heritage preservation, there is an increasing demand for calculative methods to assess and predict the long-term heat and moisture (hygrothermal) behaviour of building envelope systems. Controlling the temperature and humidity conditions in a particular wall or roof component is a critical task to prevent damage or premature ageing of building materials. The demand for better moisture control has been created by numerous moisture-related failures. Since there is no doubt that moisture transfer has an important influence on performance and service life of building components, the prediction of the hygrothermal behaviour becomes a prerequisite for damage-free and durable design. In the past these predictions were largely based on experiments, practical experience and simplified calculation tools, such as the steady-state dew-point method. Today there are a variety of transient calculation tools available (Trechsel, 2001) which provide reliable results if they meet certain specifications. These specifications have been laid down for the first time in the WTA Guideline 6-2 (2002) (WTA is the International Association for Science and Technology of Building Maintenance and Monuments Preservation). The German version of this guideline was issued in 2002 because existing


58


calculation methods were unable to predict the hygrothermal consequences of renovation measures or indoor climate changes in the case of traditional architecture. Given its international importance, an English version of this guideline was published only two years later. In the meantime the demand for hygrothermal simulation tools for design and forensic investigation purposes has increased considerably. As a consequence, a European Standard BS EN 15026 (2007) which is largely based on WTA Guideline 6-2 has been published. In order to comply with the requirements in BS EN 15026, transient hygrothermal simulation tools have to include the following transport and storage phenomena: ∑ ∑ ∑ ∑ ∑ ∑

heat storage of the dry building material and of the contained moisture heat transport by thermal conduction with moisture-dependent thermal conductivity latent heat transport by vapour diffusion with phase change (vapour evaporation/condensation) moisture storage by water vapour sorption and capillary forces water vapour transport by diffusion liquid transport by surface diffusion and capillary conduction.

As an example of the implementation of these hygrothermal phenomena, the differential equations employed for the well-established WUFI® model (Künzel, 1995; Karagiozis et al., 2001) are presented here: Moisture balance

∂w · ∂f = — · (D —f + d —(f p )) f p sat ∂f ∂t

2.1

Energy balance

∂H · ∂T = — · (l —T ) + h — · (d —(f p )) v p sat ∂T ∂t

2.2

where f is relative humidity, t is time, T is temperature, w is moisture content, psat is saturation vapor pressure, l is thermal conductivity, H is enthalpy, Df is liquid conduction coefficient, dp is vapour permeability and hv is latent heat of evaporation/condensation. The left-hand side of the moisture balance represents the moisture storage which is proportional to the derivative of the water retention curve (∂w/∂f). The transport terms on the right-hand side are described by the divergence of liquid and vapour flow. While vapour pressure (pv = f · psat), which is the driving force for vapour flow, depends strongly on temperature (saturation pressure psat ~ exp(T)), liquid flow is governed by capillary forces which are a function of relative humidity f only (Kelvin equation). The divergence



59

of vapour flow multiplied by the specific heat of evaporation represents the latent heat term on the right-hand side of the energy balance. The sensible heat flow is represented by the divergence of heat conduction. The storage term on the left-hand side of the energy equation – described as derivative of total enthalpy – contains the heat capacity of the dry material plus the enthalpy of absorbed water which depends on its state (solid, liquid or gaseous phase).

2.4

Input data for hygrothermal calculations

The execution of hygrothermal simulations including inputs and outputs is best described by a flow chart from prEN 15026 (2004) in Fig. 2.2. As first Hygrothermal material parameters & functions

Assembly, orientation & inclination of building component

Boundary condition surface transfer (indoor & outdoor climate)

Initial condition, calculation period, numerical control parameters

Input

Hygrothermal building envelope simulation

Output Water content, RH & moisture flux distributions & evolutions

Temperature & heat flux distributions & evolutions Post processing Energy consumption economy ecology

Biological growth rot, corrosion

Moisture related damage & degradation

2.2 Flow chart for hygrothermal simulations in prEN 15026 (2004). This chart has been dropped from the final version of this standard only because charts are discouraged due to additional efforts required for translation into the various European languages.


60


input, composition and exposure (orientation and inclination) of the building assembly under investigation are required. Then the material parameters for all construction layers have to be selected from a material database. The necessary material properties include: ∑ ∑ ∑ ∑ ∑

∑ ∑

Bulk density r in kg/m³, serves to convert the specific heat by mass to that by volume. Specific heat capacity c in J/(kg K). Thermal conductivity l in W/(m K) of the dry material and its moisturedependence. Porosity e in m³/m³, which determines the maximum water content wmax. Moisture storage, i.e. sorption res. suction isotherms w = f(f) in kg/m³ that give the equilibrium moisture content of materials as a function of relative humidity in both the hygroscopic and the capillary water (overhygroscopic) range. Vapour permeability in kg/(m² s Pa) which may depend on the ambient air humidity. Liquid diffusivity Dw in m²/s both for water uptake and redistribution res. drying of materials as a function of moisture content. Multiplying Dw by the derivative of the water retention curve gives the liquid conduction coefficient Df.

If present, liquid transport may dominate vapour diffusion by some orders of magnitude. Therefore it has to be considered carefully when liquid water has an impact on the building component, e.g. when wind-driven rain hits a cavity wall or a solid wall made of natural stone. In contrast to thermal conduction or vapour diffusion, liquid transport is highly non-linear because the diffusivity often shows an exponential increase with water content. As an example for typical liquid diffusivity functions of mineral building materials, the functions Dw = f(w) of calcium silicate brick (CSB) are plotted in Fig. 2.3. The liquid diffusivity used to simulate water absorption varies by a factor of almost 1000 between the equilibrium moisture content at 80% RH (w80) and the free water saturation (wf) of the material. Therefore liquid transport is very important when a porous material is wet while its influence decreases rapidly when the material dries out. After all materials and their hygrothermal properties in the investigated assembly are specified, the boundary conditions have to be selected. In order to obtain a realistic simulation of the hygrothermal performance of building components exposed to natural weather, the following climatic parameters should be provided: ∑ ∑

exterior air temperature exterior relative humidity



61

Liquid transport properties of CSB masonry units

10–6

Free (capillary) water saturation Wf Liquid diffusivity (m2/s)

W80 (water content at 80% RH) 10–7 Water absorption 10

–8

Drying, water redistribution

10–9

10–10 0

(2.5)

5

10 15 20 Water content (vol.–%)

25

(27.5)

30

2.3 From transient water absorption and redistribution measurements by NMR scanning, Krus (1996) approximated liquid diffusivity functions of calcium silicate brick (CSB) used for rainscreens or load bearing masonry. The diffusivity (separate functions are used for simulating water absorption and drying processes) depends exponentially (logarithmic scale of y-axis) on the water content of the material.

∑ ∑ ∑ ∑ ∑ ∑

short-wave radiation (global and diffuse solar radiation) long-wave radiation (thermal sky radiation) precipitation wind speed and direction interior air temperature interior relative humidity.

Hourly meteorological data sets are generally required owing to the diurnal changes of the exterior climate conditions. Depending on the purpose of the hygrothermal analysis, data from extreme years (BS EN 15026, 2007) may be more suitable than average meteorological data (e.g. test reference years) that are normally used for energy calculations. Since the interior climate shows less fluctuation, daily or even monthly mean data are sufficient for most applications. In many European countries indoor climate surveys of residential buildings have been carried out which lead to the development of representative indoor air conditions depending on the outdoor temperature (WTA 6-2; BS EN 15026). The impact of the exterior and interior climate conditions on the building envelope is also governed by the so-called surface transfer that describes the transport processes at the exterior and interior surfaces. Usually there


62


is a stagnant film of air at the surface which acts as a resistance to heat and vapour flow. This film resistance depends inversely on the air flow velocity at the surface and is therefore usually higher at the interior side of the building. The heat transfer coefficients often include the heat exchange by long-wave radiation which is sufficient for the interior surface but may lead to inaccurate results at the exterior side. In particular, when exterior condensation (and subsequent façade defacement due to algae and fungi) is an issue, an explicit radiation balance including long-wave radiation to the sky is required. The effect of solar radiation on the exterior surface is determined by the short-wave absorptivity as well as the long-wave emissivity of the surface layer. Recently, the impact of air cavity ventilation has been successfully integrated in hygrothermal models (see Künzel et al. (2008) and this type of air flow can allow a substantial amount of passive drying without mechanical equipment. The inclusion of this type of air flow in hygrothermal models has advanced the predictability of the design tools. The last input block in Fig. 2.2 contains the initial conditions and calculation specifications (duration, numerical accuracy, etc.). Since the temperature distribution over the building component normally adapts very quickly to the boundary conditions, a uniform temperature close to the expected mean can be selected as start value. Moisture transport processes are, however, comparatively slow. Therefore a realistic initial distribution may be important for the outcome of the simulation. If construction moisture is present it must be specified. For initially dry building materials, it is good practice to start with their equilibrium moisture content at 80% relative humidity. The new ANSI/ASHRAE Standard 160-2009 on ‘Criteria for MoistureControl Design Analysis in Buildings’ is aimed at providing guidance on how to best design buildings with adequate moisture control features. Given its mission, ASHRAE is uniquely qualified to provide such guidance. Computer simulation tools have become available and are used to predict the thermal and moisture conditions in buildings and the building envelope. These computer models are increasingly used to make recommendations for building design in various climates, and used as forensic tools in the investigation of building failures. However, results obtained with these models are extremely sensitive to the assumed moisture boundary conditions. Thus, a consistent approach to moisture design demands a consistent framework for design assumptions, or assumed ‘loads’. The ANSI/ASHRAE Standard 160-2009 formulates design assumptions for moisture design analysis and criteria for acceptable performance. As sufficient data are often not available to make a full statistical treatment practical, a moisture design protocol will have to be based on a combination of statistical data and professional judgement where only limited data exist. Damage is also another difficult area to address with high precision even with our state-of-the-art understanding. The standard has adopted the international consensus predicated on loads that will not be



63

exceeded 90% of the time. The standard uses a number of design indoor climate definitions that are based on engineering principles, independent of construction, and that include the impact of ventilation and air-conditioning equipment and controls that may or may not be part of the building design. The standard is flexible enough to encourage designers to use their own design parameter values if they are known. The standard includes default values for loads and parameters that are unknown. Appropriate measures are assumed to have been taken by the designer to limit bulk water entry into the building and building envelope. One major breakthrough has been to implement real loads in the design of wall systems. For example, the standard requires the design to include a 1% water penetration on the exterior side of the weather resistive barrier (building paper/membrane/sheet/liquid applied). This alone allows the design to examine the sensitivity to even small moisture loading, as a function of climate. ASTM E06 (2008) has concentrated on the development of a ‘Standard Guide for Documentation of Hygrothermal Models for Moisture Control Design in Building Envelopes’. Each hygrothermal model has specific capabilities and limitations. Determining the most appropriate hygrothermal model for a particular application requires a thorough analysis of the problem at hand, understanding the required transport processes involved, and available resources to conduct the analysis. Users of the standard can describe the functionality of the hygrothermal model used in an analysis in a consistent manner. This standard applies to hygrothermal models that range from complex research tools to simple design tools. A protocol for matching the analysis needs and the capabilities of candidate models is offered in this standard. This standard provides the needed characterization of the hygrothermal model to assess its credibility and suitability. This becomes even more important because of the increasing complexity of the building envelope systems for which new hygrothermal models are being developed. There are many different hygrothermal models available, each with specific capabilities, operational characteristics and limitations. If modelling is considered for a project, it is important to determine whether a hygrothermal model is appropriate for that project, or whether a model exists that can perform the simulations required in the project. This guide is intended to provide the framework for characterizing the functions of the hygrothermal model and the level of sophistication used as inputs for each analysis.

2.5

Hygrothermal calculation results

Hygrothermal simulation results shown in two output blocks in Fig. 2.2 consist of heat and moisture fluxes and transient cross-sectional distributions of temperature, relative humidity and water content. They may be presented as profiles for specific points in time or as temporal variations at a specific


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location within the building component as shown in Fig. 2.4. A common presentation of the results is the stepwise visualization of the transient distributions in a movie-like fashion. Before the results can be analysed they should be checked for plausibility. This means all input and output data ought to be visualized in order to detect any inconsistencies. If possible the results should be compared to suitable experimental data. If the calculation results do not correspond to practical experience, the simulation should be repeated with different numerical grid sizes and/or time steps as well as modifications of key input parameters. If all data inspire confidence the next steps include analysis and interpretation of the resulting hygrothermal conditions within the building envelope systems. At first it should be checked that the resulting temperature, relative humidity or water content in the different materials do not exceed the limits specified for these materials. Then the annual moisture balance of the whole envelope system should be analysed. Building components containing some initial moisture should dry out. Continuous moisture accumulation is usually a sign of failure and calls for redesign of the component. However, these failure criteria may not be sufficient to ensure a good long-term performance of the building component. Therefore special post-process models would be helpful for satisfactory result interpretation. Figure 2.2 tentatively (blocks in grey) shows the need for tools that use the hygrothermal results as input data which are fed into another model whose output is translated into different performance criteria such as energy consumption, risk of corrosion, mould growth or rot. Another important aspect is material ageing which may alter the hygrothermal behaviour of building envelope systems altogether. Until now, only few post Moisture content variation of building component

ial init iour v a h be

l rma the m o r hyg ilibriu equ

Time

Temporal variation at location xp

Local distribution at time tp

RH RH

Time

Width

2.4 Examples for typical simulation result outputs. Temporal variations of the integral water content (left) and of temperature and relative humidity at a certain location within the component (middle). The same variables can also be plotted over the cross section of the component at a certain point in time (right).



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process models have been developed and experimentally validated. Yet, with some expert knowledge it is still possible to predict the risk of failure of a particular building component without additional models, by comparing the hygrothermal behaviour of the investigated system with systems that have undergone experimental performance tests or have proven track records.

2.6

Model validation and practical applications

The validation of hygrothermal simulation models can be done in various ways. In order to make sure that model errors are not hidden by any uncertainties of input data or test results, a rigorous validation should comprise three steps. The first step has to be confirmation of correct implementation of physical fundamentals by comparison with analytical solutions. For moisture uptake in a semi-infinite region, there is a benchmark example in BS EN 15026 (2007). As second step it is useful to compare the calculation results to laboratory tests with well-defined boundary conditions and material data. The third step should be close to the real thing, e.g. simulating the transient hygrothermal behaviour of a building envelope component exposed to natural climate. It is important that all data of the component as well as the initial conditions be determined before starting the test. During the experiment boundary conditions as well as temperature and humidity fluctuations at predefined locations in the building component should be recorded carefully. If simulation and experimental results agree well the model validation for the selected envelope component was successful. This does not mean that the model will provide accurate predictions for all kinds of building components and boundary conditions. But it can be expected that simulation results are close to reality if building component and climatic loads are similar to the validation case. Due to increasing practical applications of hygrothermal simulation models, numerous validation cases have already been reported from which two examples are chosen, a flat roof and a natural stone façade.

2.6.1 Hygrothermal conditions in a flat roof At the field test site of the Fraunhofer Institute for Building Physics (IBP) in Holzkirchen, a 40 m² lightweight flat roof was erected. The load bearing structure of the roof is sealed with an aluminium vapour barrier. The insulation layer on top of the vapour barrier consists of high density glass fibre boards (90 mm) which is covered by a dark coloured impermeable roofing membrane. The aim of the study was to determine the hygrothermal loads caused by water trapped between vapour barrier and roofing membrane, as the fibre glass boards were spray wetted before closing up the roof. For a period of one year the temperature and humidity conditions at different locations in the insulation layer were measured (Fig. 2.5). The indoor conditions were


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1

2

3

4

Assembly from inside to outside: 1 2 3 4

Wooden sheathing Vapour barrier Glass wool Roofing felt (bitumen) Position of sensors

2.5 Cross section of the flat roof with temperature and humidity sensor positions.

Temperature (°C)

80

Summer

Winter

60 40 20 0 –20

Rel. humidity (%)

100 80 60 40 Measurement Calculation

20 0 15.8

21.8 15.12

21.12

2.6 Comparison of measured and calculated temperature and humidity variations at the exterior sensor position in the roof for two selected weeks in August and December.

kept at room temperature while an on-site meteorological station recorded all necessary weather data. After closing the roof, good weather conditions resulted in large temperature and humidity fluctuations at the top of the insulation layer beneath the roofing membrane. Figure 2.6 plots these results for a period of one week in August. While the roof surface temperature can exceed 60 °C during daytime, it drops to 0 °C during the night. The steep temperature changes result in even higher changes in vapour pressure beneath the roofing membrane (saturation vapour pressure increases exponentially with temperature) which drives the



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moisture up and down in diurnal cycles. When the sun heats up the roof, the humidity at the top of the insulation layer drops to 20% RH. In the evening the vapour flow inverses and condensation occurs on the inner side of the roofing membrane (RH = 100%). In winter the roof surface temperature is too low to drive the moisture towards the interior. Therefore, the relative humidity below the roofing membrane will remain at 100% all day long (Fig. 2.6, right). Comparison of the measured and simulated temperature and humidity conditions show excellent agreement in all graphs, as depicted in Fig. 2.6. The simulations included the use of the actual weather data from the on-site meteorological station and long-wave radiation to the sky was also used. This explains why the calculation model is able to capture the overcooling of the roof’s surface by approximately 5–8 K below ambient air temperature in August. The deviations in measured and simulated surface temperature during the last three days of the observation period in winter can be explained by the effect of snowfall during the night of 18 December. The snow cover prevented a rise in roof surface temperature above 0 °C until it had melted away two days later. The simulation cannot capture this effect because the recorded precipitation data do not differentiate between rain and snow (standard rain gauges are heated to ensure snow melt for accurate droplet counter and tipping bucket measurement). The durability of building materials is often adversely affected by the coincidence of high temperature and humidity conditions, as can be found for the glass fibre insulation located in the flat roof. For accurate assessment of its service life the maximum temperature and humidity occurring at the same time play an important part. Therefore the coinciding temperature and relative humidity conditions recorded from August until January at the different sensor positions in the insulation layer of the roof has been plotted in Fig. 2.7. Again almost no difference is found between measured and calculated results. Regarding the three sensor locations, the worst scenario occurs at the exterior (top) side of the insulation layer directly under the roofing membrane. The dashed line in the three graphs represents the maximum limit for temperature and humidity coincidence in the whole insulation layer. The line clearly shows a significant decrease in humidity when a temperature of 40 °C is exceeded. When temperatures rise above 60 °C at a certain location in the roof, the humidity drops to less than 50% RH. The humidity decreases at the same time as the temperature rises, which can clearly be observed by looking at the diurnal fluctuations of temperature and humidity beneath the roofing membrane in summer (Fig. 2.6). Investigations such as this help to select appropriate temperature and humidity conditions for durability tests in the laboratory. Since extensive tests on real objects are expensive, hygrothermal simulation models can be very helpful once they have been validated for the specific building application. They can also be employed to study the influence of hot and humid climate


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Exterior

Interjacent

Interior

Rel. humidity (%)

100 80 60 40 Measurement Calculation

20 0 – 20 0

20 40 60 80 –20 0 20 40 60 80 –20 0 20 40 60 80 Temperature Temperature Temperature

2.7 Coinciding temperature and humidity conditions on an hourly basis measured and calculated at three sensor positions. The dashed line represents the upper limit for the hygrothermal load within the insulation layer of the roof.

zones on building envelope performance since most practical experience has been obtained by trial-and-error in the moderate climate zones of Europe and North America. This allows construction materials to be tested before being used for building envelope components subject to operating conditions they have not been designed for.

2.6.2 Moisture behaviour of natural stone façade To develop better means to preserve heritage buildings such as castles and churches, the moisture behaviour and related degradation processes of natural stone have to be analysed. In the frame of a comprehensive project the hygrothermal properties of several species of natural stone were determined as a basis for numerical simulations. Parallel to investigations in the laboratory, a field test was carried out exposing stone samples at one side to natural weather and at the other side to a well-defined interior climate. The detailed recordings of the boundary conditions on both sides of the stone samples also served as input data for the hygrothermal simulations. During the field test the natural stone samples which formed part of a wall were periodically removed and weighed to determine their water content. In addition the samples were scanned from time to time by a nuclear magnetic resonance (NMR) apparatus in order to obtain an instant moisture distribution profile over the cross section of the façade. A comparison of experimental results with hygrothermal simulation results is presented in Fig. 2.8. The top graph shows the total water content variations of three samples of the same natural stone species (Sander sandstone) compared to the calculated curve. The steep increases in water content at several occasions during the observation period are the result of driving rain events which, interrupted by bright spells, result in a slow moisture accumulation of the originally



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Total water content (kg/m3)

40 Measurement: Sample 1 Sample 2 Sample 3

30

Calculation

20

10

0

0

10

20

30 40 50 Observation time (d)

60

70

80

(a) Bright spell

Rainy weather

12.5

Local water content (vol-%)

Calculation NMR-Scans 10.5

7.5

5.0

2.5

0.0 0

5

10

15

20 25 0 5 Wall cross-section (cm)

10

15

20

25

(b)

2.8 (a) Temporal variations of the integral water content of the exposed natural stone samples determined by weighing in comparison with simulation results. (b) Moisture distributions in the samples determined by NMR scanning at two points in time compared to simulated moisture profiles for the same dates.

dry samples. Figure 2.8(b) presents a comparison of the measured and calculated moisture distributions over the cross section of the natural stone wall (the samples are part of the wall) for two distinct points in time. The presented temporal variations as well as the moisture distributions show an excellent agreement between hygrothermal simulation and experiment, which proves that the hygrothermal model describes the moisture behaviour of exposed natural stone walls accurately. It should be noted, however, that these results are based on a detailed analysis of the hygrothermal properties in the laboratory and on-site registration of the interior and exterior climate


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conditions including hourly measurements of the driving rain load by a rain gauge integrated in the test façade. Additionally, natural stone represents an ideal material concerning homogeneity and variability which means the hygrothermal properties show little or no variation with location and time. The comparison with the experiment inspires confidence in the simulation results. Since walls of heritage buildings are usually thicker than the wall in the field test (25 cm), the simulations were repeated with the same material properties of Sander sandstone and with a typical meteorological dataset for a wall of 60 cm thickness. The aim of this study was to get an average moisture and temperature distribution in such a wall as a basis for further analysis of degradation processes and remedial measures. Therefore the simulations were run with the same annual climate conditions until a dynamic equilibrium was achieved. The dynamic equilibrium is the condition most likely to be found in existing buildings with continuous operation. It means that heat and moisture gains or losses of a building envelope component integrated over one year are zero. In principle this can only be attained when the same meteorological dataset is applied. Since problems often occur at walls exposed to wind-driven rain, a western orientation has been selected. The results are plotted in Fig. 2.9 as annual variations and mean profiles of temperature and water content. Due to the dark beige colour of the sandstone, the maximum surface temperature in summer may exceed 40 °C. The lowest temperature coincides with the minimum air temperature in winter. Towards the interior of the building the temperature variations are gradually dampened and the annual mean profile (solid line in Fig. 2.9(a)) forms a straight line between the average exterior and interior conditions which can be expected because heat transfer is described by a linear differential equation. However, the picture changes when the moisture distributions in Fig. 2.9(b) are analysed. First of all, the annual variations in moisture content due to natural weather do not reach the interior side of the wall; in fact the climatic influence does not go beyond the exterior third of the wall. Even more surprising is the shape of the annual variation range and the mean distribution which do not form a straight line. Similar to the thermal pro cesses, the fluctuations in water content due to direct impact of hygrothermal load cycles (wind-driven rain/solar radiation) peak at the exterior surface. However, the average water content at the surface lies below 3 vol.-% which is almost the same as at the interior surface. Some millimetres beneath the exterior surface, the average water content rises steeply and reaches a maximum of approx. 7 vol.-% at a depth of about 5 cm. This phenomenon is caused by the strong – non-linear – dependence of liquid transport on water content which has already been discussed by referring to the liquid diffusivity functions in Fig. 2.3. The practical consequences of this moisture behaviour for porous materials



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Temperature (°C)

60

40

20

0

–20

(a)

Temperature (°C)

16

12

8

4

0

0

10

20 30 40 Wall cross section (cm)

50

60

(b)

2.9 Annual variations (shaded areas) and mean profiles (solid lines) of temperature and moisture content in an external wall made of Sander sandstone. These results are obtained by running the simulations with the same meteorological dataset of one year until a dynamic equilibrium is reached, i.e. the mean profiles do not change from one year to the next.

exposed to wind-driven rain are manifold. Material degradation is often caused by dilatation processes in the microstructure due to the expansion of freezing water or crystallizing salts. Especially in the case of frost, the risk of damage increases with water content. This would explain why the material layer beneath the surface is often more degraded than the exposed surface layer, which may lead to spalling or delamination of the exterior crusts. There are also positive aspects. Rainwater hitting the surface is absorbed and stored in a layer some distance beneath the surface which is not accessible to microorganisms. Stored rainwater in concrete façades prevents carbon dioxide diffusion and therefore protects the layer with the reinforcing steel from carbonation and consequently the steel from corrosion.


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As an example for a two-dimensional calculation, the moisture distribution in an exposed natural stone wall with poured mortar joints is investigated. Poured mortar is often used between big blocks of natural stone which would unevenly compress normal mortar due to their weight. Therefore, gauges are used to produce a gap between two blocks and a mortar with low viscosity is poured in between. Once hardened the mortar is rather porous and its water absorption capacity (liquid diffusivity) as well as its vapour permeability are much higher than those of the natural stone blocks. This may cause damage of the stone blocks starting at the interface with the mortar joint, as demonstrated by a photograph of an exposed part of the cathedral in Cologne (Fig. 2.10(c)). The simulated moisture distribution in a wall section with a mortar joint between two differently orientated anisotropic sandstones (inherent structure results in directionally different water absorption) is presented in Fig. 2.10(a). The resulting isohygric zones (zones with identical water content) after three hours of intensive wind-driven rain show the rapid water absorption of the mortar joint. Its comparatively high liquid diffusivity facilitates the water infiltration while the adjacent stone blocks lag behind. However, in the vicinity of the joints the initially dry sandstone blocks extract water from the mortar. In particular, the vertically layered block seems to draw a considerable amount of moisture from the joint. This becomes even more evident by looking at the resulting moisture distributions after another 48 hours of dry weather presented in Fig. 2.10(b). 3 hours of driving rain

48 hours of dry weather

Sandstone (vertically layered)

Mortar joint

Sandstone (horizontally layered)

(a)

(b)

(c)

2.10 Moisture distribution in a wall section made of anisotropic natural sandstone blocks and poured mortar joints after 3 hours of intensive rain (a) and after another 48 hours of dry weather (b) simulated with WUFI®-2D. The zones of elevated residual moisture close to the mortar joint may explain the mechanism leading to the observed damage case at the cathedral of Cologne (c).



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The mortar joint is now drier than the stone blocks next to it because of the high vapour permeability of the mortar. The residual moisture maxima in the sandstone blocks are again located beneath the exterior surface. But now there is an additional zone of elevated moisture next to the mortar joint. This phenomenon helps to explain why the sandstone degradation observed at Cologne cathedral appears to start at the mortar joints.

2.7

Limitations of current hygrothermal models

While there has been a considerable amount of progress in developing hygrothermal models for the architect or building envelope designer, a number of overall limitations may still be present. First, mainstream commercial hygrothermal models are based on deterministic, rather than statistical distribution of the material properties and loads (Salonvaara et al., 2001). Most of the available hygrothermal models are implemented in one dimension and do not include air flow through the envelope structure. Being one dimensional precludes the transport phenomena associated with groundwater, hydrostatic pressures and gravity. Air flow is not yet captured by most hygrothermal models, with the exception of air cavity ventilation. However, research is on-going that aims at specifying the moisture sources which result from humid air infiltration into building envelope components. Currently hygrothermal models do not include the impact of ageing, material properties that change with respect to time or cumulative damage. Dimensional changes such as swelling and shrinkage, the eventual cracking of the material, and loss of mass are not accounted for in the current versions of the models. Indeed all material properties are considered constant in time. The loads due to wind-driven rain are also crudely approximated with very simple expressions that do not take into account the interference of other buildings or architectural details and the geometry of the building. While these limitations exist in most design applications, the user is interested in performance ranges, for example, does a particular wall system perform better with a vapour tight or vapour open weather resistive barrier? Is adding insulation beneficial for moisture control, and if so which type of insulation? These answers can be readily addressed by the current hygrothermal models.

2.8

Conclusions and future trends

In recent years, hygrothermal simulation models have become useful tools for moisture control design of new and existing buildings. This includes the moisture analysis of walls and roofs of heritage constructions that must be handled with special care when restoration or retrofit measures are envisaged. These tools have been successfully employed to predict the hygrothermal


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performance of building materials and envelope systems under different indoor and outdoor climate conditions. There are even some examples where hygrothermal simulations have resulted in the development of innovative solutions for better building enclosures, e.g. a humidity controlled vapour retarder (Künzel 1998) for wooden wall and roof constructions. There are a number of ways moisture can enter the building enclosure. Interstitial condensation due to vapour diffusion – which has been dealt with extensively in the past – is hardly the most important one. In fact, the most important and unexpected damage cases of the last decade were all caused by leaky windows or problematic flashings that introduced rainwater penetration and harmful code vapour retarder requirements. An example were the problems with exterior wall insulation systems (EIFS) in North America (Cheple & Huelman 2000) and the current trouble with EIFS for Scandinavia. (Samuelson et al., 2008). Equally important may be moisture intrusion as a result of air flow in lightweight wood frame construction, as well as commercial metal frame construction. Infiltration of humid air into a building component due to total pressure differences amplifies the effect of vapour diffusion. Air pressures created by the direct wind or adjacent environment (other buildings, stack effect due to buoyancy forces and mechanical pressures due to the operation of equipment) can create positive or negative flows across different parts of the building. Moisture may be accumulating at one part of the building while drying at another part due to infiltrating or exfiltrating air flows. An adequate drying potential towards both sides of the building component is therefore extremely beneficial for the reduction of moisture damage risks. Hygrothermal simulations offer the opportunity to find the most effective solution between two opposing tasks: to prevent or at least limit the moisture entry into a building component and to let moisture that has entered dry out again as fast as possible. Design optimizations that result in more moisture tolerant constructions are already encouraged by the European Standard EN 15026 (2007) and even more importantly by the recently issued ANSI/ ASHRAE Standard 160-2009. Last but not least there is also a working group at ASTM which will issue a new Standard on hygrothermal simulation models in the near future. From a practical point of view there is still a lack of post-processing models that will translate the hygrothermal simulation results into pass/fail criteria or into a service life prediction. This is a scientific challenge for everybody involved in damage prevention. Most degradation processes in the construction sector are somehow related to moisture and temperature, such as: ∑ ∑

chemical reactions like corrosion or hydration biological processes like rot, mould growth and algae formation



∑ ∑

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physical damage of the microstructure due to frost or salt crystallization shape changes or warping due to hygrothermal dilatation.

This list is far from complete and yet there are only very few criteria that establish a quantitative correlation between the hygrothermal conditions and the durability of building materials and components. One reason is that durability itself is difficult to define. In the context of hygrothermics it means a low risk of damage and a known progress of degradation until a limit state is reached where maintenance or replacement is required. Another reason is the lack of specific knowledge concerning hygrothermal degradation processes. However, performing hygrothermal simulations is the first step towards a better solution. Therefore more scientists should be encouraged to get involved in this subject.

2.9

References

ANSI/ASHRAE (2009) Standard 160-2009 Criteria for Moisture Control Design Analysis in Buildings. ASHRAE (2009) Handbook of Fundamentals, chapter 25 ‘heat, air and moisture control in building assemblies – fundamentals’. ASTM E06.41 (2008) Standard Guide for Documentation of Hygrothermal Models for Moisture Control Design in Building Envelopes, Sept. BS EN 15026 (2007) Hygrothermal performance of building components and building elements – Assessment of moisture transfer by numerical simulation. April. Cheple, M. and Huelman, P. (2000) Literature Review of Exterior Insulation Finish Sys tems and Stucco Finishes. Report MNDC/RP B80-0130, University of Minnesota. Karagiozis, A.N. (1998) Applied Moisture Engineering, Thermal Performance of the Exterior Envelopes of Building VII. Clearwater Beach, FL, ASHRAE Proceedings; pp. 239–251, December 6–10 1998. Karagiozis, A.N. (2001) Advanced Hygrothermal Models and Design Models. The Canadian Conference on Building Simulation, Ottawa, International Building Performance Simulation Association (IBPSA), pp. 547–554, 13–14 June. Karagiozis, A.N., Künzel H.M. and Holm A. (2001) WUFI-ORNL/IBP – A North American Hygrothermal Model. Performance of Exterior Envelopes of Whole Buildings VIII: Integration of Building Envelopes, 2–7 December. Krus, M. (1996) Moisture Transport and Storage Coefficients of Porous Mineral Building Materials – Theoretical Principles and New Test Methods. IRB-Verlag, Stuttgart. Künzel H.M. (1995) Simultaneous Heat and Moisture Transport in Building Components – One-and two-dimensional calculation using simple parameters. http://publica. fraunhofer.de/eprints/urn:nbn:de:0011-px-566563.pdf. Künzel, H.M. (1998) The Smart Vapor Retarder: An Innovation Inspired by Computer Simulations. ASHRAE Transactions 104/2, 903–907. Künzel, H.M., Karagiozis, A. and Kehrer, M. (2008) Assessing the benefits of cavity ventilation by hygrothermal simulation. Proceedings Building Physics Symposium, Leuven, pp. 17–20. prEn 15026 (2004) Hygrothermal performance of building components and building elements – Assessment of moisture transfer by numerical simulation. August. © Woodhead Publishing Limited, 2010

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Salonvaara, M., Karagiozis, A. and Holm, A. (2001) Stochastic Building Envelope Modeling – The Influence of Material Properties. Performance of Exterior Envelopes of Whole Buildings VIII: Integration of Building Envelopes, 2–7 December. Samuelson, I., Mjörnell, K. and Jansson, A. (2008) Moisture damage in rendered, undrained, well insulated stud walls. Proc. 8th Symposium of Building Physics in the Nordic Countries, Copenhagen, 1253–1260. Trechsel, H.R. (2001) Moisture Analysis and Condensation Control in Building Envelopes. American Society for Testing and Materials (ASTM) Manual 40. WTA (2002) Merkblatt 6-2-01: Simulation of Heat and Moisture Transfer, May. English version Oct. 2004. WTA-Publications, www.wta.de.


3

Ventilation, air quality and airtightness in buildings D. W. E t h e r i d g e, University of Nottingham, UK

Abstract: This chapter gives an overview of ventilation and air quality in low-energy buildings, leading to a description of how materials can influence the performance of ventilation systems. Emphasis is placed on natural ventilation, since, in principle, it offers a passive low-energy solution. Potentially important contributions from materials relate to the control of envelope leakage, thermal storage with phase-change materials, ‘smart’ glazing for controlling solar gains and, possibly, porous materials for insulation and night cooling. Key words: natural ventilation, building design, low-energy ventilation, phase-change materials, air quality.

3.1

Introduction

The primary purpose of a ventilation system is to contribute to the provision of a safe, healthy and comfortable indoor environment. Nowadays, this purpose needs to be satisfied with minimal energy consumption, i.e. the systems should be ‘low-energy’. The aims of this chapter are to give an overview of building ventilation systems and to describe areas where materials can play a role in the development of low-energy systems. Thus Section 3.2 describes the basic types of system (natural and mechanical) and what constitutes a low-energy system. Section 3.3 explains the main physical mechanisms involved. In principle, natural ventilation offers a passive low-energy solution and therefore Sections 3.4 and 3.5 deal respectively with the feasibility and the design of natural ventilation systems. It is here that some important roles of materials are introduced. These aspects are then dealt with in more detail in Section 3.6 for both natural and mechanical systems. The emphasis is on buildings where the occupants are the determinants of ventilation rates, e.g. commercial buildings and dwellings, rather than industrial buildings.

3.2

Ventilation systems

3.2.1 Types of system There are basically two types of ventilation system, namely natural and 77 © Woodhead Publishing Limited, 2010

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mechanical. A pure natural ventilation system is one where the ventilation arises from flows through openings in the envelope, which are generated by the natural forces due to wind and buoyancy (gravity). The envelope is the fabric that encloses the air, e.g. external and internal walls. Natural ventilation has of course been the sole means of ventilation for thousands of years in all climates. The underlying aim of modern design is to improve and adapt systems to cope with changing circumstances (climate change and the desire for low-energy buildings). Another aim is to expand the application of natural ventilation to non-domestic buildings where mechanical ventilation is usually adopted. A pure mechanical ventilation system is one where the ventilation is provided by a network of ducts, powered by a fan or fans. Mechanical ventilation is a relatively recent phenomenon, which is suited to the particular demands of large non-domestic buildings. However, in some countries mechanical ventilation in dwellings is not uncommon, particularly for multi-family apartments. Modern design of mechanical systems has the same underlying aim as for natural systems. There are three basic types of mechanical system: ∑

Extract only – the air is extracted through ducts, with openings in the envelope providing the route for air supply. ∑ Supply only – air is supplied through the ducts, and openings in the envelope provide the exhaust route. ∑ Supply and extract system – separate duct networks perform the supply and extract functions, which, if the mass flow rates are equal, is referred to as a balanced system. Not surprisingly, there are variants of the above. For example, a building that is primarily naturally ventilated may have local mechanical ventilation devices, e.g. extract fans in some rooms, cooker hoods, desk fans. Systems that combine natural and mechanical ventilation in some way are known as mixed-mode systems (the terms hybrid and assisted natural are also used). A relevant example is where both natural and mechanical systems are installed, but the mechanical system is used only when the natural system is (or is believed to be) insufficient. This is known as a complementary or changeover mixed-mode system.

3.2.2 Differences between systems Mechanical systems offer close control over the magnitude and direction of the flow rates of air into a space and over the positions where the air enters and leaves. With natural ventilation, the positions of openings in the envelope form part of the design, but the magnitude and direction of the flow rates can vary widely with time. A specific aim of natural ventilation



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design is to build in some element of control, particularly with respect to flow directions (Section 3.5). The high level of control with mechanical systems stems from the use of fans for driving air through ducts. This advantage comes at a financial cost, namely higher capital and maintenance costs. Moreover, with long duct lengths and high flow rates, the electrical energy consumed by the fans can be significant. However, with a balanced system, heat recovery from ventilation air can more than compensate for this. Another important difference is that a mechanical ventilation system can be used to distribute treated air, i.e. heated, cooled and/or dehumidified air (air conditioning system). In such systems, high flow rates are usually required and the provision of fresh air is a relatively minor function of the system with a correspondingly low marginal cost. The incorporation of heating and cooling devices into a natural ventilation system is more restricted, due to the low driving pressures. The major appeal of natural ventilation is of course that it is a passive system and, in principle, it is to be preferred to a mechanical system, provided that it can perform satisfactorily. This proviso is of course a key issue, which will depend on many factors (subjective and objective). It can be argued that for many applications, close control of the internal environment is unnecessary (since human beings are adaptable) and unjustified (since the specification of required ventilation rates is subject to considerable uncertainty). Nowadays it is not uncommon for design briefs to specify natural ventilation as the preferred system for commercial buildings.

3.2.3 Low-energy ventilation systems There is no precise definition of what is meant by a low-energy ventilation system, and even less for what constitutes a ‘sustainable’ or a ‘green’ system. Many factors need to be taken into account (Liddament, 1996). In the following we concentrate on the more important ways in which the energy consumption directly associated with the provision of ventilation can be reduced. It is relevant to consider heating and cooling separately and in that order. For a given flow rate of fresh air, Q (m3/s), the heating load, Hv (W), imposed on a building is

Hv = rcpQ(Tin – Tout)

3.1 3

where r denotes density (kg/m ), cp the specific heat (J/kg K), Tin the temperature of the air flowing into the building and Tout the temperature of the air leaving the envelope. The outlet temperature will normally be controlled to a fixed value (room temperature), so the heating load can only be reduced by increasing Tin above the external temperature. This can be done by a heat


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recovery system, whereby the inlet and outlet flows are ducted through a heat exchanger. In this way the ventilation heat load can be reduced by as much as 70%. A balanced mechanical system is ideally suited for heat recovery for a complete building. However, there are two problems. First, energy savings are reduced by the extra fan power required to induce the flow through the heat exchanger and associated ducts. Second, with a balanced system there is no pressurization, so any natural ventilation through openings in the envelope simply adds to the ventilation heat load. It is therefore necessary that the envelope should not be too leaky (see Section 3.4.3). Another way of increasing Tin is to arrange for the external air to pass through an underground duct before it enters the envelope. The ground temperature remains reasonably constant, so pre-heating takes place when Text is lower. This approach raises questions about the quality of the air and is not common. The above remarks apply equally to the case when cooling is required (although in Eq. 3.1, the temperature difference is reversed and is smaller). However, there is a major difference, namely that ventilation itself can provide cooling. In certain climates it is possible to obtain acceptable conditions in the cooling season, either by achieving very high flow rates during the occupied period, or by making use of ‘night cooling’. These approaches are discussed in Sections 3.4.5 and 3.4.6. An important point to note here is that a natural ventilation system is often taken to mean one which reduces (or eliminates) the need for mechanical refrigerative cooling. This issue has been brought to the fore in some countries by global warming. For example, in the UK, it is now not uncommon for the cooling requirement of a building to exceed the heating requirement. A natural ventilation system is sometimes viewed as a requirement for a building to be classed as low-energy by virtue of the fact that fans are not used. This is not necessarily correct, since it assumes that the natural flow rates that occur during the heating season are not excessive, i.e. the Q term in Eq. 3.1. Nevertheless, it seems to be generally accepted that a well-designed natural ventilation system can be an important component of a low-energy building, particularly if it also eliminates the need for mechanical cooling. Low-energy forms of mechanical ventilation systems can be achieved by reducing fan energy consumption and by the use of heat recovery. In simple terms the fan energy consumed, EF (J), is given by

EF = 100topQFDp/h

3.2

where top is the operating time (s), QF the fan flow rate (m3/s), Dp the pressure rise (Pa) and h the overall fan efficiency (%). There are various ways that the first three parameters can be reduced. For example, the operating time and the volume flow rate can be reduced by intelligent controls, or by a mixedmode system. Good duct design with bypasses for unused components will



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reduce Dp. The use of variable speed motors and aerodynamic fan blades can increase h.

3.2.4 Internal air motion Internal air motion plays an important role in determining both the air quality and the comfort level in a space (provided sufficient fresh air is supplied). In this respect, there are basically two types of mechanical systems. In one, the aim is to thoroughly mix the internal air, so that uniform conditions (e.g. pollutant concentrations, temperature) occur throughout the occupied space. This type is known as mixing ventilation. The second type is known as displacement ventilation and is virtually the opposite. The underlying aim is that pollutants and internal heat gains should rise to a high level, without mixing with the air in the occupied space. This requires the use of a low-velocity, low-level supply and a high-level extract, so that the air is displaced upward. For natural ventilation, the mixing process is more complicated and less predictable. Depending on the wind and temperature conditions, either mixing or displacement ventilation may be evident, or some combination of the two. For example, the former tends to occur with high flow rates, such as encountered with large window openings used for cooling and when wind is the dominant driving force. Displacement ventilation is more likely to occur with low ventilation rates, particularly when buoyancy is dominant. However, both the positions of the openings and buoyancy generated by heat transfer at surfaces can disrupt the classical displacement flow pattern.

3.3

Physical mechanisms

Natural ventilation of a building can be considered as comprising two physical processes, namely envelope flows (the entry and exit of air through openings in the envelope) and internal air motion within the envelope.

3.3.1 Envelope flows In simple terms, flows through openings in the envelope arise as a result of pressure differences acting across the envelope. These pressure differences are generated by temperature differences (buoyancy), by the wind and by mechanical fans. The external velocity field around an opening can also play a role, particularly with large openings, but only the pressure fields are considered here. When describing envelope flows, the term buoyancy refers to the difference between the density of the internal air and the external air. Under the action of gravity, the different densities generate different hydrostatic pressure


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variations with height inside and outside the envelope. When wind is present, the variations in air velocity that occur around the building are accompanied by a variation in pressure on the external surface of the building. The absolute pressure at a point on the external surface of the building, PE, is given by the sum of the ‘hydrostatic’ pressure and the pressure due to wind, pw. Thus for a point at height z

PE = PE0 – rEgz + pw

3.3

where PE0 denotes the hydrostatic pressure at z = 0 in the exterior. The density of the external air is uniform and is denoted by rE. The internal pressure on the internal surface is given by the hydrostatic equation. In general there will be air movement inside the building, but the velocities are usually low enough for the pressures due to motion to be neglected. Thus the internal surface pressure, PI, at height z is given by

PI = PI0 – rIgz

3.4

where PI0 denotes the internal hydrostatic pressure at z = 0. (When the internal air temperature and density are not uniform, Eq. 3.3 has a different form.) The pressure difference across an opening, Dp, at height z (inlet and outlet at same height) is thus given by

Dpi = Dp0 – Drgzi + pwi

3.5

where the following substitutions have been made

Dr ∫ rE – rI

3.6

Dp0 ∫ rE0 – rI0

3.7

The equations for the special cases of buoyancy alone and wind alone are obtained by putting pw = 0 and Dr = 0 respectively in Eq. 3.5. Pressure differences generated by natural ventilation are quite small. For a low-rise building they are typically less than 10 Pa and may often be less than 5 Pa. The actual values encountered depend largely on the height of the building and how exposed it is to the wind. When a mechanical fan is used to extract or supply air to a space, it generates a pressure that is uniform over the internal surfaces of the envelope. The magnitude of this pressure is also likely to be less than 10 Pa, which means that natural ventilation can play a role in such systems. A balanced system is particularly susceptible, because it generates no pressure difference (see Section 3.6.1). Internal fans, such as a desk fan, are often used to provide convective cooling of occupants or to enhance mixing of the air. This type of fan primarily affects the internal air motion, rather than the flow through envelope openings.



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3.3.2 Flow through envelope openings The flow rate through an opening, qi, can often be expressed in terms of the pressure difference across it, Dpi, by means of the discharge coefficient, Cdi,

qi = Cdi Ai

2Dpi r

3.8

where Ai denotes a defined area of the opening. There will generally be multiple openings in an envelope, and conservation of mass applied to the flow into and out of the envelope provides a relationship between all of the individual flow rates

∑ rqi = 0

3.9

If there are N openings in an envelope, there are N equations of the form 3.5 and 3.8 and one of the form in Eq. 3.9. If the areas, discharge coefficients and positions are known for each opening, the equations can be solved to give Dp0 and the flow rates. This is known as the implicit method of solution. Generally speaking this is an iterative process, whereby Dp0 is altered until the mass conservation equation is satisfied. To some extent, this process mimics what physically happens. When the flow rates change, due to a change of wind speed, for example, the internal pressure changes to re-establish the mass balance.

3.3.3 Characteristics of natural ventilation The ventilation rate of a building, Q, is usually defined as the rate at which external air (fresh air) flows into the building. The qualitative characteristics of a natural ventilation system can be seen by plotting Q against wind speed, U, as illustrated in Fig. 3.1. For the wind alone case, Q increases with U at a rate that depends on the wind direction (the wind pressure distribution depends on wind direction). For the buoyancy alone case, Q is independent of U (Q increases roughly in proportion to the square root of DT). The buoyancy alone values thus appear as horizontal lines. The curve for the general case of wind and buoyancy combined can often be obtained by treating the wind-alone line and buoyancy-alone line as asymptotes and by joining them with a smooth curve, as shown in Fig. 3.1. This approximation is often acceptable. Bearing in mind the changes that occur in wind speed, wind direction and external and internal temperatures with time, it can be appreciated that the magnitude of Q can vary over a wide range in quite short time scales (in the order of hours). Equally important, the direction of the flow through individual openings can change. For example, with one wind direction the flows into a room may all be inward, whereas with another direction they


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Q


Wind alone, two values of wind direction

Wind and buoyancy together

Buoyancy alone, two values of DT

0

U

3.1 Illustrative variation of natural ventilation rate, Q, with wind speed, U, wind direction and temperature difference, DT.

may all be outward (i.e. no fresh air supply). For this reason, one aim of natural ventilation design is to fix the flow directions (see Section 3.5.1). The magnitudes are assumed to be controlled by adjustment of the openings (by the occupants or a control system).

3.3.4 Types of envelope openings Openings in an envelope take a wide variety of forms, but they can be classified under two headings, namely purpose-provided and adventitious. As the name implies, purpose-provided openings are ones that form part of the design. Typical examples are air vents, openable windows and chimneys. Adventitious openings are simply all the openings that are not purpose-provided. Typical examples are gaps in the frames of windows, gaps around pipe penetrations and cracks in walls and ceilings. The two types are distinguished by the fact that the position and the geometry (size and shape) of purpose-provided openings are known, whereas this information is rarely known for adventitious openings. Knowledge of the position and geometry of purpose-provided openings means that such openings can be included in a mathematical model of the envelope and the flows through the openings can be calculated. This requires knowledge of the flow characteristic of each opening (discharge coefficient), which can be found either from experimental measurement or from the known geometry. Ventilation through adventitious openings (referred to as infiltration), is much harder to deal with, basically because the openings are unknown. The adventitious leakage of an envelope can be measured (Section 3.4.3) and this allows some account to be taken of infiltration. When measurement is not possible, methods have been developed that enable the leakage to be estimated, but such estimates are subject to considerable uncertainty.



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3.3.5 Internal air motion At the detailed level, internal air motion is a particularly complex process, because the motion is three-dimensional, unsteady (turbulent) and often involves an interaction between dynamic and buoyancy forces. In the past, theoretical treatments have relied heavily on empirical relationships for the bulk properties of the flows. Nowadays it is possible to calculate the properties at all points in the space by numerical solutions of the basic equations of motion and heat transfer. The numerical methods are known as computational fluid dynamics (CFD). The methods have been extended to the calculation of air quality and comfort indices. Care and skill are required in the use of CFD, because there are many sources of uncertainty, not the least of which is the specification of the boundary conditions that are required to obtain a solution. One of the underlying problems in describing (and calculating) internal motion is that the flows usually involve recirculation, which basically means that there is no obvious starting point within the flow. In addition, the interaction between the temperature and velocity fields can lead to flow instabilities. Also, the heat transfer at surfaces can play a major role in determining the temperature and velocity fields and this implies that account needs to be taken of the thermal properties of the envelope. In tall spaces, such as atria, temperature stratification can be expected, e.g. the temperature at high level is greater than the temperature at low level in summer time. This can be beneficial, because it will increase the buoyancy pressures and hence the flow rates at lower levels, when wind speeds are low. Negative buoyancy can be used to generate so-called ‘topdown’ ventilation. In summer, pre-cooled (and denser) air at high level is induced to flow downwards, thereby providing cooler air at low levels. A potential problem with this approach is that wind pressures will tend to oppose it.

3.3.6 Air quality A primary role of ventilation is to maintain acceptable air quality inside the building and this is often used for determining flow rates for design. Pollutants emitted inside the building mix with the internal air and are eventually transported out of the building. The mixing process is determined by the internal air motion and the nature of the emissions. However, the bulk characteristics of the process can be seen by considering a single space of volume V (m3), in which the concentration is assumed to be uniform at all times (mixing ventilation), with a steady (constant) ventilation rate, Q (m3/s) and a steady emission rate of contaminant qc (m3/s). Under these conditions, the contaminant concentration at time t (s) is given by (CIBSE, 2005):


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C (t ) = Cext +

qc 6 Ê q ˆ Ê Q ˆ 10 – Á Cext – C (0) + c 10 6 ˜ exp Á – t˜ Q Q Ë ¯ Ë V ¯

3.10

where C(t) is the volumetric concentration in parts per million (ppmv), Cext is the concentration in the external air (ppmv) and C(0) is the concentration at time t = 0 (ppmv). The steady state concentration, Cst, is given by: qCO 2 6 3.11 10 Q For commercial buildings the underlying aim is often to maintain body odours at an acceptable level. The CO2 concentration is directly related to the number of occupants and it can be used as a measure of air quality. For example, air quality may be considered to be met if the concentration is kept below 1000 ppmv. Equation 3.11 can then be used to determine the required ventilation rate. If the space is occupied for a short period of time (e.g. a theatre), Eq. 3.10 can be used to give a lower design ventilation rate. The use of CO2 in the above way is based on the assumption that any other contaminants require lower ventilation rates. If this is not the case, the aim should be to reduce the other emissions rather than increase the ventilation rate. A particular problem here is emissions from building materials (Section 3.6.5). Another problem is contamination from external sources. It is clear from Eq. 6.11 that ventilation can do nothing to reduce the steady state internal concentration below Cext. Cst = Cext +

3.4

Feasibility of natural ventilation

Modern natural ventilation systems are direct descendants of techniques that have evolved over thousands of years, e.g. systems based on wind towers owe much to the badgir and malqaf used in the Middle East (Battle McCarthy, 2001). However, modern systems are judged by different standards, particularly for commercial buildings, where mechanical ventilation and airconditioning have led to tight control over the internal environment. Many factors have to be considered when determining whether natural ventilation is feasible, not least the views of the client. Only the main technical ones are considered here.

3.4.1 Climate Some climates can be very challenging for natural ventilation. The two main problems are cooling and humidity. Natural ventilation alone can provide cooling (see Section 3.4.5), but it can do nothing to reduce humidity levels. A climate where humidity is high and the diurnal variation of temperature



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is small is perhaps the most challenging for natural ventilation. Naturally ventilated buildings (commercial and domestic) are not uncommon in such climates, although some form of mechanical cooling and dehumidification may be necessary.

3.4.2 Building envelope The building envelope is one of the most important factors in natural ventilation design. It contains the purpose-provided openings and the manner in which they are incorporated can be a major design issue, e.g. double-skin façades. The envelope also contains the adventitious openings and it influences such factors as thermal storage and internal heat gains. These issues are discussed in Sections 3.4.3, 3.4.6 and 3.4.8.

3.4.3 Airtightness (air leakage) It has long been recognized that flow through adventitious openings (known as infiltration) can lead to significant energy loss when a building is heated. This is a problem for both natural and mechanical ventilation and some examples are given in Section 3.6.1. In many countries, standards exist that set down the maximum allowable adventitious leakage of building envelopes. As far as is known there are no requirements for the minimum leakage. It could be argued that the leakage should not be reduced to too low a value, because purpose-provided openings can be sealed (and mechanical systems switched off) by the unwary. Some types of domestic fuel-burning appliances rely on adventitious leakage to provide combustion air. The leakage is commonly defined as the volume flow rate required to generate a pressure difference of 50 Pa across the envelope. Leakage standards are often expressed in terms of permeability, i.e. the leakage per m2 of envelope surface area. For example, in the UK an air permeability of 10 m3/hm2 at 50 Pa is currently the acceptable level, although values of 7 or 5 are considered to be good practice. In countries with more severe winter climates, lower values are used. In many countries there are recommended procedures for measuring leakage. The equipment consists of a fan, a flow meter and a manometer. The fan generates the flow and the pressure difference across the envelope, which are measured by the flow meter and the manometer. By using a pressure of 50 Pa, the measurements should not be greatly affected by the naturally occurring pressures at the time of the test. The disadvantage of this approach is that the leakage measured at 50 Pa is not necessarily representative of the leakage at the low pressures of practical interest (usually in the range 0 to 10 Pa). Adventitious leakage in a naturally ventilated building during the cooling season is of no consequence, because the purpose-provided openings will be


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large. However, if some form of mechanical cooling is employed, adventitious leakage can be important.

3.4.4 Occupants The success of natural ventilation design relies heavily on the occupants for three reasons, two of which are active and one that is passive. First, the occupants are assumed to modify the sizes of openings and to take other actions (e.g. use desk fans to promote cooling; reduce internal gains) in order to obtain acceptable conditions. Second, achievement of thermal comfort requires that occupants vary their dress and activities accordingly. Third, it is now generally recognized that occupants passively adapt to the climate, e.g. the acceptable comfort temperature increases as the external temperature increases. However, satisfying thermal comfort requirements in the summer is still a major challenge. This has led to passive and active cooling techniques for natural ventilation systems (Sections 3.4.5–3.4.7). It has also highlighted the importance of reducing internal heat gains (Section 3.4.8).

3.4.5 Cooling (prevention of overheating) A distinction needs to be made between cooling, which implies that the internal air temperature is reduced below the external air temperature, and the prevention of overheating, which means that excessive internal air temperatures are prevented. Prevention of overheating by natural ventilation relies on convective heat transfer to remove internal heat gains. The main features can be seen by considering the simple case used by Dabidian and Etheridge (2007), i.e. a well-mixed space with a constant ventilation rate. Since the air temperature within the room, Tair, is uniform at all times, the rate of change of Tair with time is given to a close approximation by the following equation (conservation of thermal energy for air in room):

rcV

dTair = H f + H int + H v dt

3.12

The terms on the right-hand side are the heat flows to or from the air (taken as positive when the flow is to the air), i.e. fabric Hf, internal heat gain Hint, ventilation Hv, respectively. The rate at which heat is removed by a ventilation rate Q is

Hv = rQ(Text – Tair)

3.13

Clearly, the higher the value of Q, the higher is the value of Hv. However, it is also necessary for Hv to be negative and this requires the internal temperature to be greater than the external temperature. The temperature rise, DT, needs



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to be small enough for Tair to lie within the comfort boundary (e.g. DT < 4 K). With high flow rates, such as those achieved with high wind speeds and large open windows, the temperature rise is likely to be well within this limit. In addition, the high internal velocities lead to convective and evaporative cooling of occupants. This is the basic idea behind wind-driven ‘crossflow’ ventilation of dwellings that is popular in hot climates (Ohba et al., 2006). Of course, high wind speeds cannot be relied upon. With buoyancy alone, the low value of DT means that flow rates will be small and the cooling available directly from natural ventilation will be small. This emphasizes that the internal heat gains Hint need to be kept as small as possible (Section 3.4.8). The fabric heat transfer Hf in Eq. 3.13 also plays a part. This can be positive or negative, depending, amongst other things, on the dynamic thermal behaviour of the envelope. In fact, thermal storage by the envelope offers an alternative method of cooling to direct convective cooling.

3.4.6 Thermal storage (night cooling) Night cooling refers to the use of high ventilation rates at night, with the aim of removing heat from the fabric that has been stored during the day. During the following day the cooled fabric can again absorb heat from the internal air, thereby providing passive cooling. For this strategy to be effective, several conditions need to be met. The external temperature change from day to night should be greater than about 10 K. The thermal storage capacity of the envelope should be high (i.e. ‘heavyweight’ construction) and the heat transfer rate between the internal air and the thermal mass needs to be high. The latter requirement is a challenge, because heat transfer by natural convection is relatively low, leading to the need for large exposed areas. In addition the internal heat gains should be low (preferably less than 40 W per m2 floor area). Modern buildings can be designed to enhance night cooling, e.g. an exposed concrete ceiling to give direct thermal contact between the mass of concrete and the air or an underfloor duct to give thermal contact with a concrete floor. These are purely passive systems. Active systems that use fans or pumps to pass air or chilled water through channels in beams are also used. The heat transfer rates are increased by forced convection, but at the expense of fan or pump energy consumption. A more recent development is the use of phase-change materials (Section 3.6.2).

3.4.7 Evaporative cooling When water evaporates it absorbs latent heat and cools the surroundings. Evaporative cooling techniques rely on a dry climate, such as that in the


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Middle East. Indeed, the technique whereby a porous material containing water is exposed to a dry air stream has long been used to cool buildings in that area. A modern technique developed for natural ventilation systems is positive displacement evaporative cooling (PDEC) (Ford, 2001). In this system, water is sprayed into the building at high level. Evaporation lowers the temperature and density of the air, which falls under the action of negative buoyancy and flows into rooms at lower level.

3.4.8 Internal heat gains Minimizing the internal heat gains during the cooling season can be crucial to the success or failure of a natural ventilation system. For example, in the UK climate, and as a rough guide, the internal heat gains should be less than 20–30 W per m2 of floor area for purely natural ventilation. Larger values probably require some form of additional cooling (passive or active). Internal heat gains arise from lighting, occupants, electric equipment and solar gains. The reduction of solar gains is an area where materials can make a significant contribution (Section 3.6.3)

3.4.9 Control In its most basic form, a natural ventilation system is controlled by the occupants, but there are other options. Air vents are available that automatically adjust their area in response to the pressures acting across them. They are known as ‘constant flow ventilators’, because the aim is to maintain a preset flow rate above a certain pressure difference. Control can be by some form of BMS (Building Management System), e.g. adjustment of stack flow rates by varying the position of a damper in response to a measurement of CO2 concentration.

3.5

Natural ventilation design

3.5.1 Basic procedures The design of a naturally ventilated building can be a lengthy, complex and iterative process, particularly for a large commercial building, because the ventilation system interacts with other aspects of the design. The specific procedures adopted may be unique to a particular building and/or to design teams. Nevertheless, there are basic procedures that are likely to be employed in the system design and it is these that are summarized here, by dividing the design process into four stages. The first stage is to assess the feasibility of natural ventilation. This will involve consideration of the factors referred to in Section 3.4.



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The next stage is to choose a ventilation strategy. In essence this means choosing a flow pattern for the flows through the envelope openings (internal as well as external). The building may be divided into several parts, each with a different strategy. Some rooms or floors in the building may be isolated from the rest of the building. At the other extreme, rooms or floors may form a single cell (in ventilation terms) by the presence of very large openings between them. Figure 3.2 illustrates such a case. The floors are ventilated by cross-ventilation, with fresh air being drawn through the openings by the chimney (or atrium). The advantage of this strategy is that wind and buoyancy act together to maintain the flow pattern, under most weather conditions. This occurs when the internal temperature is greater than the external temperature and the chimney outlet lies in a region of negative wind pressure. The third stage consists of initial design calculations. For the chosen strategy, the initial design aim could be to determine the sizes of openings required to give the required flow rates (magnitude as well as direction) under specified design conditions. This can be done using an envelope flow model, by employing what is known as the explicit method of solution, i.e. the sizes and positions of openings are calculated for specified flow rates. In a North European climate there are usually two basic design conditions, corresponding to winter (heating) and summer (cooling). The first determines the minimum sizes of the openings and the other determines the maximum sizes. The occupants (or other control system) should then be able to exercise the control to satisfy most weather conditions throughout the year. In the later design stage, off-design conditions can be investigated using more sophisticated tools. There is a wide range of tools available, e.g. multicell envelope flow models, computational fluid dynamics (CFD), combined U

TE

TI

3.2 Example of ventilation strategy (flow pattern).


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thermal and ventilation models, physical scale modelling. This stage could well be the most time-consuming.

3.5.2 Design criteria In the heating season, where the aim is to minimize ventilation heat loss, the design criterion will probably be to provide satisfactory indoor air quality. In an office, the requirement will often lie in the range from 5 l/s to 10 l/s of fresh air per person, corresponding to control of CO2 and body odours. In the cooling season, the ventilation rate required to prevent overheating will be much larger than that for air quality. Specifying the design rate is more difficult, because it is necessary to take account of the thermal characteristics of the building. Some form of dynamic thermal model can be used to determine the ventilation rate required to give acceptable internal temperatures. An important point to make here is that design flow rates are not precise. Thus the inherent variability of natural ventilation rates is not necessarily a problem, provided that some broad level of control is exercised, either by the occupants or by a control system.

3.6

Issues concerning materials

In the following, those areas where materials can play a significant role are discussed in more detail.

3.6.1 Airtightness (adventitious leakage) The adverse effect that adventitious leakage can have on natural and mechanical systems is illustrated here by results from an envelope model. Figure 3.3 shows results for a naturally ventilated building in the heating season. The air change rate (h–1) is defined as Q (m3/h) divided by V (m3) and is shown plotted against wind speed for a fixed temperature difference of 15 K. The purpose-provided openings have been designed to provide an air change rate of 0.4 h–1 at a relatively severe condition (U = 8 m/s and DT = 20 K). For less severe weather conditions, the openings can be increased in size as required. The four curves show the air change rates that occur when the purpose-provided openings are kept at their minimum value, with additional adventitious openings (corresponding to permeabilities of 0, 5, 10 and 15 m3/h m2). Clearly, permeability has a significant effect on ventilation rates. However, with a permeability of 5, the ‘design’ air change rate is hardly exceeded. Thus, if the purpose-provided openings were kept at their minimum value by the occupants (or by some other control system), the ventilation heat loss would not be excessive.



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2.0 Permeability = 0

1.8

Permeability = 5

Air change rate (per h)

1.6

Permeability = 10 Permeability = 15

1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0

0

2

4 Wind speed (m/s)

6

8

3.3 Effect of adventitious leakage on a natural ventilation system. 2.0 Permeability = 0

1.8

Permeability = 5

Air change rate (per h)

1.6

Permeability = 10 Permeability = 15

1.4

Zero line

1.2 1.0 0.8 0.6 0.4 0.2 0.0 0

2

4 Wind speed (m/s)

6

8

3.4 Effect of adventitious leakage on a balanced mechanical system.

The same calculations have been carried out for a balanced mechanical system with heat recovery and a design air change rate of 0.4. The actual ventilation rate is now simply the sum of the infiltration rate and the mechanical supply rate. The results are shown in Fig. 3.4. The effects of adventitious leakage on air change rate are similar to those in Fig. 3.3. However, with


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70% heat recovery from the air passing through the heat exchanger, the actual air change rate needs to exceed a value of 0.68 for the benefit of the heat recovery system to be completely negated (shown by ‘zero line’ in the figure). This occurs with a permeability of 10. With a permeability of 5, the benefit of heat recovery is significantly reduced and when allowance is made for the fan energy consumption the effective reduction will be larger. Ideally, the permeability needs to be less than 2 and such low values are achievable (Ford et al., 2007). Achieving an airtight envelope requires consideration at the design and the construction stages. This means specifying quality components and construction techniques in the design stage and ensuring that the techniques are properly followed on site. It is generally recognized that poor construction can be a major source of adventitious openings. Pre-fabrication of components followed by on-site assembly has been found to be a way of achieving tight envelopes, presumably because quality control is easier. A range of products has been developed to assist in reducing leakage. These include proprietary boards and paints. If a building fails the leakage test, it can be time-consuming and expensive to correct the matter. There are recognized procedures for identifying and eliminating leakage sites. This usually involves the use of various sealants. An important issue that does not appear to be covered by current regulations is ageing. A newly built envelope may be satisfactory, but it is known that the leakage of an envelope can increase with time. Increases can occur due to shrinkage or expansion of components, hardening of seals and sealants, settling of foundations and the actions of occupants. Materials that retain their relevant properties over long periods of time could be an important issue here.

3.6.2 Night cooling with phase-change materials (PCM) Conventional night cooling makes use of the storage of sensible heat in the building fabric. To turn a lightweight building into a heavyweight one requires quite large masses of material (e.g. concrete). Phase-change materials (PCM) offer the possibility of storing thermal energy in a much smaller mass. This enhanced energy density makes PCM more suitable for retro-fitting to existing buildings and for incorporation into stand-alone storage systems. However, the use of PCM for night cooling is still in its infancy. There are basically two types of system, namely passive and active. In a purely passive system, the PCM is installed in the building and left to operate with no further actions. A good example of this is plasterboard that contains a layer of micro-encapsulated PCM as used in some Passivhaus designs (Ford et al., 2007). In active systems, the heat transfer to and from



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the PCM is enhanced by mechanical means, usually by a fan to provide forced convection (Etheridge et al., 2006). Passive systems rely on natural convection for heat transfer, which is not controllable and tends to be low. Active systems provide higher heat transfer rates at the expense of the energy consumed by the fans or pumps. However, active systems can be controlled and in this way the energy consumed can be minimized, thereby offering a relatively high coefficient of performance (COP) compared to conventional refrigerative cooling. The COP of a cooling device is defined as the cooling rate divided by the power supplied to the device. In addition, the fan can be used to ensure high ventilation rates at night. This is by no means guaranteed with a passive system, simply because the natural driving forces are unreliable. In fact, an active system allows the room temperature to be controlled, as illustrated by curve C in Fig. 3.5. The results shown are theoretical, using the model described in Dabidian et al. (2007), but with a more realistic PCM simulation. Curves A and B correspond respectively to natural ventilation (air change rate = 1.0 h–1 throughout the day) and natural ventilation with night cooling (air change rate increased to 10 h–1 at night). The main requirements for the thermal properties of a PCM for night cooling are as follows. The transition temperature needs to be around 20 °C.

35 A

Temperature (°C)

30

B

25 A 20

C B

15

10

0

4

8

12 Time (h)

16

20

24

3.5 Variation of room air temperature over 24 hours for three cooling strategies: A – natural ventilation, B – natural ventilation with night cooling, C – natural ventilation with night cooling and controlled PCM system.


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As well as a high latent heat capacity, the thermal conductivity needs to be high. The reason for this is that the performance of the system depends on the heat transfer into the material. There is little point in having sufficient mass when the heat transfer rates are too low to make use of it. In addition, the properties need to be stable over a long period. When used for night cooling, a lifetime of 10 years implies a lifetime of about 3000 transition cycles. Other important factors for building usage are flammability, toxicity, corrosiveness, cost (capital and maintenance), environmental impact (recycling, embodied energy). Air quality is an important factor when choosing a PCM. The air should not come into contact with the PCM and this imposes requirements on the manner in which the PCM is contained. A particular issue is that of expansion and contraction of the PCM, which could lead to leakage. PCMs that have been used for cooling in buildings are organic (paraffin waxes, fatty acids) and inorganics (hydrated salts).

3.6.3 Reducing solar gains Solar gains can be beneficial in the heating season, but they are undesirable in the cooling season. The thermal capacity of air is low, so a small increase of thermal energy gives rise to a large increase in air temperature and consequent discomfort. Solar gain is often a significant part of the total internal heat gain of a space, and attention has to be paid to reducing it during the cooling season. There are already well-established ways of doing this, e.g. external solar shading devices (CIBSE, 2005). More recently ‘smart’ glazing has appeared, and such developments are promising because of the potential for control, e.g. changing the transmission through the glass as required (Wiggington, 1996).

3.6.4 Porous materials and dynamic insulation Porous materials (strictly speaking permeable materials) may have a role to play in ventilation systems. They can be used as a medium for evaporative cooling. They can also provide what is known as dynamic insulation. With dynamic insulation, the envelope is formed from a permeable material through which the ventilation air is induced to flow by pressure differential. In so doing, the heat transmitted by conductance through the material is transferred to the air (Dalehaug, 1993). This can be appreciated by considering the temperature distribution within the material for the case when the flows of air and heat are opposed (counter-flow). With quite small air speeds (1 m/h), the temperature gradient, dT/dx, at the external surface can be reduced virtually to zero, i.e. the thermal transmittance (U-value) is virtually zero and the only heat loss is that due to ventilation. Provided the



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ventilation rate with dynamic insulation is close to that with conventional insulation, dynamic insulation offers considerable reductions in heat loss. Despite the potential advantages of dynamic insulation, there have been very few examples of its use, other than for research. One reason for this is the need to maintain the required flow through the material. The obvious way is to use a mechanical extract system (counter-flow), but the energy required for the fan can significantly offset the gain. In theory, it is possible for dynamic insulation to be driven by natural ventilation, with a wind-driven extractor fan (Etheridge and Zhang, 1998), but this has not been put into practice. Perhaps the most important reason is that the presence of other openings in the envelope will reduce the flow through the porous material. Adventitious leakage is therefore a potential problem, as are purpose-provided openings. It can also be mentioned here that porous materials could be useful for thermal storage with night cooling. The passage of air through the material should enhance the transfer of heat to and from the material.

3.6.5 Air quality In recent years, it has been recognized that building and furnishing materials can be a significant source of pollution in the form of VOCs (volatile organic compounds). The best way of dealing with this type of pollutant (i.e. one that is not related to the number of occupants) is to reduce the emission rate, rather than to increase the ventilation rate above that required for CO 2 control. Many countries now have standards both for emission rates and for testing materials (see Chapter 6). External concentrations of pollutants can be a concern for buildings in urban areas with traffic pollution. In principle, filters can be used to trap particulates and to adsorb some gaseous pollutants (Liddament, 1996). More recently catalytic devices have been developed. These approaches are more suited to mechanical ventilation. An approach that is more appropriate for natural ventilation is to ensure that air inlets are placed well away from the external source. This is not easy and the benefits may be marginal.

3.7

Future trends

The need to reduce carbon emissions and the projected increase in external temperatures associated with global warming are perhaps the two most important factors that will determine future trends in ventilation systems. The first factor increases the demand for low-energy ventilation systems, both natural and mechanical (and mixed-mode). The second factor makes it more difficult to achieve a successful natural ventilation system in the cooling season. Another adverse factor here is the increasing use of electronic equipment in homes and commerce, leading to higher internal gains.


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Whether or not the demand for low-energy systems in commercial buildings can be increasingly met by natural ventilation remains to be seen. Emphasis will need to be placed on features that prevent overheating. It is still unclear how successful PCM and smart glazing will be in this respect, but both have potential. During the heating season, a low-energy system requires that the ventilation rate should not significantly exceed that required for air quality. Achieving a tight envelope and maintaining it over the life of the property is important for natural and mechanical systems, particularly for a balanced mechanical system with heat recovery. The materials used in construction are clearly important here and the development of materials that are resistant to ageing could have an impact.

3.8

Sources of further information and advice

There is a vast and growing literature on ventilation (natural and mechanical) and air quality. The following sources can be regarded as a starting point. Textbooks on ventilation Awbi (1991) and Etheridge and Sandberg (1996) cover most aspects of the theory and measurement of natural and mechanical ventilation. Design guides for ventilation Detailed design guidance on many aspects of ventilation is available from professional societies. The American Society of Heating and Refrigeration Engineers (ASHRAE) in the USA, the Chartered Institution of Building Services Engineers (CIBSE) in the UK and the Japanese Architectural Association in Japan are good examples. Self-contained design guides specifically for natural ventilation are relatively scarce. A good example is CIBSE (2005). Liddament (1996) gives guidance on many aspects of low-energy ventilation systems. Standards, regulations Many countries have their own standards and regulations for ventilation, leakage and air quality. A good starting point is AIVC TN55 (2001). Scientific journals The following journals regularly contain technical papers on ventilation and



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air quality: ASHRAE Trans, Build Serv Eng Res Tech, Bldg & Env, Energy & Bldgs, Indoor Air, Int J Vent. International conferences The Proceedings of international conferences are a good way of finding latest developments. There is no shortage of them and the short titles of some are AIVC, IAQVEC, Indoor Air, ISHVAC, RoomVent. Further details can be found from their websites. International organisations The International Energy Agency (IEA) promotes and supports research by the member countries of the Organisation for Economic Co-operation and Development (OECD) in a number of areas. One area is energy conservation in buildings and community systems (ECBS). Projects are denoted by Annex numbers and there are several that relate specifically to ventilation, e.g. IEA ECBS Annex 35, IEA ECBS Annex 5. The latter is the Air Infiltration and Ventilation Centre (AIVC) http://www.aivc.org. The AIVC organizes an annual conference and publishes a range of material on ventilation (including a bibliographic database). The European Commission also funds research within the European Community under the framework of the JOULE Programme, e.g. NatVent (1998).

3.9

References and further reading

AIVC TN55, A review of international ventilation, airtightness, building insulation and air quality criteria. Brussels, AIVC, 2001. Awbi H B, Ventilation of buildings. London, E & F Spon, 1991. Battle McCarthy Consulting Engineers, Wind towers. Chichester, Academy Editions, 2001. CIBSE, Natural ventilation in non-domestic buildings. Applications Manual AM10:2005, London, CIBSE, 2005. Dabidian N and Etheridge D W, Simulation of a novel heat pipe/pcm system for cooling of naturally ventilated buildings. Proc. of IAQVEC2007, Sendai, Japan, 2007. Dalehaug A, Development and survey of a wall construction using dynamic insulation. Proc. of Building Physics ’93 – 3rd Nordic Symposium, 1993. Dorer et al., Airtightness of buildings. VIP Note 8, Brussels, AIVC, 2004. Etheridge D W, Murphy K and Reay D, ‘A PCM/heat pipe cooling system for reducing air conditioning in buildings: review of options and report on field tests’. Building Serv. Eng. Res. Tech., 27(1) 27–39, 2006. Etheridge D W and Sandberg M, Building ventilation – Theory and measurement. Chichester, John Wiley and Sons, 1996. Etheridge D W and Zhang JJ, Dynamic insulation and natural ventilation: Feasibility study. Building Serv. Eng. Res. Tech., 19(4) 203–212, 1998.


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Ford B, Passive downdraught evaporative cooling: principles and practice. Architectural Research Quarterly, 5(3), 271–280, 2001. Ford B, Schiano-Phan R and Zhongchen D (eds), The Passivhaus Standard in European Warm Climates, European Commission IEE Programme ‘Passive-on’, July 2007. IEA ECBS Annex 5, Air Infiltration and Ventilation Centre, Coventry, ESSU. IEA ECBS Annex 28, Low Energy Cooling, Technical Synthesis Report, Coventry, ESSU, 2000. IEA ECBS Annex 35, Design of Energy Efficient Hybrid Ventilation, www. hybridventilation.dk Liddament M, A guide to energy efficient ventilation. Coventry, AIVC, 1996. NatVent, Overcoming technical barriers to natural ventilation, 1998. Reports available at http://projects.bre.co.uk/natven Ohba M, Goto T, Kurabuchi T, Endo T and Akamine Y, Experimental study on predicting wind-driven cross-ventilation flow rates and discharge coefficients based on the local dynamic similarity model, Int. J. Ventilation, 5(1), 105–114, 2006. Wiggington M, Glass in Architecture. London, Phaidon Press Ltd, 1996.


4


S. W u, University of Nottingham, UK

Abstract: This chapter introduces the technology of heat storage and cooling and its applications in buildings. It discusses the psychrometrics and air conditioning which are relevant to the thermal energy storage for building thermal comfort applications. The discussion then goes on to three heat storage methods, namely, the sensible, the latent and the chemical heat storage technologies and the issues that are related to the performance of the thermal energy storage. It also presents some of the latest developments in thermal energy storage for energy-saving building thermal comfort applications. Key words: heat storage, thermal energy storage, cooling, building energy.

4.1

Introduction

A considerable amount of energy is consumed in buildings via heating, cooling and dehumidifying to provide thermal comfort. However, the shortage of energy resources and the excessive emission of greenhouse gases resulting from the use of carbon-based energy have rendered such a consumption rate unsustainable. The solution to these unsustainable practices is, perhaps, through increasing the efficiency of our energy usage and utilising renewable energies in order to avoid compromising our living standards. In this aspect, thermal energy storage can play an important role for the following reasons: 1. Thermal energy storage can improve energy efficiency by conserving energy, that otherwise would have been wasted, for later use. Additionally the fluctuations of temperature that compromise the efficiency of thermal systems may also be minimised. 2. Thermal energy storage can also maximise the output from an intermittent renewable energy supply source, such as solar radiation, by increasing its accessibility to applications such as building cooling and heating. As the regulations on the energy performance of buildings tighten, thermal energy storage technologies and their applications in buildings are becoming an increasingly important subject. This chapter covers the contents of psychrometrics and air conditioning that are related to thermal comfort; the principles of the thermal energy storage technologies and their applications for building energy saving. 101 © Woodhead Publishing Limited, 2010

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4.2

Psychrometrics

4.2.1

State of air

The air in the atmosphere is a mixture of gases. It contains 78.08% nitrogen, 20.95% oxygen plus a small amount of other gases such as argon and carbon dioxide, etc. Air also contains a variable amount of water vapour. Air that contains no water vapour is called dry air. When air contains water vapour, humidity is a term that is used to describe the amount of water vapour in it. There are two ways to describe how humid the air is: the quantity of water vapour contained in the air (absolute humidity) and the degree to which the air is close to its saturated state (relative humidity). Absolute humidity is expressed in terms of the mass of water vapour contained in one kilogram of dry air, i.e.

w=

mw (kg water ater vapour vapour/kg /kg dr dryy air air)) mg

Here, mw and mg are the quantities of water vapour and dry air, respectively. Since the water vapour and air can be treated as an ideal gas under the relevant conditions, the absolute humidity can also be written in the following form:

w=

0.622 Pw (kg/kg) P – Pw

4.1

Here, P is the total pressure which equals the sum of the partial pressures of dry air and water vapour in the air. Pw is water vapour pressure. Relative humidity is the ratio of mass of water vapour in the air to the mass of water vapour when the air is in saturated state, i.e. the maximum amount of water vapour the air can hold under the same temperature and pressure. If the maximum amount of water vapour is denoted as mw,s, then its vapour pressure is Pw,s, thus according to the definition, relative humidity can be written as:

j=

mw P = w mw,s Pw, s

4.2

Absolute humidity and relative humidity have the following relationship according to Eqs 4.1 and 4.2:

j=

wP (0.662 + w ) Pg

4.3

Both absolute and relative humidity are of dimensionless units. Relative



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humidity ranges from 0 to 1 and is usually expressed as a percentage, where 0% indicates that the air is dry and 100% indicates that the air is saturated. When the humidity of air reaches 100%, water vapour in the air begins to condense. This point is called the dew-point. The temperature at which condensation begins as a result of cooling the air (at constant pressure) is called the dew-point temperature. A simple method to measure the humidity of the air is by measuring the temperature difference before and after it undergoes the adiabatic saturation process. The temperature measured before the process is the dry-bulb temperature and the temperature measured after is the wet-bulb temperature. Note that, theoretically, the wet bulb temperature is equal to the adiabatic saturation temperature. The larger the difference between the dry-bulb and wet-bulb temperatures, the lower the humidity of the air. The water vapour in atmospheric air, temperature and pressure are variables in determining the state of the atmospheric air. However, since atmospheric pressure varies within a small range, the pressure is generally represented with the standard atmosphere pressure of 1.013 kPa. Thus, in practice, only temperature and humidity are required for determining the state of atmospheric air. The energy stored in atmospheric air is measured in enthalpy which is a combination of properties u + Pv, in which, u is internal energy and the product Pv is associated with the work potential of the air flow. The enthalpy of a pure substance is a function of temperature and pressure and is given the symbol H. The enthalpy of a unit mass is called specific enthalpy and given the symbol h. Since atmospheric air is a mixture of dry air and water vapour, the enthalpy of atmospheric air is a sum of the enthalpies of dry air and water vapour in the air, i.e., Hm = Hg + Hw, where Hg = mghg, Hw = mwhw, and the mass of the mixture mm = mg + mw. Thus the enthalpy of air can also be expressed in unit mass enthalpy of the atmospheric air as below: m mh m = m gh g + m wh w

4.4

Rearranging Eq. 4.4 and using the definition of absolute humidity, we have: mw hw mg hg + w hw = m 1+w 1+ w mg

hg + hm =

4.5

Equation 4.5 can be used to calculate the specific enthalpy of the atmospheric air if the temperature, pressure and humidity are known. The enthalpy of atmospheric air based on per unit dry air is commonly used. In this case,


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h=

Hm = hg + w hw mg

4.6

In engineering practice, a psychrometric chart is constructed to show the relationship between the dry-bulb temperature, web-bulb temperature, dewpoint temperature, absolute humidity, relative humidity, volume and enthalpy of atmospheric air per kilogram dry air. Among the seven variables, only two are independent, i.e., if both the independent variables are known, the rest are also known. The psychrometric chart is very useful in analysis of the air processes in air-conditioning and evaporative cooling.

4.2.2

Surface and interstitial condensation

The moisture in the air can cause surface and interstitial condensation to building structure. When the surface temperature of walls, floors and ceilings is below the air’s dew point, water vapour will condense on them and the condensate can diffuse into the materials. For materials with large pores, moisture penetration can occur. When the material temperature falls below the dew-point temperature of the air, interstitial condensation can occur within the pore structure of the materials. Surface and interstitial condensation are closely related to the properties of the materials such as thermal conductivity, vapour permeability, water sorption and diffusivity, density and specific heat capacity. The long-term effects of condensation can have a significant influence on the strength of building structures.

4.2.3

Principles of air conditioning

Air conditioning is the process of cooling and dehumidifying indoor air to meet the requirements of thermal comfort or other purposes. A common method of cooling and dehumidifying is by contacting the air to a cold surface such as an evaporator. Thus whilst the temperature of the cold surface is usually below the dew-point of air, some of the water vapour in the air condenses as the air is cooled to below dew-point, thereby removing water from the air. It should be noted that while this process decreases the absolute humidity, it actually increases the relative humidity of the air to 100%. In order to reduce the relative humidity, a heating process is needed. This can be achieved through warming the air by simply heating it or by mixing it with another stream of warm air. This process can be drawn on a psychrometric chart as shown in Fig. 4.1. The lines of b–2 and b–2¢ in Fig. 4.1 represent the processes of the air being warmed up without water vapour taken in or being mixed with another stream of warm air with higher humidity, respectively. Cooling devices used for air conditioning can be electrically powered


1

a

w1

2¢ b

2

w2

105

Absolute humidity


T1 T2 Dry-bulb temperature

4.1 Processes of air conditioning on psychrometric chart.

mechanical refrigerators, heat powered absorption refrigerators or any systems that can produce cooling below the dew-point temperature of the air. Dehumidification and cooling of air can also be achieved using the desiccant and evaporative cooling method. This method involves using a substance which has a strong affiliation with the water vapour in the air as a solid adsorbent, or liquid absorbent, to remove water vapour from the air. upon contact with the absorbent (adsorbent), water vapour is taken into the liquid absorbent via diffusion or bound onto the surface of the solid absorbent. This sorption is an exothermal process. The heat generated in the process has to be removed in order to sustain the sorption. Typically, the heat is removed as it is generated. This is an isothermal sorption process as both the air and the absorbent (adsorbent) remain at a constant temperature. The heat can also be removed after the sorption. In this case, sorption takes place adiabatically and the heat generated in the process is held by both the air and the absorbent (adsorbent). As a result, the air temperature rises in the process. Cooling the air may be required to reduce the temperature to the desired level for air conditioning. This can be achieved by adiabatically evaporating water into the desiccated air. The amount of water evaporated, mf, is determined by the decrease in air temperature and the flow rate of the air if ignoring the heat absorbed by water: maCp(T1 – T2) = mf hfg

4.7

If the absolute humidity of the air after the desiccant process is denoted as w1, then the temperature decrease of the air, and the increase in the absolute humidity of air after the cooling process, can be approximately determined from the following:

w 2 – w1 ª

Cp (T1 – T2 ) h fg


4.8

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Since T2 and w2 have to meet the requirements for air conditioning, Eq. 4.8 sets the minimum requirements for the temperature (T1) and humidity (w1) that the air should reach after passing through the desiccant process. Since the sorption is an exothermal process, the heat generated in the process will increase the temperatures of the air and the sorbent. The temperature rise of the air depends on the heat distribution between the air and the absorbent (or adsorbent). The distribution can be expressed by a ratio of the heat received by the air to that by the absorbent or adsorbent. A high distribution ratio indicates that more of the heat is transferred to the air. If the distribution ratio is too large, the subsequent evaporative cooling may be unable to cool down the air temperature at the desired humidity. In this case, the air must be pre-cooled before it enters the evaporative cooling process in order to achieve the required air conditioning. If the humidity of the atmospheric air is low enough that water can directly evaporate in the air to cool down the air, the desiccant process is not necessary.

4.3

Fundamentals of thermal energy storage

4.3.1 Energy in substances A substance contains energy which can be divided into macroscopic and microscopic forms. The former refers to the energy that a substance possesses as a whole with respect to the outside reference frame; the latter refers to the energy that is related to the molecular structure and activity of a substance, independent of the outside reference frame. The microscopic form of energy is called internal energy in thermodynamics. The transfer of this energy between the systems is called heat. Strictly speaking, heat is not a form of energy but a quantity of internal energy transferred, although the term heat energy is widely accepted. So, heat storage is the storage of additional internal energy that has been acquired by heat absorption. The internal energy can be divided into two categories according to what it is associated with: those relating to the kinetic energy of the molecules, atoms and electrons, and those relating to the binding forces between the molecules, atoms and particles of a substance. If the change of internal energy is a consequence of kinetic energy change, then the change is accompanied by a change in temperature. The heat associated with kinetic energy change is called sensible heat. If, however, the change of internal energy is a consequence of a change in the binding forces, then the change is accompanied by a form change of the substance, which could be the result of a phase change relating to weak physical bonds or the result of a chemical reaction relating to strong chemical bonds. It is called ‘potential heat’, when the heat is associated with a phase change, or ‘reaction heat’, when the heat is associated with a chemical reaction. These heats are not associated with temperature change. © Woodhead Publishing Limited, 2010


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Heat storage is realised by converting input heat into the internal energy of the materials. Since input heat can increase kinetic energy and/or change the physical or chemical bonds present within the materials, the methods of heat storage can be characterised accordingly as: sensible heat storage, latent heat storage or chemical heat storage.

4.3.2

Sensible heat storage

Sensible heat storage stores heat in the form of kinetic energy in materials. The heat excites the molecules and/or atoms of the storage media so that the heat is converted into internal kinetic energy. The temperature of the storage media will increase as a result of increase in kinetic energy. For a thermal system like a sensible heat storage device, the change in the internal energy of the thermal system, if considering only the heat exchange and the work done between the thermal system and the surroundings, can be described by the first law of thermodynamics: dU = dQ – dW If the heat transfer is a reversible process and the work is done as a result of change of volume, the first law becomes: dU = TdS – pdV

4.9

Entropy may be described as a function of temperature T and volume V, i.e. S = S(T, V) The change in entropy can be expressed as follows: Ê ∂S ˆ Ê ∂S ˆ dS = Á ˜ dT + Á ˜ dV Ë ∂T ¯ V Ë ∂V ¯ T Substituting dS in Eq. 4.9 with the above equation and using the Maxwell relations (Cengel and Boles, 1998), Eq. 4.9 can be rewritten as follows: Ê Ê ∂P ˆ ˆ dU = Cv dT dT + Á T Á ˜ – P˜ ddV Ë ¯ ∂ T Ë ¯ V The first term on the right represents internal energy varying with the temperature – this is proportional to the specific heat, Cv, of the materials provided that volume is constant. The second term on the right represents the volume effect on the internal energy. Since the volume of solid or liquid does not vary greatly with pressure, this effect can be ignored without significant sacrifices in accuracy, provided that the materials used as the storage medium


108


are liquid or solid. This leads to a simpler relationship between the internal energy and temperature:

dU = CvdT

It should be noted that the specific heat Cv is not a constant in general. Therefore, the relationship between temperature and internal energy is not linear. Specific heat and temperature The specific heat discussed until now is a thermal property that can be understood as the change in the internal energy per degree temperature change of 1 mol (or 1 kg) substance under constant volume. As mentioned previously, this part of internal energy associated with sensible heat is stored in the form of kinetic energy, relating to the translation, rotation and vibration of the molecules. These activities are temperature dependent; therefore, the specific heat is a function of temperature. Since the rotation and vibration of the molecules only start to become more vigorous at higher temperatures, one can expect that more energy is required per degree temperature rise at higher temperatures. This results in higher values of specific heat at high temperatures. Figure 4.2 shows how the specific heat of water varies with temperature. It can be seen from the figure that specific heat increases with temperatures from 20 °C upward. The initial rise of the specific heat is small until the temperature reaches 300 ºC after which a dramatic increase follows. The increase at temperatures above 300 ºC reflects the significant contribution of the rotation and vibration of the molecules in absorbing the energy. It should be noted that there is another specific heat which is defined under 16 14

Cp (kJ/kg°C)

12 10 8 6 4 2 0 0

100

200 T (°C)

300

400

4.2 The specific heat of water varying with temperature.



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constant pressure and denoted as Cp. The constant pressure specific heat, Cp, is used in the processes where the volume change becomes significant. In such cases, the energy change of a system as a result of temperature change includes work done to the surroundings plus internal energy. This energy change is usually measured by the combined thermal property of a substance, termed enthalpy – the sum of work and internal energy for convenience. However, in the case of liquids and solids, the difference between ‘constant pressure’ and ‘constant volume’ specific heats becomes less significant as the volume of liquid and solid substances vary insignificantly. More general theoretical relationships between the specific heat and temperature for solid and liquid can be found in White (1999). Knowing how the specific heat of a substance varies with the temperature is important when choosing a storage medium for a sensible heat storage system in order to optimise heat storage performance. Specific heat and molecule structure According to the theory of equipartition energy (White, 1999), the kinetic energy part of internal energy is associated with the number of degrees of freedom of motion. The specific heat of a substance therefore varies with the structure of the molecules. Since a monatomic species only has translational motion, it has three translational degrees of freedom. In addition to translational motion, a diatomic molecule has two rotational degrees of freedom (the rotation on its own axis cannot be counted as a degree of freedom) and one vibrational degree of freedom, and thus a diatomic molecule has six degrees of freedom in total. Since each degree of freedom holds kinetic energy independently, the total kinetic energy of a molecule is the sum of these. A diatomic molecule can possess two times the kinetic energy of a monatomic species at the same temperature. In other words, the specific heat per mole of a diatomic molecule substance is twice that of the monatomic species. Thus, a more complicated molecule has a higher molar specific heat. For example, the specific heat of helium (a monatomic gas) is 12.47 kJ/(kmol K) or 3.1156 kJ/(kg K) while the molar specific heat of hydrogen (a diatomic gas) is 20.53 kJ/(kmol K) or 10.183 kJ/(kg K) (Cengel and Boles, 1998). However, it should be noted that this is not always true when the specific heat is mass based. For example, the specific heat of carbon monoxide gas is 0.744kJ/(kg K) which is much lower than that of helium 3.1156 kJ/(kg K). This is because the high molar specific heat is weighted down by its heavy molar mass. Capacity of sensible heat storage The sensible heat storage stores thermal energy in a heat storage medium whose temperature change is the result of the addition or removal of heat.


110


The capacity of sensible heat storage is determined by its specific heat and the mass of the heat storage medium as well as the allowed temperature change in operation: Q = mCvDT

4.10

Since m = rV, the heat capacity of the sensible storage can also be written as follows: Q = rVCvDT

4.11

Analogous to the definition of density, we can define the thermal density of a thermal storage medium by rearranging Eq. 4.11: Q = r Cv VDT

4.12

Thermal storage density is an important parameter when choosing a storage medium for sensible heat storage systems. It is desirable to have a heat storage medium with a high value of rCv in order to reduce the volume of the heat storage system. The heat capacity of a sensible storage system can also be expanded by increasing its operation temperature difference. However, this is limited by the operational temperatures of the application and the heat source.

4.3.3

Latent heat storage

Change of phase We are all familiar with the phenomenon that under normal atmospheric conditions, water appears in solid, liquid and vapour states when the temperature of water is below 0 ºC, between 0 ºC and 100 ºC and above 100 ºC, respectively. We use the term ‘phase’ to describe the physical state of a substance. The transition of a substance between the states is called change of phase which is the result of a change in the physical bonds between the particles of a substance. Change of phase is unrelated to the kinetic energy of molecules as was discussed; therefore, there is no change of temperature during transition between phases. The state of a single component substance can be determined by three variables: pressure (P), temperature (T) and volume (V). Figure 4.3 shows the phases of a substance changing with P, T and V. In Fig. 4.3(a), the lines show that there are three possible ways for a substance to change its phase, namely, sublimation (from solid to vapour), melting (from solid to liquid) and vaporisation (from liquid to vapour). Further observation of Fig. 4.3(a) and (b) will determine that sublimation can take place only at low pressures and results in significant volumetric



111

P

Meltin Liquid

p Va

ion imat Subl Triple point

is or

at

io

n

Critical point Liquid

Critical point

Solid

Solid

Solid-Liquid

g

P

Vapour

Liquid-Vapour

Vapour

Solid-Vapour T

(a)

(b)

V

4.3 A typical phase diagram of a substance under (a) P–T and (b) P–V coordination.

changes. This is problematic in practice for a heat energy storage system and thus a phase change heat storage based on sublimation is usually not considered. Similarly, vaporisation also causes large volumetric change but at high temperature and pressure. Thus, heat storage systems based on vaporisation are required to withstand high temperatures and pressures. This is, therefore, unfavourable as a result of the high costs. The melting of a substance occurs at temperatures between sublimation and vaporisation and with the least volumetric change. Therefore, heat storage systems based on melting are very suitable for building applications where modest heat storage temperatures are required. Condition of phase change From a micro perspective, a change of phase is a change in the strength of the physical bonds between molecules or atoms of a substance. Melting, for example, is the result of a decrease in the number of physical bonds between molecules or atoms. In order for this to occur, the molecules or atoms must gain sufficient momentum to overcome the bonds between them. Thus, changes of phase take place only at certain temperatures. When a substance is changing its phase, it can be considered as particle exchange between two systems of different phases. The driving force that causes the exchange is called chemical potential – a measure that is equivalent to the temperature in heat transfer. Chemical potential For a single component system, if a substance exchanges its particles (or mass) with the surroundings, the change in internal energy as a result of the loss


112


or gain of the particles can be included in the first law of thermodynamics in the following form: dU = TdS – PdV + mdN

4.13

where N is the number of particles in the system. If the change in internal energy is caused solely by the exchange of the particles, i.e., dS = 0 and dV = 0, we have the following equation:

m = ÊÁ ∂U ˆ˜ Ë ∂N ∂N ¯ S ,V The above equation defines the chemical potential of a substance or the change of internal energy with respect to the variation of particles in the system. In practice, the condition of keeping entropy S and volume V constant is difficult to maintain. For phase change processes with constant temperature and pressure, use of the Gibbs function to define the chemical potential is convenient. The Gibbs function is defined as: G = U + PV – TS Differentiation of the Gibbs function results in: dG = dU + PdV + VdP – TdS – SdT Substitute dU with Eq. 4.13 and the above equation becomes: dG = VdP – SdT + mdN

4.14

under the condition of constant pressure and temperature, we have the chemical potential in the following form:

m = ÊÁ ∂Gˆ˜ Ë ∂N ∂N ¯ P , T

4.15

The above equation indicates that the exchange of particles in a process under constant pressure and temperature is proportional to the change in the Gibbs function. It is known that a system is stable when the Gibbs free energy of the system is minimal. This can be used to decide the direction of a phase change process. The following is a derivative of the Gibbs function: Ê ∂G ˆ Ê ∂G ˆ Ê ∂G ˆ dG = Á ˜ dT + Á ˜ dP + Á ˜ dN Ë ∂T ¯ P , N Ë ∂P ¯ T , N Ë ∂N N ¯ P, T Comparing Eqs 4.14 and 4.16, we have Ê ∂G ˆ Ê ∂G ˆ Ê ∂G ˆ –S = Á ˜ , V = Á ˜ , m = Á ˜ Ë ∂T ¯ P , N Ë ∂T ¯ T , N Ë ∂N N ¯ P, T


4.16


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For a phase change process with constant temperature and pressure, the phase change should lead to a falling Gibbs free energy, i.e., dG < 0, until the equilibrium between the phases is established, i.e., dG = 0. This means that for a two-phase system whose phases are denoted as 1 and 2, the process can be described by the following equation: Ê ∂G ˆ Ê ∂G ˆ dG = Á 1 ˜ dN1 + Á 2 ˜ dN 2 ≤ 0 Ë ∂N Ë ∂N N1 ¯ p, T N 2 ¯ p, T Since the loss of a particle in one phase should be equal to the gain in the other phase, i.e., dN1 = –dN2, this gives: ÈÊ ∂G ˆ Ê ∂G ˆ ˘ dG = ÍÁ 1 ˜ – Á 2 ˜ ˙ dN dN1 ≤ 0 N1 ¯ P , T Ë ∂N N 2 ¯ P, T ˙ ÍÎË ∂N ˚ If dN1 is greater than zero, whereby phase 1 receives the particles, the chemical potential difference in the bracket must be equal to or less than zero, i.e., m1 – m2 ≤ 0. This indicates that the phase change happens when there is a difference of the chemical potentials between the phases and that the particles are transferred towards the phase with the lower chemical potential. The phase change stops when both phases have the same chemical potential, i.e., m1 – m2 = 0. Figure 4.4 shows a general specific Gibbs function versus temperature curve under constant pressure (Finn, 1993). The two curves in the figure represent the specific Gibbs functions of two phases in a system. The cross point of the two curves is the state at which the two phases are in equilibrium with the temperature at this point TP. Assume that the two phases under g

gL gs gs

gL Solid

Liquid Tp

T

4.4 Gibbs functions of two phases around the equilibrium state.


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consideration are solid and liquid, respectively. It can be seen from the diagram that the liquid phase is the stable phase when T > Tp. The solid phase becomes the stable phase when T < Tp. Charge heat to the two-phase system will shift the temperature to T > Tp, thus there will be a transition from the solid phase to the stable liquid phase until the two phases return to the original equilibrium state Tp. A difference of Gibbs function between the phases is necessary for the phase change to occur. However, this has to be kept low in order to avoid lowering the usefulness of the energy in the phase change. It is potentially a challenge to maintain the phases in equilibrium in a melting/solidification energy storage system. When the storage medium in such a system is being charged with heat, the solid phase close to the heated surface will begin to melt first. This results in a heated surface surrounded by the liquid. Continuing the charge of heat to the heat storage system will increase the temperature of the liquid phase. Since there is no solid phase, no phase change occurs until the heat reaches the liquid and solid interface. This will drive up the charging temperature and slow down the charging rate – and vice versa in the reverse process. The latent heat The phase change of a substance is always accompanied by heat transfer and volume change. Their relationship can be expressed using the Clausius– Clapeyron equation:

g dP = dT T (v2 – v1) Here, g is the heat transfer involved in the phase change and is called latent heat. The latent heat is an important parameter for the heat storage medium of an energy storage system where the energy is stored through phase change. The Clausius–Clapeyron equation links the slope of the phase boundary with the change in volume of the substance. It provides a way of determining the latent heat by measuring the slope of the saturation curve on a P–T diagram and also the specific volume at the given temperature. A positive slope, dP/dT > 0, indicates that the substance contracts when it changes phase from liquid to solid. A negative slope, however, means that the substance expands when it changes phase from liquid to solid. Water is one of the few substances belonging to the latter group. Capacity of latent heat storage Latent heat storage stores heat in a storage medium in the form of potential energy between the particles of the substance. The conversion between the



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heat and the potential energy within the substance involves a phase change – thus heat storage occurs without significant temperature changes in the storage medium. The capacity of latent heat storage can be determined by the following: Q = mg where g is latent heat in kJ/kg. The thermal storage density of the storage medium can be expressed as below, Q = rg V Since the latent heat of a substance is much greater than the specific heat of the same substance, latent heat storage can store the same amount of heat in a much smaller volume. This is a significant advantage of latent heat storage systems.

4.3.4

Chemical heat storage

Chemical heat storage occurs via endothermic reactions between substances through which thermal energy is stored as the potential energy either between atoms (chemical bonds) or between molecules (van der Waals interactions – a physical bond). Chemical heat storage that involves chemical bond change is called reaction-type storage while one that involves van der Waals interactions is sorption-type storage. The discharge of heat from chemical heat storage systems is an exothermic process through which the substances involved return to their original states before the endothermic reaction takes place. Sorption heat storage The sorption heat storage technology involves at least two components: one as the sorbent and the other as the sorbate. upon contact, the sorbate undergoes a phase change that releases heat. This is the discharge process of a heat storage system that provides the energy for applications. The heat storage is achieved by a reverse process which separates the sorbate from the sorbent in a sorption heat storage system. By heating the sorbent the sorbate gains sufficient momentum to overcome the bonds of the sorbent and as a result, the sorbate changes its phase in the process. This process is sometimes described as desorption. This process brings the sorbent back to the initial state so that it can take the sorbate again. In the sorption heat storage process, the sorbent does not store the heat but changes its chemical potential. Most of the input heat is carried away by the sorbate during its


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phase change. This type of heat storage does not require thermal insulation as long as the content of the sorbate in the sorbent does not increase. There are two different sorption processes: one is called absorption – a process in which the sorbate diffuses into the liquid or solid sorbent accompanying a phase change; the other is adsorption – a process where the sorbent does not diffuse into the sorbent but changes phase on the surface of the sorbent. In practice, absorption uses a liquid sorbent (usually called absorbent) while adsorbent uses solid sorbent (called adsorbent). Absorption is characterised by two independent variables among temperature, pressure and concentration. The force behind the absorption is the difference of the chemical potential between the absorbent and the sorbate. For an ideal binary solution system consisting of components A and B, the chemical potential of component A in the solution can be expressed by the following (Blundell and Blundell, 2006):

m A¢ = m A0 + RT ln x A If this solution is in equilibrium with the pure component A which has the chemical potential:

m A = m A0 + RT ln

PA PA0

then under constant temperature and pressure, the chemical difference between them is zero, i.e., m A¢ = m A . Adding heat to the solution will increase the temperature of the solution and thus break the equilibrium; however, the system can only be in a stable state when the Gibbs free energy reaches a minimum, i.e., the system is in a new equilibrium. This can be achieved only by reducing the molar fraction of the component in the solution. As a result, some of the component A in the solution turns into the pure component A. This process will continue until a new equilibrium is reached. Once the heating ceases and the temperature of the solution decreases, the solution will be in a state with lower chemical potential. This enables the solution to absorb component A. The heat recovered from the absorption can be calculated from the concentration changes of the absorbent in the solution. If the concentration of the sorbate in the solution changes from x1 to x2 during an absorption process, then the amount of sorbate taken by the solution is m = m 2x 2 – m 1x 1 Since the amount of the absorbent does not change during the absorption, the following relationship exists: m2(1 – x2) = m1(1 – x1)



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From the above two equations, the amount of the sorbate can be derived as the following: m=

x2 – x1 m1 x1

Therefore, the heat released from condensation of the sorbate is Qc = m (h2 – h1) =

x2 – x1 m1 (h2 – h1 ) x1

Solution heat, which is the enthalpy difference as a result of the variation in concentration, will also be generated in the absorption: Qs = m2 h2,s – m1h1,s =

1 – x1 m1 (h2,s – h1,s ) 1 – x2

The total heat that can be recovered is: Èx – x1 ˘ 1 – x1 Qc + Qs = m1 Í 2 (h2 – h1) + (h2,s – h1,s )˙ x 1 – x 2 Î 1 ˚ The heat storage capacity of a unit absorbent can be written as the following: Qc + Qs x – x1 1 – x1 = 2 (h2 – h1) + (h2,s – h1,s ) m1 x1 1 – x2 Since the sorbate in the absorption storage system involves the phase change from vapour to liquid, a large heat storage capacity can be obtained. unlike the sensible heat storage, this method does not require storing the absorbent at high temperature. It only requires the absorbent be kept at a low concentration of the sorbate. Therefore, insulation is not needed to prevent the heat loss from the storage tank. The adsorption storage systems store thermal energy in a similar way to absorption storage systems. However, since adsorption only takes place on the surface of the adsorbent, the adsorbent must have a large surface area for adsorption. An adsorbent with a large ratio of surface to volume will help to maximise the amount adsorbed, and therefore the heat released. However, in order for the inner surfaces of the adsorbent to be accessible to the vapour which has a large specific volume, sufficient passages between the adsorbent must be provided. This will add a significant volume to the adsorbent. So, adsorption thermal storage systems are generally associated with large volume.


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Reaction heat storage Reaction heat storage involves a thermo-chemical process in which the participating reactants undergo a reversible chemical reaction. The process can be generally expressed as following:

A + B ¤ AB + DH

Here, DH is the reaction heat. Depending on the type of the reaction, DH can be positive or negative. Since a chemical reaction involves change of chemical bonds, one can expect the heat storage capacity of systems using reaction heat storage technology to be of high density. For example, the carbon dioxide reforming of methane can provide a reaction heat of 247 kJ/ mol (Lovegrove and Luzzi, 1996).

4.3.5 Heat transfer in heat storage systems Previous discussions have shown that heat storage is achieved by changing the internal energy of the materials by transferring heat to them. In sensible heat storage systems, the temperature difference between the heat source and the storage media is the force for the heat charging or discharging process. For this to happen, a force induced by heat is necessary regardless of the technologies (the sensible, phase change or chemical heat storage) that are used in the systems. The forces are the temperature difference in the case of sensible heat storage and the chemical potential difference in the case of phase change and chemical heat storage systems. Heat transfer in heat storage systems can have a significant effect upon them.

4.4

Materials for thermal energy storage

4.4.1 Materials for sensible heat storage As discussed previously, gas is not a suitable thermal storage medium for sensible thermal storage systems due to its large specific volume. The materials used as storage media in sensible thermal storage systems are usually in either solid or liquid phases. In order to increase thermal storage density, thermal storage systems need high thermal density materials as the storage media. These materials also have to be low cost and easy to source since storage systems need large quantities of the material to meet the required storage capacity. Table 4.1 gives some common building materials that are used as storage media in sensible heat storage systems and could be used in buildings. Table 4.1 also indicates that solid materials possess thermal density values far higher than liquid media. The thermal density of concrete is approximately 466 times greater than that of water at 20 ºC. Thus, solid materials are generally better thermal energy storage media. © Woodhead Publishing Limited, 2010


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Table 4.1 Thermal density of some storage media at 20 °C Name

Density (kg/m3)

Specific heat (kJ/kg K)

Thermal density (kJ/m3 K)

Solid Concrete Brickwork Wood Granite Glass Sandstone Aluminium Steel Coarse-grained earth

2200 1600 600 2750 2800 2200 2707 7833 2040

0.88 0.84 2.4 0.89 0.8 0.71 0.896 0.465 1.84

1936 1344 1400 2448 2240 1562 2425 3642 3754

Liquid Water Engine oil

0.9982 0.884

4.158 1.91

4.151 1.688

Source: Bejan (1993)

Thermal conduction is a major issue with solid non-metallic storage media. The rate of thermal conduction inside the materials is potentially far slower than the rate at which heat would be charged or discharged on their surfaces. This could result in large temperature differences between the surface and inside of the materials. For a large block of the solid materials, this means the heat takes a much longer time to reach the innermost parts of the materials. Therefore, the large size, non-metallic solid blocks are not suitable for applications that require high rate heat storage with a frequent charge/discharge cycle. Metallic solid materials perform much better in this respect, though the high cost of the metals makes it uneconomic. Practically, nevertheless, heat transfer in non-metallic heat storage media can be improved by using smaller size non-metallic solid particles instead of large ones. This shortens the heat travelling distance to the innermost parts of the material. The use of the small size particles will slightly reduce the thermal density of the materials due to the additional voids between them; however, the voids could be used as fluid paths to enhance the heat transfer on the surface of the particles by convection heat transfer. This arrangement can allow nonmetallic solid sensible heat storage systems to simultaneously provide the desired high thermal storage densities as well as high charging/discharging rates, although a match between the size of particles and the velocity of the fluid must be made with care.

4.4.2 Materials for latent heat storage Many materials can be used as latent heat storage media if the phase changing temperature of the material is within the required operating range. Materials © Woodhead Publishing Limited, 2010

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with high latent heat are advantageous in pure latent heat operation, since this increases the potential thermal storage density. Some latent heat storage systems may be required to operate in a wide range of temperatures. In such cases, sensible heat can be a significant part of the heat stored and the specific heat of the storage media may need to be taken into account along with the latent heat if the heat storage capacity is to be maximised. The term ‘phase change material’ (PCM) refers particularly to materials that store thermal energy in the phase change from solid to liquid. PCMs can broadly be arranged into three categories: eutectics, salt hydrates and organic materials. For building thermal energy storage applications where the operating temperature falls in the range between 0 ºC and 100 ºC, only salt hydrates and organic materials are suitable. Generally, PCMs for such low temperature applications (below 100 ºC) have low thermal conductivities – a range between 0.1 W/m K and 1 W/m K (Lane, 1980). The slow heat transfer due to the low thermal conductivities of the materials used can potentially prevent heat from reaching all of the phase change materials. This effectively reduces the total capacity of the heat storage. This problem can be addressed by reducing the size of PCMs in the storage systems to reduce the heat transfer resistance within the PCM. Encapsulation of the PCM is a way to divide a large quantity of PCM into small individual particles. The encapsulated small PCM particles reduce the heat travel distance within the PCM and provide a larger specific surface to volume ratio; this is an effective way to improve the overall heat transfer of the latent heat storage systems. Encapsulated PCM particles can be packed into a heat storage bed or formed into slurries for use in liquid applications. Encapsulation can also be achieved from the integration of PCMs with solid materials to form functional construction materials.

4.4.3 Materials for chemical heat storage Different from the sensible or latent heat storage materials, chemical thermal storage materials work as pairs in thermal storage systems. The sorbent is a component of the working pair in a sorption type chemical thermal storage system. The sorbent is not responsible for storing the thermal energy, and thus, its thermal mass is not of major concern. Contrarily, a low sorbent thermal mass will reduce the thermal inertia of the thermal energy storage system and so the speed at which the system responds to demand increases. Chemical thermal storage is very suitable for incorporating with certain heat powered refrigeration systems, such as vapour absorption and adsorption, to provide cooling to buildings. The working pairs used in these systems can be used directly as thermal storage materials. Table 4.2 lists some commonly used working pairs in these systems.



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Table 4.2 Some common working pairs suitable for thermal storage for heat powered sorption cooling systems Storage type

Working pairs

Adsorption

water/zeolite, water/silica gel, methanol/activated carbon and ammonia/activated carbon

Absorption

Water/lithium bromide, ammonia/water

Chemical reaction

Ammonia/inorganic salts

Mugnier and Goetz (2001) theoretically compared the storage capacities of 41 other working pairs that are potentially suitable for sorption or reaction cooling systems. They categorised these working pairs into three groups: absorption, adsorption and solid/gas reaction. Their results show that the solid/gas reaction group of working pairs have the highest thermal storage capacity in terms of Wh/kg (ranging from 84 Wh/kg to 233 Wh/kg) with the absorption working pairs second (ranging from 77 Wh/kg to 277 Wh/kg) and the adsorption working pairs third (ranging from 23 Wh/kg to 82 Wh/kg). PCMs with 85 Wh/kg storage capacities are only slightly better than the adsorption working pairs, but much worse than the other two. This is because all chemical thermal storage working pairs in the three groups involve liquid–vapour phase change of high latent heat, but the surface sorption mechanism potentially compromises the sorption capacity of the adsorption working pairs. Lahmidi et al. (2006) studied a commercially available powder of SrBr2·6H2O as a solid/gas working pair for thermal storage. The reactive solid grains were mixed with a chemically inert binder – expanded natural graphite. Their experimental results show that this working pair can deliver approximately 49 kW/m3 cooling power and 36 kW/m3 heating power. They pointed out that a thermal storage system based on this working pair is superior to a hot water storage tank in terms of storage capacity for a direct floor heating application: 250 kW/m3 of a SrBr2·6H2O working pair at 353 K against 52 kW/m3 of water between 353 K and 308 K. Chemical thermal storage is promising for building heating and cooling applications due to its potential high storage capacity and wide operating temperature range. With the development of the research on chemical thermal storage working pairs, more working pairs will appear for practical applications.

4.4.4 Heat transfer in heat storage materials Thermal energy storage density and storage capacity are very important specifications of a thermal energy storage system. In applications, how quickly the heat can be charged/discharged to/from a thermal energy storage system is equally significant, since a compact high thermal storage capacity is not


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necessarily able to deliver high rates of thermal power. Contrarily, a high thermal storage density is likely to compromise it. This dynamic performance of thermal power delivery is closely associated with heat transfer within the heat storage materials – that is, high thermal power charge and discharge signify fast heat transfer within the storage materials. However, they do not come together naturally since most good, practical thermal energy storage materials, particularly those with high thermal density, do not have favourable thermal properties such as high thermal conductivity, low viscosity, etc. for heat transfer. Thus, measures have to be taken to improve heat transfer within the storage materials in order to achieve the desired high charging/ discharging rates from thermal energy storage systems. There are several ways to address this issue. For solid sensible heat storage materials, heat transfer can be improved by using smaller particles instead of larger ones to shorten the heat travel distance in the materials. The same method can also be applied to PCMs by encapsulating them into small units. Mixing PCMs with good thermal conducting materials such as powder of graphite can also enhance heat transfer. Recently, Ho and Gao (2009) investigated the thermophysical properties of nanoparticles-in-paraffin emulsion as a phasechange material. They emulsified alumina (Al2O3) nanoparticles in paraffin (n-octadecane) by means of a non-ionic surfactant. The experimental results show that both thermal conductivity and viscosity non-linearly increase with the mass fraction of the nanoparticles. The increase in thermal conductivity becomes significant at high temperatures. However, the increased viscosity is, by no means, advantageous for natural convection-dominated thermal storage applications. Khodadadi and Hosseinizadeh (2007) simulated the phase-change process of a nanofluid (water + copper nanoparticles) within a differently heated square cavity. The simulation results revealed that, with the addition of nanoparticles, the nanofluid froze faster than without, and that the freezing speed increased with the percentage of nanoparticles added. Both these works demonstrate the potential of nanoparticles in enhancing heat transfer in phase-change materials.

4.5

Thermal storage applications for building heating and cooling

Thermal storage materials can be used to improve the energy performance of a building with respect to building heating or cooling in different ways. Some of the applications, as discussed below, are easily implemented.

4.5.1 Structural thermal energy storage materials The indoor temperature of buildings constructed using lightweight materials can be easily influenced by the conditions outside, since these buildings have



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low thermal masses and therefore low thermal capacities for heat storage. Even with good insulation, solar insolation through windows and glass doors can still be a significant source of overheating and thus processes such as air conditioning become necessary. This increases the energy consumption of the building. A current way of addressing such a problem is to incorporate PCMs into the structural materials of walls and ceilings. This development increases thermal storage capacities whilst maintaining the desired light weight, using a phase-change wallboard, for example. Common structural materials such as gypsum or concrete can be incorporated with PCMs. Both PCM incorporated gypsum and concrete materials have improved thermal storage capacities compared to those without. Nevertheless, PCM incorporated gypsum is superior to PCM incorporated concrete since only low percentages of PCM can be integrated into concrete when compared to gypsum (around 25%). Consequently, the resultant PCM–concrete composite has a lower latent heat capacity than its gypsum counterpart. Liu and Awbi (2009) and Kuznik and Virgone (2009) tested DuPontTM EnergainTM panels containing 60% micro-encapsulated paraffin and compared the results to wallboards without PCM incorporated. Their test results show that the PCM incorporated panels enhanced the natural convection of the air and had lower wall temperatures in the cooling period than the panels without. Thus, PCM incorporated wallboards are potentially very useful for improving the thermal storage capacity of existing buildings without the need for major work. Despite their promise, porous thermal storage composite materials such as PCM incorporated gypsum weaken over time. Endeavours to address this issue include the study by Li et al. (2009), whereby micro-encapsulated paraffin was incorporated into high-density polyethylene (HDPE)/wood flour rather than gypsum. The research claims that the use of HDPE/wood flour in place of gypsum results in a stronger composite and also that the addition of 8.8 wt% Micro-mist graphite into the paraffin increases the thermal conductivity of the storage material by 17.7%. Although the integration of composite PCM storage materials in building thermal storage applications is a promising advancement, the current technologies have far to go. There is still a pressing need for further improvements before they can be treated as conventional materials from a practical perspective.

4.5.2 Thermal energy storage materials for passive cooling The variation of temperature throughout the day can be exploited to provide buildings with ‘free cooling’. The term ‘free cooling’ refers to the storage of cold in the night when temperatures are low and the absorption of heat during the day when room temperatures are high. By incorporating cold


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storage into ventilation systems, one can supply the building with ‘free cooling’. In such applications, any thermal storage material (be it sensible or phase change) can be used for the storage of cold. Zalba et al. (2004) studied and designed a four-module storage system using PCMs (RT25 and C22) to store and supply the free cooling. It was reported that the PCM storage system needs an extra 9% investment but uses 9.4 times less electrical power (equivalent to 3–4 years pay-off period) compared with similar cooling power conventional split refrigeration units. The effective transfer of heat between PCMs and the air is a persistent challenge when large quantities of PCM are involved in the case of free cooling systems. Turnpenny et al. (2000) proposed the use of heat pipes as a method of heat transfer and reported that a heat transfer rate of 40 W over a melt period of 19 hours was achieved when the temperature difference between the air and the PCM is 5 ºC. Their experiments were carried out using a heat pipe with fins extending 20 mm into PCM, leaving 49% of the PCM more than 20 mm from any fin. The authors believe that further extending the fins into the PCM can potentially increase the heat transfer rate. If a ventilation system is incorporated with ground thermal energy storage (where the earth is used as the sensible heat storage material), the provision of a stable air temperature all year round becomes a possibility. As an open system, fresh air can be supplied at a stable temperature to buildings, i.e., supplying cool air in summer and warm air in winter.

4.5.3 Thermal energy storage incorporated with active cooling/heating Thermal storage can also be incorporated with heat pump systems to avoid the extreme high or low temperature conditions that compromise the energy performance of the systems. By acting as a stabiliser of a heat source or sink, thermal storage can provide heat pumps with a more stable evaporating or condensing temperature all year round. This reduces the temperature difference upon which the heat pumps operate and thus reduces energy consumption. For this kind of application, the ground is an excellent thermal energy storage body. Nagano (2005) studied the use of the foundation piles of buildings as a medium with which to exchange heat with the earth. The example discussed in the report consists of a high-rise building, with a 2800 m2 floor area, supported by 51 steel piles with diameters ranging from 600 mm to 800 mm. Such steel piles provide an effective total of 240 m length for heat exchange with the earth and are able to handle 50 kW of heat. Compared with a gas powered cooling and heating system (a gas boiler plus cooling/ heating machine), the study concluded that the annual operating cost of this system was less than half of the gas powered one and had an annual CO2 emission that was approximately 63% of the gas powered one.



4.6

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The following journals regularly contain technical papers on heat energy storage and cooling: Applied Energy, Applied Thermal Engineering, Building and Environment, International Communications in Heat and Mass Transfer and Solar Energy.

4.7

References

Bejan, A. (1993), Heat Transfer, John Wiley & Sons, Inc. Blundell, S. and Blundell, K. (2006), Concepts in Thermal Physics, Oxford University Press. Cengel, Y. and Boles, M. (1998), Thermodynamics: An Engineering Approach, 3rd edn, McGraw-Hill. Finn, C. (1993), Thermal Physics, 2nd edn, Chapman & Hall. Ho, C.J. and Gao, J.Y. (2009), Preparation and thermophysical properties of nanoparticlesin-parafin emulsion as phase change material. International Communications in Heat and Mass Transfer, 36, 460–470. Khodadadi, J.M. and Hosseinizadeh, S.F. (2007), Nanoparticle-enhanced phase change materials (NEPCM) with great potential for improved thermal energy storage. International Communications in Heat and Mass Transfer, 34, 534–543. Kuznik, F. and Virgone, J. (2009), Experimental assessment of a phase change material for wall building use. Applied Energy, 86, 2038–2046. Lahmidi, H., Mauran, S. and Goetz, V. (2006), Definition, test and simulation of a thermochemical storage process adapted to solar thermal systems. Solar Energy, 80, 883–893. Lane, G.A. (1980), Low temperature heat storage with phase change materials. International Journal of Energy Research, 5, 155–160. Li, J., Xue, P., Ding, W., Han, J. and Sun, G. (2009), Micro-encapsulated paraffin/ high-density polyethylene/wood flour composite as form-stable phase change material for thermal energy storage. Solar Energy Materials and Solar Cells, 93(10), 1761–1767. Liu, H. and Awbi, H. (2009), Performance of phase change material boards under natural convection. Building and Environment, 44, 1788–1793. Lovegrove, K. and Luzzi, A. (1996), Endothermic reactors for an ammonia based thermochemical solar energy storage and transport system. Solar Energy, 56, 361–371. Mugnier, D. and Goetz, V. (2001), Energy storage comparison of sorption systems for cooling and refrigeration. Solar Energy, 71, 47–55. Nagano, K. (2005), Development of the PCM floor supply air-conditioning system, in Paksoy, H. (ed.), Thermal Energy Storage for Sustainable Energy Consumption, Springer, 367–373. Turnpenny, J., Etheridge, D. and Ray, D. (2000), Novel ventilation cooling system for reducing air conditioning in buildings. Part I: testing and theoretical modelling. Applied Thermal Engineering, 20, 1019–1037. White, M. (1999), Properties of Materials, Oxford University Press. Zalba, B., Marin, J., Cabeza, L. and Mehling, H. (2004), Free cooling of buildings with phase change materials. International Journal of Refrigeration, 27, 839–849.


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4.8 C p C v g G h hfg H m N P Q R S T u U v V W x

Appendix: Nomenclature specific heat under constant pressure, kJ/kg K specific heat under constant volume, kJ/kg K or kJ/(mol· K) specific Gibbs free energy or Gibbs function, kJ/kg Gibbs free energy or Gibbs function, kJ specific enthalpy, kJ/kg latent heat of evaporation or condensation, kJ/kg enthalpy, kJ mass of substance, kg number of particles pressure, Pa, N/m2 heat, kJ universal gas constant, kJ/(mol· K) entropy, kJ/K temperature, °C or K specific internal energy, kJ/kg internal energy, kJ specific volume, m3/kg volume, m3 work, kJ concentration of solution, % mass friction, kg/kg

Greek w r m g f

absolute humidity, kg/kg density, kg/m3 chemical potential, J/mol latent heat, kJ/kg relative humidity, %

Subscript 1,2.. state point a air f liquid g dry air m mixture v vapour w water vapour


5


K. P a r s o n s, Loughborough University, UK

Abstract: The basic principles of creating environments for thermal comfort are well understood. This chapter presents those principles and uses them to address the new challenge of specifying conditions for sustainable thermal comfort. Currently accepted methods and Standards are presented which include the predicted mean vote (PMV) and the predicted percentage dissatisfied (PPD) thermal indices. A description of behavioural models leads to the equivalent clothing index (IEQUIV) and a suggestion that it is not necessary to heat offices above 20 oC nor cool them below 25 oC. If this practice were followed it would save significant resources and make a step change in progress towards sustainable thermal comfort. It is concluded that while much can be done, our philosophical, technical and scientific understanding of sustainable thermal comfort is still in its infancy. Key words: sustainable, thermal, comfort, heat, cold.

5.1

Introduction

There are many ways of creating indoor environments that provide thermal comfort and the factors involved are generally well understood. The reduction of air temperature in an office during a heat wave using air conditioning systems can provide thermal comfort for the people in that office. The increase in air movement using fans or natural air flows, however, can also provide comfort even if the air temperature is not reduced. The conditions that provide thermal comfort are well understood, from a century of systematic laboratory and field research, involving studies in psychology, psychophysics, biophysics and physiology. However, the relationships among designing for thermal comfort and energy use, human behaviour, and sustainability have not been studied extensively and are not well understood. The principles for creating thermal comfort are a prerequisite to sustainable design. Use of the principles to achieve requirements for energy and sustainability is a subject of on-going investigation. Both the principles of thermal comfort and the practical design for thermal comfort in sustainable environments are addressed in this chapter.

5.2

Thermal comfort

Thermal comfort is a term that is generally regarded as a desirable or positive state of a person. It is used in relation to how warm or cold a person feels 127 © Woodhead Publishing Limited, 2010

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and is clearly related to the environment a person occupies. There are many levels of discussion regarding the meaning and nature of thermal comfort and there was much activity and debate in the 1960s and 1970s on this topic (see McIntyre, 1980, for a review). Much has been achieved since 1970 in understanding the conditions that create thermal comfort. However, because of the need to take our understanding to a higher level in order to meet increased and new requirements, these discussions are again coming to the fore. A starting point is the generally accepted basic definition of thermal comfort from ASHRAE (1966) and now adopted internationally: ‘Thermal comfort is that condition of mind which expresses satisfaction with the thermal environment’. The emphasis is on a state of mind clearly linking thermal comfort with a ‘psychological’ response. So how do we know what state of mind a person is in? The obvious answer is that we ask them. Subjective scales for use in measuring thermal comfort are provided in Section 5.3. For most cases the definition above is acceptable. There is debate, however, about whether it is entirely correct. What if the state of mind expresses satisfaction but it is clear that the conditions could not be regarded as comfortable or even acceptable? If a person is not familiar with the concept of thermal comfort then he or she may not be disposed to report discomfort. Satisfaction may be expressed because dissatisfaction is not the state of mind. This could apply even where a person is sweating, sticky, hot and so on; a state of mind which expresses satisfaction but surely the person is not in thermal comfort. This should be distinguished from expression of satisfaction and hence implied thermal comfort due to expectation. The state of mind is then ‘uncomfortable’ but satisfaction is expressed because the person is not used to, nor expects, ‘any better’. A further group of people where it is difficult to interpret the definition is for those who may have a limited concept of comfort or an inability to express it. Babies, young children, people with mental disabilities and others, would generally not be able to express a condition of mind. An alternative definition may then apply as it is clear (at least reasonable to assume) that all people in thermal environments would regard certain conditions as undesirable and negative and others desirable and positive (i.e. whatever their condition of mind). The issue of whether the concept of thermal comfort is an international phenomenon is of interest if we are to specify optimal conditions for indoor environments worldwide. If the concept exists how is it expressed in terms of the culture and language of the population? It has generally been considered that the concept of thermal comfort does exist internationally and that conditions for thermal comfort can, for practical purposes, be assumed to be identical across the world. Arguments against this assumption have generally pointed to the likelihood, or natural expectation, that people in hot conditions would prefer ‘warm’ or higher temperature conditions for thermal



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comfort than those that live in temperate or cold climates. In fact, evidence suggests that this is not the case and that although people who live in hot or cold climates are better able to survive heat or cold respectively, conditions for thermal comfort do not vary (Fanger, 1970; Parsons, 2003). In field studies of tropical climates it is often concluded that thermal comfort conditions are at higher temperatures than those for people who live in temperate climates. Supporting evidence often cites indoor temperatures which exist in buildings in tropical countries and assume that they are comfort temperatures. First, there is no reason why we can assume that these are comfort temperatures and second, even if people say they are satisfied, it may be for one of the reasons outlined above and probably if ‘actual’ comfort conditions were presented then this would be preferred. A less neutral and more positive concept related to thermal comfort is thermal pleasure. A contributor to good feng shui is a refreshing wind providing a stimulating and desirable environment. Thermal pleasure can also be found in cool temperatures compensated for by the heat of the sun. Such conditions are usually beyond thermal comfort. Thermal pleasure can also be found where a person who is cold moves to a warmer environment and a person who is hot moves to a cooler environment. This is a transient, short-lived phenomenon and after a few minutes pleasure will disappear as the person adapts to the new conditions. Designing for thermal pleasure therefore is a function of the interaction of the thermal environment with other factors and the recognition that environments are dynamic.

5.3

Measurement of thermal comfort

One of the best ways of determining whether a group of people are comfortable is to ask them. Thermal sensation, comfort, pleasure, pain, as well as behavioural responses, are all psychological phenomena. There have been many useful studies to ‘correlate’ physical conditions and physiological responses with psychological responses. However, no model provides a more accurate prediction than measuring psychological responses directly. Methods for measuring psychological responses range from psychophysical techniques (method of limits, method of magnitude estimation, multidimensional scaling, etc. – see Guildford, 1954) and simple (seven-point) scales often used in laboratory experiments, to the integration of techniques into questionnaires for practical surveys as well as behavioural measures.

5.3.1 Subjective measures There are a number of subjective scales which have been used in the assessment of thermal environments; the most common of these are the seven-point scales of Bedford (1936) and ASHRAE (1966) (see Table 5.1).


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Materials for energy efficiency and thermal comfort in buildings Table 5.1 Scales of warmth sensation, from Bedford (1936) and ASHRAE (1966) Bedford comfort scale

ASHRAE sensation scale

Much too warm Too warm Comfortably warm Comfortable Comfortably cool Too cool Much too cool

Hot Warm Slightly warm Neutral Slightly cool Cool Cold

7 6 5 4 3 2 1

7 6 5 4 3 2 1

The form and method of administering the scales is important. For example, a continuous form of the scale would be to draw a line through all points. This would allow subjects to choose points between ratings (e.g. between cool and cold, a rating of 1.6 on the ASHRAE scale, for example). In analysis of results this would enable parametric statistics to be used. However, maybe the investigator does not consider data ‘strong enough’ for this and is prepared only to use ordinal data (ranks) and non-parametric statistics. These and other points are of importance and for further information the reader is referred to a text on the design and analysis of surveys – e.g. Moser and Kalton (1971) – and on the use of subjective assessment methods – e.g. Sinclair (1990) as well as ISO DIS 28802 (2009) which describes scales for use in an environmental survey. The ‘psychological’ interaction when the scale is administered may also influence results. Subjects are usually given the scale and asked to tick the place which represents ‘how they feel now’, for example. It is important to avoid ambiguity which may lead to a person providing their own interpretation, e.g. what the environment is generally like, or how other people may perceive it, etc. Other issues include range effects – the range provided, e.g. hot to cold, influences the judgement – and leading questions ‘you are uncomfortable aren’t you?’. Sinclair (1990) identifies the following important issues to be considered when constructing questionnaires: question specificity, language, clarity, leading questions, prestige bias, embarrassing questions, hypothetical questions and impersonal questions. Other issues include whether knowledge of results is given – for example, if responses are requested over time, is the subject informed of previous ratings he made? – and whether the ratings are given in the presence of others. Investigations involving subjective measures therefore must be carefully planned. It should be emphasized that although there are many pitfalls, most can be relatively easily overcome and the use of simple subjective methods allows easy collection of important data, which can prove invaluable in the measurement of psychological responses. ISO 10551 (1995) presents the principles and methodology for the



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construction and use of scales for assessing the environment. Scales are divided into two types: personal and environmental. Those related to the personal thermal state may be perceptual – how do you feel now? (e.g. hot), affective – how do you find it? (e.g. comfortable) and preference – how would you prefer to be? (e.g. warmer). Those related to the environment fall into two types: acceptance (e.g. is the environment acceptable?) and tolerance (e.g. is the environment tolerable?) An interesting point for an international standard is translation between languages, since in French, for example, one cannot easily use together ‘warm’, ‘hot’ and ‘very hot’. The fundamental principles and psychological phenomena, however, apply over all nationalities although language and cultural difference (in some cultures subjects may be reluctant to express dissatisfaction) will be important. The selection of subjective scales will depend upon the population under investigation and an initial investigation may be necessary to identify meaningful dimensions. For example, in the investigation of the thermal comfort of clothing (Hollies et al., 1979), the seven-point thermal sensation scales are in general use; however, scales of stickiness, wetness, etc., are used for specific applications. The construction and use of simple questionnaires used in a thermal survey are given in Parsons (2003) and ISO DIS 28802 (2009) (Environmental survey).

5.3.2 Behavioural and observational measures Thermal environments can affect the behaviour of individuals (move about, curl up, put on or take off clothing, become aggressive or quiet, change thermostat setting, etc.). This behaviour can be observed and aspects of it quantified and hence measured. A number of studies have observed the behaviour of householders in controlling internal temperatures by measuring temperatures in the homes (e.g. Weston 1951; Humphreys, 1978). The causes are not always clear. They may be related to increased heating costs to the preference for wearing lighter clothing, for example. More direct observations of behaviour have been made on schoolchildren using time-lapse photography and two-way mirrors. For these methods observer interference is of great importance and should be carefully considered (Humphreys, 1972; Wyon and Halmberg, 1972). Other behavioural measures could include the occupational density of a room (where there is choice) or a measure of accidents or critical incidents. Drury (1990) provides a description of direct observational methods that he says have a high degree of face validity but a low degree of experimental control. For example, even if behavioural measures are correlated with thermal conditions, it cannot be concluded that thermal conditions are the (sole) cause. If the establishment of causality is not important, however, then observation of behaviour provides a useful measure. Behavioural measures can provide a


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method of observing both quantitative and qualitative ‘measures’ with little interference with what is being observed.

5.4

The thermal index: an assessment technique

A useful tool for describing, designing and assessing thermal environments is the thermal index. The principle is that factors that influence human response to thermal environments are integrated to provide a single index value. The aim is that the single value varies as human response varies and can be used to predict the effects of the environment. A thermal comfort index, for example, would provide a single number that is related to the thermal comfort of the occupants of an environment. It may be that two different thermal environments (i.e. with different combinations of various factors such as air temperature, air velocity, humidity and activity of the occupants) have the same thermal comfort index value. Although they are different environments, for an ideal index, identical index values would produce identical thermal comfort responses of the occupants. Hence environments can be designed and compared using the comfort index. A useful idea is that of the standard environment. Here the thermal index is the temperature of a standard environment that would provide the ‘equivalent effect’ on a subject as would the actual environment. Methods of determining equivalent effect have been developed. One of the first indices using this approach was the effective temperature (ET) index (Houghton and Yagloglou, 1923). The ET index was, in effect, the temperature of a standard environment – air temperature equal to radiant temperature, still air, 100% relative humidity for the activity and clothing of interest – which would provide the same sensation of warmth or cold felt by the human body as would the actual environment under consideration.

5.5

Thermal comfort indices

5.5.1 The six basic ‘parameters’ It is generally agreed that any specification of thermal comfort conditions must consider the six basic parameters (variables). These are air temperature, radiant temperature, humidity, air velocity (i.e. the four environmental factors) and the clothing insulation and heat produced by the activity of a person (i.e. the two personal factors). An important point is that any and all of the six factors can influence thermal comfort and that it is the integrated influence of all of the six factors that determines thermal comfort response. ISO 7726 (1985) provides information concerning specification of instruments for the measurement of the environmental factors. ISO 8996 (1990) provides methods for the estimation of heat production by a person for different activities. 1



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Met is defined as the heat produced by a sedentary person and is given the value of 58 Watts produced for every square metre of the body surface area (i.e.1.0 Met = 58 Wm–2. Higher activity levels have higher values. ISO 9920 (1992) provides information concerning the thermal properties of clothing. For dry insulation a value of 1 Clo is defined as providing an insulation of 0.155 m2 K W–1 (1.0 Clo often regarded as the insulation of a typical business suit; 0 Clo is for a naked person, 0.6 Clo light clothing and so on).

5.5.2 Fanger (1970) The most significant landmark in thermal comfort research and practice was the publication of the book Thermal Comfort by Fanger (1970), which outlines the conditions necessary for thermal comfort and methods and principles for evaluating and analysing thermal environments with respect to thermal comfort. Fanger considered that existing knowledge of thermal comfort was inadequate and unsuitable for practical application, and his book is based upon research undertaken at the Technical University of Denmark and at Kansas State University, USA. The methods that he developed are now the most influential and widely used throughout the world. The reason for this success has been the consideration of the ‘user requirements’. He had the vision to recognize that it is the combined thermal effect of all (six basic parameters) physical factors which determines human thermal comfort, and that a practical method was required which could predict conditions for ‘average thermal comfort’ and consequences (in terms of thermal discomfort, e.g. percentage of people dissatisfied) of exposure to conditions away from those for ‘average thermal comfort’.

5.5.3 The comfort equation Fanger (1970) defines three conditions for a person to be in (whole-body) thermal comfort: ∑ the body is in heat balance; ∑ sweat rate is within comfort limits; and ∑ mean skin temperature is within comfort limits. A fourth condition is the absence of local thermal discomfort (e.g. caused by draught). The objective was to produce a comfort equation requiring input of only the six basic parameters and based on the above three conditions, to calculate conditions for thermal comfort. This was achieved using a rational analysis of heat transfer between the clothed body and the environment and experimental research. Fanger’s original work was not in SI units so the SI version presented below is taken from Olesen et al. (1982), ASHRAE (1989) and ISO 7730 (2005).


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5.5.4


Heat balance

Fanger’s conceptual heat balance equation is: H – Ed – Esw – Ere – L = K = R + C

5.1

where a description of terms used is given in Table 5.2. Heat is generated in the body and lost at the skin and from the lungs. It is transferred through clothing where it is lost to the environment. Logical considerations, reasonable assumptions, and a literature review provide equations for each of the terms such that they can be calculated from the six basic parameters: air temperature (ta); mean radiant temperature (tr); partial vapour pressure (Pa pv); air velocity (v); clothing insulation (Icl); and metabolic heat production (M–W, where M is metabolic rate and W is external work). Table 5.3 provides the equations for the components of the Table 5.2 Terms used in the predicted mean vote (PMV) H Ed Esw Ere L K R C

= = = = = = = =

Internal heat production in the human body Heat loss by water vapour diffusion through skin Heat loss by evaporation of sweat from skin surface Latent respiration heat loss Dry respiration heat loss Heat transfer from skin to outer surface of clothing Heat transfer by radiation from clothing surface Heat transfer by convection from clothing surface

Source: Comfort equation of Fanger (1970) Table 5.3 Equations for components of the heat balance equation used by Fanger (1970) in determining the PMV thermal comfort equations (see Table 5.2 for terms) H=M–W Ed = 3.05 ¥ 10–3(256ts – 3373 – Pa) ts = tsk,req i.e. for thermal comfort = 35.7 – 0.0275H Esw = Ersw,req i.e. for thermal comfort = 0.42(M – W – 58.15) L = 0.0014M(34 – ta) Ere = 1.72 ¥ 10–5M(5867 – Pa) K=

(t s – tcl ) 0.155Icl

R = 3.96 ¥ 10–8fcl [(tcl + 273)4 – (tr + 273)4] C = fclhc(tcl – ta) Note: Units for all components: W m–2; Pa in Pascals; temperatures in °C and lcl in m2 KW–1.



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heat balance equation to determine the PMV thermal comfort equations. tcl is the mean surface temperature of clothing and exposed skin; ts is mean skin temperature and fcl is a non-dimensional term for the ratio of the surface area of the clothed person to the surface area of the naked person; hc is the convective heat transfer coefficient in Wm–2K–1.

5.5.5 Sweat rate and skin temperature for comfort Heat balance is a necessary but not sufficient condition for comfort. The body can be in heat balance but uncomfortably hot due to sweating or uncomfortably cold due to vasoconstriction and low skin temperatures. Skin temperatures and sweat rates required for comfort tsk,req, and Ersw,req, depend upon activity level. Rohles and Nevins (1971) provide the following equations:

tsk,req = 35.7 – 0.0275(M – W) °C

Ersw,req = 0.42(M – W – 58.15) Wm–2

5.2

By substituting tsk,req and Ersw,req terms into the heat balance equation, the method of combination of the six basic parameters which produce thermal comfort can be expressed in a comfort equation (see Fanger, 1970; Parsons, 2003).

5.5.6 Predicted mean vote (PMV) and predicted percentage dissatisfied (PPD) To provide a method for evaluating and analysing thermal environments, Fanger made the proposal that the degree of discomfort will depend on the thermal load (L). This he defined as ‘the difference between the internal heat production and the heat loss to the actual environment for a man hypothetically kept at the comfort values of the mean skin temperature and the sweat secretion at the actual activity level’. In comfort conditions the thermal load will be zero. For deviations from comfort the thermal sensation experienced will be a function of the thermal load and the activity level. For sedentary activity, Nevins et al. (1966) and Fanger (1970) provide data and McNall et al. (1968) provide data for four activity levels (from 1396 subjects exposed for 3 hours in 0.6 Clo KSU uniform). This provided an equation for the predicted mean vote (PMV) of a large group of subjects if they had rated their thermal sensation in that environment on the following scale:


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sensation

PMV

Hot Warm slightly warm Neutral slightly cool Cool Cold

+3 +2 +1 0 –1 –2 –3

PMV = (0.303e–0.036M + 0.028 ¥ [(M – W) – 3.05 ¥ 10–3{5733 – 6.99(M – W)–Pa} – 0.42{(M – W) – 58.15} – 1.7 ¥ 10–5M(5867 – Pa) – 0.0014M(34 – ta) – 3.96 ¥ 10–8fcl[(tcl + 273)4 – (tr + 273)4] – fclhc(tcl – ta)}]

5.3

where: tcl = 35.7 – 0.028(M – W) – 0.155Icl[3.96 ¥ 10–8 fcl ¥ {(tcl + 273)4 – (tr + 273)4} + fclhc(tcl – ta)]

5.4

hc = max(2.38(tcl – ta)0.25, 12.1 v) fcl = 1.0 + 0.2Icl for Icl ≤ 0.5

5.5

= 1.05 + 0.1Icl for Icl > 0.5 where Icl values are in units of Clo. Fanger (1970) presents tables showing PMV values for 3500 combinations of the variables. These are now unnecessary as the calculations of PMV can easily be made on a personal computer; see ISO 7730 (2005) and Parsons (1993). The predicted percentage of dissatisfied (PPD) provides practical information concerning the number of potential complainers. The data of nevins et al. (1966), Rohles (1970) and Fanger (1970) provided a relationship between the percentage of dissatisfied and the mean comfort vote (see Fig. 5.1 and ISO 7730, 2005): PPD = 100 – 95exp(–0.03353PMV4 – 0.2179PMV2)

5.6

Fanger (1970) describes a method for the use of the PMV and PPD in practical applications and includes examples involving an analysis of a large room and the use of a thermal non-uniformity index, lowest possible percentage of dissatisfied (LPPD), indicating the ‘best’ that can be achieved by changing the average PMV value in the room only – by changing average air temperature, for example – and hence indicating where specific areas of the room require attention.



Predicted percentage dissatisfied (PPD)

100%

Cold Cool

Slightly Neutral cool

Slightly warm

Warm

Hot

2

3

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10 5

1 –3

–2

–1 0 1 Predicted mean vote (PMV)

5.1 Relationship between the PMV and the PPD.

Data to allow the determination of some PMV and PPD values are provided in Tables 5.4, 5.5, and 5.6.

5.6

International Standards and thermal comfort

Optimum indoor air temperatures have often been the subject of debate, and suggested limit or guidance values for buildings have been proposed over many years, by a number of professional institutions and in legislation. There is an increasing international interest in providing guidance to ensure protection and good practice in areas where people are exposed to hot, moderate and cold environments and there has been recognition that air temperature is only one component of a human thermal environment. The requirements of regional (e.g. European) and international (global) markets and the recognition that systems, services and products should be designed for human use have raised the profile of ergonomics and led to a proliferation of ISO standards, including those in the area of the ergonomics of the thermal environment. These include thermal comfort and are described below. For full details and practical applications the reader is referred to the original Standards. ISO 7730 (2005) provides an analytical method for assessing moderate environments and is based on the predicted mean vote and predicted percentage of dissatisfied (PMV/PPD) index, and on criteria for local thermal discomfort. If the responses of individuals or specific groups are required, then subjective measures should be used (ISO 10551, 1995). © Woodhead Publishing Limited, 2010

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Table 5.4 Examples of estimates of clothing insulation values (Icl) for use in the PMV thermal equation of Fanger (1970) Clothing ensemble

Clo

m2KW–1

Naked

0.0

0.0

Shorts

0.1

0.016

Typical tropical clothing outfit Briefs (underpants), shorts, open neck skirt with short sleeves, light socks and sandals

0.3

0.047

Light summer clothing Briefs, long lightweight trousers, open neck shirt with short sleeves, light socks and shoes

0.5

0.078

Working clothes Underwear, cotton working shirt with long sleeves, working trousers, woollen socks and shoes

0.8

0.124

Typical indoor winter clothing combination Underwear, shirt with long sleeves, trousers, sweater with long sleeves, heavy socks and shoes

1.0

0.155

Heavy traditional European business suit Cotton underwear with long legs and sleeves, shirt, suit comprising trousers, jacket and waistcoat (US vest), woollen socks and heavy shoes

1.5

0.233

Table 5.5 Examples of estimates of metabolic rates (M) for use in the PMV thermal comfort equation of Fanger (1970) Activity

Met

Wm–2

Lying down

0.8

47

Seated quietly

1.0

58

Sedentary activity (office, home, laboratory, school)

1.2

70

Standing, relaxed

1.2

70

Light activity, standing (shopping, laboratory, light industry)

1.6

93

Medium activity, standing (shop assistant, domestic work, machine work)

2.0

116

High activity (heavy machine work, garage work)

3.0

175

5.6.1 ISO 7730: Moderate Thermal Environments – Determination of the PMV and PPD indices and specification of the conditions for thermal comfort This standard considers whole-body thermal sensation and local thermal discomfort caused by draughts. It is based on the predicted mean vote



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Table 5.6 Predicted mean vote (PMV) values from Fanger (1970). Assume RH = 50%; still air and ta = tr. PMV: +3, hot; +2, slightly warm; +1, warm; 0, neutral; –1, slightly cool; –2, cool; –3, cold ta = tr

lcl (Clo) 0.1

0.3

0.5

0.8

1.0

1.5

2.0

M = 1 Met; var = 0.1 ms 10 12 14 –2.5 16 –2.5 –1.9 18 –1.9 –1.4 20 –2.3 –1.3 –0.9 22 –2.3 –1.5 –0.7 –0.3 24 –2.3 –1.4 –0.8 –0.1 0.2 26 –1.2 –0.5 0.0 0.6 0.8 28 –0.1 0.4 0.8 1.2 1.4 30 1.0 1.3 1.6 1.8 1.9 32 2.0 2.2 2.3 2.4 2.5

–2.2 –1.8 –1.4 –1.0 –0.5 –0.1 0.4 0.8 1.2 1.7 2.1 2.6

–1.4 –1.0 –0.7 –0.3 0.0 0.4 0.8 1.1 1.5 1.9 2.3 2.6

M = 1.2 Met; var = 0.1 ms–1 10 12 –2.8 14 –2.3 16 –2.8 –1.8 18 –2.9 –2.1 –1.2 20 –2.2 –1.5 –0.7 22 –2.3 –1.4 –0.8 –0.2 24 –1.4 –0.7 –0.2 0.3 26 –0.5 0.1 0.4 0.8 28 0.4 0.8 1.1 1.3 30 1.3 1.5 1.7 1.8 32 2.0 2.1 2.2 2.3

–2.7 –2.2 –1.8 –1.3 –0.8 –0.4 0.1 0.6 1.0 1.5 1.9 2.3

–1.6 –1.2 –0.9 –0.5 –0.1 0.2 0.6 1.0 1.4 1.7 2.1 2.4

–0.9 –0.6 –0.3 0.0 0.3 0.6 0.9 1.3 1.6 1.9 2.2 2.4

M = 1.6 Met; var = 0.1 ms–1 10 12 –2.6 14 –2.9 –2.1 16 –2.4 –1.7 18 –2.8 –1.8 –1.2 20 –2.1 –1.3 –0.7 22 –1.4 –0.7 –0.2 24 –0.7 –0.2 0.2 26 0.0 0.4 0.7 28 0.7 1.0 1.2 30 1.4 1.6 1.7 32 2.1 2.2 2.2

–1.5 –1.2 –0.9 –0.5 –0.2 0.2 0.5 0.8 1.2 1.6 1.9 2.3

–0.7 –0.4 –0.2 0.1 0.4 0.6 0.9 1.2 1.5 1.8 2.0 2.3

–0.2 0.0 0.3 0.5 0.7 0.9 1.2 1.4 1.6 1.9 2.1 2.4

–1

–2.0 –1.6 –1.3 –0.9 –0.5 –0.1 0.3 0.7 1.1 1.5 1.9 2.3

(PMV) and the predicted percentage of dissatisfied (PPD) indices (Fanger, 1970), and more recent work concerning draughts (Olesen, 1985; Fanger et al., 1989).


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The PMV is the predicted mean vote of a large group of persons, on the following thermal sensation scale, if they had been exposed to the thermal conditions under assessment.

+3 +2 +1 0 –1 –2 –3

hot warm slightly warm neutral slightly cool cool cold

The PMV is calculated from the air temperature, mean radiant temperature, humidity and air velocity of the environment and estimates of metabolic rate and clothing insulation. It is derived from a heat balance equation for the human body combined with empirically determined equations which define sweat rates and mean skin temperatures which are within comfort limits. The equation is provided in Section 5.5.4 above. It is based on data from the exposure of 1300 subjects to various thermal environments. The draught rating (DR) is expressed as the percentage of people to be bothered by draughts, where

DR = (34 – ta)(v – 0.05)0.62(0.37vTu + 3.14)

5.7

where ta is local air temperature (°C), v is local mean air velocity (ms–1), Tu is local turbulence intensity (%) defined as the ratio of the standard deviation of the local air velocity to the local mean air velocity. The DR model is based on experiments on 150 human subjects for the following range of conditions: ∑ ∑ ∑

ta : 20–26 °C v : 0.05–0.4 ms–1 Tu: 0–70%

It applies to people performing mainly sedentary activity with whole-body sensations close to neutral. Risk of draught is lower at higher activities and if people are warmer than neutral. Thermal comfort is defined in the standard as ‘that condition of mind which expresses satisfaction with the thermal environment’. Dissatisfaction may be caused by whole-body or local discomfort. An annex (included, but labelled as not part of the standard) provides guidance in terms of levels of dissatisfaction. This includes dissatisfaction caused by whole-body discomfort and by draughts and other local effects (e.g. thermal gradients, asymmetric radiation, etc.). The Standard also considers the long-term thermal comfort performance of buildings and also provides guidance on appropriate conditions for different quality and spaces. Tables of metabolic rate and clothing



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insulation values are included. For more detailed estimates the user can use ISO 8996 (1990) and ISO 9920 (1992). A computer program is provided to allow ease of calculation and efficient use of the standard.

5.6.2 ISO 7726: Thermal Environment – Instruments and methods for measuring physical quantities This Standard provides definitions of the basic parameters (air temperature, mean radiant temperature, humidity, air velocity) and derived parameters (natural wet bulb temperature, globe temperature). It also provides methods of measurement and specifications of measuring appliances. No specific instrument is standardized, only specifications. The Standard can therefore serve as a guide to manufacturers of instruments as well as specifying measuring requirements, in a contract between investigator and a client.

5.6.3 ISO 10551: Assessing the influence of the thermal environment using subjective judgement scales ISO 10551 (1995) presents the principles and methodology behind the construction and use of subjective scales, and provides examples of scales that can be used to assess thermal environments (see Table 5.7). A practical example and a discussion of methods of data analysis are provided. The principle of the Standard is to provide background information to allow ergonomists to construct and use subjective scales as part of the assessment of thermal environments.

5.7

Behavioural thermoregulation, thermal comfort and the adaptive model

People adapt to preserve comfort. When there is a ‘heat wave’ in the United Kingdom, enquiries are often received (from the press, public, etc.) on how to ‘keep cool’ in the hot conditions. Advice in the context of the six basic parameters could be to reduce clothing, increase air movement using fans, Table 5.7 Subjective scales considered in ISO 10551 (1995) Judgement

Example

Related to

Perceptual Affective Thermal preference Personal acceptance Personal tolerance

How do you feel now? (e.g. hot) How do you find it? (e.g. comfortable) How would you prefer to be? (e.g. warmer) Is the environment acceptable/unacceptable? Is the environment tolerable?

Personal Thermal State Environment


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stay in the shade and be inactive. In extreme cases take a cool bath. It became clear, however, that there are simpler solutions based upon a paradigm shift in thermal comfort from that of traditional laboratory and field research. Cool places provide the opportunity to avoid becoming uncomfortably hot. People are not passive receptors of discomfort; if the opportunity is available to them they can take action including moving to more comfortable surroundings. The most effective form of thermoregulation to ensure survival, comfort and performance is classically known as behavioural thermoregulation. Moving to more desirable thermal conditions, adjusting clothing, seeking shelter, opening windows, changing posture, cuddling, lighting of fires, switching on air conditioners or fans, and more are all examples of behavioural thermoregulation. The profound change in the six basic parameters due to behavioural thermoregulation demonstrates the potential of behavioural thermoregulation and emphasizes the continuous dynamic interaction between people and their thermal environments. Although it has always been accepted that people are not passive, little account has been taken of human behaviour in design and assessment for thermal comfort. Attempts to influence accepted methods for the design and assessment of thermal comfort have used the term ‘adaptive modelling’ and proposals to use adaptive models of thermal comfort have been discussed for over forty years. The term ‘adaptive’ is unfortunate because it implies longer term, even evolutionary adaptation; however, it is now widely used in the context of thermal comfort and although requiring more stringent definition, it does not seem to cause confusion. Early ‘adaptive modellers’ include Auliciems (1981) from Australia and Humphreys and Nicol (1970) from the United Kingdom. The drive for adaptive modelling has continued with researchers from Australia, the United Kingdom, and the United States involving theoretical issues and worldwide field studies. The debate is ongoing; however, the ASHRAE (1997) global database of thermal comfort field experiments and associated adaptive model (de Dear, 1998; de Dear and Brager, 1998, 2002) and the interest of the International Standards Organization (ISO) ensured that adaptive models are being given serious consideration. Clearly if we are to improve our understanding of ‘adaptive modelling’ there needs to be rigorous scientific investigation of this area based on the method of null hypothesis. However, the most important point about behavioural or adaptive modelling is the paradigm shift and the new opportunities that it affords to develop our knowledge of thermoregulation.

5.8

Equivalent clothing index (IEQUIV)

In any environment there will be opportunity to adapt to maintain thermal comfort and these adaptive adjustments (behaviours) can take many forms and can occur in combination (e.g. adjust clothing, change activity, open a



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window, change posture). Each of the actions and their combination will have an effect on human heat exchange. It is therefore possible to represent these effects in terms of the equivalent effect of changing one of the parameters in the heat balance equation. This could be any of the six basic parameters; however, a convenient approach would be to relate the total effect of all adaptive behaviour to the equivalent effect of adjusting clothing. An equivalent clothing index (IEQUIV) can be described as follows. The equivalent clothing index (IEQUIV) is the clothing insulation that would give equivalent thermal comfort to people with no adaptation as the thermal comfort of people who adapt to their thermal conditions. A group of people initially wearing 1.0 Clo who change clothing, change activity, change posture and open a window to maintain a neutral thermal sensation may have an equivalent clothing index value of 0.2 Clo, where 0.2 Clo would represent the clothing insulation required to maintain a neutral thermal sensation in the original conditions. The total of all adaptive behaviour therefore summates to a reduction of 0.8 Clo. The equivalent clothing index value can then be substituted into the PMV equation, instead of the clothing insulation value, to give a PMV that takes account of adaptive behaviour. Table 5.8 shows a possible relationship between adaptive opportunity and IEQUIV for a person dressed in 1.0 Clo.

IEQUIV = ISTART – (IADJ ¥ ISTART) in the heat

IEQUIV = ISTART + (IADJ ¥ ISTART) in the cold

5.8

where Istart is the clothes worn before adaptation and IADJ is the change in clothing that would have equivalent effect on thermal comfort to the sum of all adaptive measures. For cases where no adaptation is possible, then clothing, posture, activity, physical environment (e.g. windows) cannot be adjusted (IADJ = 0). Therefore the equivalent clothing is the actual clothing worn at the beginning of the exposure period (IEQUIV = ISTART). For maximum adaptation in the heat, it will be possible to take off all clothing as well as making other adaptive adjustments (open windows, reduce activity). For these conditions it may be useful to adjust parameters separately (e.g. metabolic rate) in the rational index (e.g. PMV) as well as using IEQUIV = 0. In these circumstances, thermal Table 5.8 IADJ values for a range of adaptive opportunities Adaptive opportunity

IADJ

Minimum Low Medium High Maximum

0 0.25 0.5 0.75 1.0


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comfort is an unusual concept and, in practice, adaptive effects greater than taking off all clothing would be unusual. It may therefore be reasonable to assume IEQUIV = 0 is a minimum practical value. A similar argument applies for IEQUIV in cold conditions where doubling of clothing insulation is a realistic maximum with more detailed analysis required for special cases. It should be noted that IADJ may be different for hot conditions than for cold conditions as different adaptation may be required; for example, if opening windows is the only adaptive opportunity, then IADJ may be ‘medium’ in hot conditions but ‘minimum’ in the cold, etc.

5.9

Equivalent clothing index, the comfort temperature range and temperature limits in offices

Although a rational index such as the PMV provides a versatile tool with which to determine thermal comfort conditions in terms of the six basic parameters (variables), there is a requirement to provide temperature ranges within which people can maintain comfort. Suppose people in a building wear 1.0 Clo, perform sedentary activity in conditions with no radiant load (ta = tr), and vapour pressure of 1.0 kPa in still air. They can reduce their clothing but cannot increase it, they can move around and open windows. We could assess the adaptive opportunity as high in the heat and low in the cold. A PMV of 0 (neutral) is then obtained at 24 °C for IEQUIV = ISTART = 1.0 Clo; 28.5 °C for IEQUIV = 0.25 Clo and 22.8 °C for IEQUIV = 1.25 Clo. This provides a comfort temperature range of 22.8 °C to 28.5 °C. If a range between +1 (slightly warm) and –1 (slightly cool) is regarded as acceptable then an acceptable range would be 18.2 °C to 30.5 °C (see Table 5.6). The PMV is an example of a rational index and its validity does not affect the principles of the above. The IEQUIV method is a behavioural adjustment which is a practical way forward. It is preferable to ‘expectancy’ adjustments where statements such as, ‘It is what they are used to and do not expect better’, border on the unethical and, in any case, are flawed as if people claim comfort at 30 °C, how do we know that they would not also claim comfort at PMV = 0 (e.g. 24 °C).

5.10

Sustainable thermal comfort

If we consider sustainability to be the use of resources in such a way that they do not become exhausted, or even reduced at all, this will provide a context for achieving sustainable thermal comfort. We have seen in this chapter that there are many ways of attaining thermal comfort. A useful starting point is to consider the many combinations of air temperature, radiant temperature, humidity, air velocity, clothing and activity that will create



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comfort conditions. Practical constraints and context will leave a subset of the conditions to provide thermal comfort in any design or application. It is reasonable to assume that a further subset of those conditions will provide sustainable thermal comfort. Knowledge of what constitutes a sustainable ‘system’ or environment is in its infancy and enthusiasm for ‘saving the planet’ has not always been based upon ‘solid science’, clear rationale and evidence. By definition the degree of sustainability requires a view on resources used, resources available and the dynamics of the relationship between them. To specify conditions for sustainable thermal comfort therefore requires much more analysis and research than is currently available. There are, however, practical suggestions for ‘good housekeeping’ that can greatly reduce the use of resources and energy and decrease carbon production. Two obvious measures towards sustainable thermal comfort are to reduce use of resources due to technological control (e.g. air conditioning) and design environments that make it less resource intensive to create thermal comfort and complement, for example, human behaviour and adaptive opportunity to allow individuals to achieve thermal comfort. The use of the IEQUIV index to take account of adaptive opportunities provides a rational approach to specifying thermal comfort. If the IEQUIV index is used in conjunction with the PMV thermal comfort model then a range of conditions can be found within which thermal comfort can be maintained. This simple approach can have dramatic effects in saving resources. For example, there is a tendency for people to over-react when faced with hot or cold conditions. In a heat wave, air temperatures are reduced to levels unnecessary for the achievement of thermal comfort when simple measures such as air movement or reduced clothing (e.g. taking off a tie) could be used. Indeed, such is the over-reaction that complaints of being too cold in rooms, due to air conditioning, are not uncommon when the outside weather is hot. There seems to be an over-reaction in this scenario which could significantly contribute to an increase in total cooling demand during a heat wave. In cold outdoor conditions it is not necessary to overheat a space when a simple increase in clothing can provide comfort. Parsons (2002) demonstrated that people could maintain comfort at below 20 oC air temperature using clothing, although local effects (particularly among females) suggested below 19 oC started to cause discomfort. The over-reliance on technology to provide thermal comfort solutions leads to high energy costs, due to excessive cooling with air conditioning or unnecessary use of heating; this can be considered poor housekeeping and represents an unsustainable approach to achieving thermal comfort. There are clearly limits to adaptive behaviour for thermal comfort. However, a significant saving in energy and resources could be made and sustainable thermal comfort provided if we did not heat offices above 20 oC


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or cool them below 25 oC. This can be demonstrated by an IEQUIV and PMV analysis (see Table 5.6). For more detailed values a full analysis, involving all six parameters, is required. However, cooling below 20 oC begins to cause local discomfort and above 25 oC an unacceptable reduction in clothing, or a tendency to become warm and sticky. Using simple measures, some major savings can already be made and help move towards sustainable thermal comfort. Further work is needed to our philosophical approach and technical and scientific understanding.

5.11

References

ASHRAE (1966), Thermal comfort conditions, ASHRAE standard 55.66, New York. ASHRAE (1989), Physiological principles, comfort and health, in Fundamentals Handbook, Atlanta, USA. ASHRAE (1997), Thermal comfort. ASHRAE Handbook of Fundamentals, Chapter 8, Atlanta, USA. Auliciems A (1981), Towards a psychophysiological model of thermal perception, International Journal of Biometerology, 25, 109–122. Bedford T (1936), The warmth factor in comfort at work: a physiological study of heating and ventilation, Industrial Health Research Board Report No. 76, London, HMSO. de Dear R J (1998), A global database of thermal comfort field experiments, ASHRAE Transactions, 104, Part 1. de Dear R J and Brager G S (1998), Developing an adaptive model of thermal comfort and preference, ASHRAE Transactions, Atlanta, USA. de Dear R J and Brager G S (2002), Thermal comfort in naturally ventilated buildings: revisions to ASHRAE Standard 55, Energy and Buildings, 34(6), 549–572. Drury C G (1990), Methods for direct observation of performance, in J R Wilson and E N Corlett (eds), Evaluation of Human Work – A Practical Ergonomics Methodology, London, Taylor & Francis, 35–57. Fanger P O (1970), Thermal Comfort, Copenhagen, Danish Technical Press. Fanger P O, Melikov A K, Hanzawa H and Ring J (1989), Turbulence and draught, ASHRAE Journal, April. Guildford J P (1954), Psychometric Methods, 2nd edn, New York, McGraw-Hill. Hollies N R S, Custer A G, Morin C J and Howard M E (1979), A human perception analysis approach to clothing comfort, Textile Research Journal, 49, 557–564. Houghton F C and Yagloglou C P (1923), Determining equal comfort lines, Journal of ASHVE, 29, 165–176. Humphreys M A (1972), Clothing and thermal comfort of secondary school children in summertime, CIB Commission W45 symposium thermal comfort and moderate heat stress, Watford, London, HMSO. Humphreys M A (1978), Outdoor temperatures and comfort indoors, Building Research and Practice, 6(2) 92–105. Humphreys M A and Nicol J F (1970), An investigation into the thermal comfort of office workers, Journal of the Institution of Heating and Ventilation Engineers, 30, 181–189. ISO 7726 (1985), Thermal Environments – Instruments and methods for measuring physical quantities, Geneva, International Standards Organization. © Woodhead Publishing Limited, 2010


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ISO 7730 (2005), Moderate Thermal Environments – Determination of the PMV and PPD indices and specification of the conditions for thermal comfort, Geneva, International Standards Organization. ISO 8996 (1990), Ergonomics of the Thermal Environment: Estimation of metabolic heat production, Geneva, International Standards Organization. ISO 9920 (1992), Estimation of the thermal characteristics of a clothing ensemble, Geneva, International Standards Organization. ISO 10551 (1995) Assessment of the influence of the thermal environment using subjective judgement scales, Geneva, International Standards Organization. ISO DIS 28802 (2009) Ergonomics of the Physical Environment – The assessment of environments by means of an environmental survey involving physical measurements of the environment and subjective responses of people, Geneva, International Standards Organization. McIntyre D A (1980), Indoor Climate, London, Applied Science. McNall P E, Ryan P W, Rohles F W, Nevins R G and Springer W E (1968), Metabolic rates at four activity levels and their relationship to thermal comfort, ASHRAE Transactions, 74, Pt I. IV.3.1. Moser C and Kalton G (1971), Survey Methods in Social Investigation, 2nd edn, London, Heinemann. Nevins R G, Rohles F H, Springer W E and Feyerherm A M (1966), A temperature humidity chart for thermal comfort of seated persons, ASHRAE Transactions, 72(I), 283–291. Olesen B W (1985), Local thermal discomfort, in Bruel and Kjaer, Technical Review No. 1, Copenhagen. Olesen B W, Sliwinska E, Madsen T L and Fanger P O (1982), Effects of body posture and activity on the thermal insulation of clothing: measurements by a movable thermal manikin, ASHRAE Transactions, 88(2), 791–805. Parsons K C (1993), Safe surface temperatures, in E J Lovesey (ed.), Contemporary Ergonomics, London, Taylor & Francis. Parsons K C (2002), The effects of gender, acclimation state, the opportunity to adjust clothing and physical disability on requirements for thermal comfort, Energy and Buildings, 34, 593–599. Parsons K (2003), Human Thermal Environments, 2nd edn, London, Taylor & Francis. Rohles F H (1970), Thermal sensations of sedentary man in moderate temperature, Institute for Environmental Research: Special Report, Kansas State University, USA. Rohles F H and Nevins R G (1971), The nature of thermal comfort for sedentary man, ASHRAE Transactions, 77(1), 239–246. Sinclair M A (1990), Subjective assessment, in J R Wilson and E N Corlett (eds), Evaluation of Human Work – a practical ergonomics methodology, London, Taylor & Francis, pp. 58–88. Weston J C (1951), Heating research in occupied houses, Journal of the Institute of Heating and Ventilating Engineers, 19, 47–108. Wyon D P and Halmberg I (1972), Systematic observation of classroom behaviour during moderate heat stress, in Thermal Comfort and Moderate Heat Stress, Proceedings of CIB, W45 Symposium, Watford, London, HMSO.


6


P. B r i m b l e c o m b e, University of East Anglia, UK

Abstract: A surprising range of environmental hazards are present in commercial and residential interiors. These range from issues of fire safety and the problems of combustible materials through to the hazards created by indoor air pollution. Decreased ventilation and the widening variety of indoor materials have enhanced the number of compounds to which we are exposed to indoors. Sick building syndrome and respiratory illnesses have been attributed to these exposures, so the management of occupied spaces has been an increased focus of regulation. Key words: fire, indoor air pollution, sick building syndrome, outgassing.

6.1

Introduction

The question of dangers in occupied spaces can seem somewhat surprising, especially when they refer to our homes which are familiar environments. Typically these are spaces we retire to to escape the noise and bustle of the outside world. They are environments where we usually feel in control, yet these have a range of problems in the modern world, that has made them an increasing area of study and even regulatory concern. The very notion of regulation raises issues of personal freedom and the problem is often blurred as it can be unclear which government agency is appropriate for indoor spaces. Indoor air, for example, is often not treated by the same government agency as outdoor air, thus departments of the environment are frequently replaced by departments of health, housing, safety or industry. The heterogeneous nature of the indoor environment has added to the difficulties of monitoring and regulating these volumes. Some issues have been rather mechanical and easy to approach, such as the flammability of indoor materials, where regulation is widespread, while others such as sick building syndrome, where the sociological context becomes an important factor, remain problematic. Hazards in the residential environment have frequently been linked with adverse health outcomes, especially among children (Klitzman et al. 2005). These include: the problems of lead-based paint hazards that can be associated with elevated blood lead levels, illnesses that arise from dampness and fungi and the microbial volatile organic compounds (MVOCs), dust mite allergens 148 © Woodhead Publishing Limited, 2010


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with asthma and allergy and a range of risks associated with electrical hazards, fire hazards and emissions of materials that are flammable. Infants spend much time inside so there are issues here (Franklin 2007), and elderly or infirm people are also exposed to similar hazards. There have been increasing pressures from buildings regulations to set standards for the design and construction of buildings to ensure the safety and health of people in or about those buildings. In the UK the Building Regulations are made under powers provided in the Building Act 1984, and apply in England and Wales (http://www.planningportal.gov.uk/england/ genpub/en/1115313929034.html). These reflect concern over sustainability and climate change, but the Regulations also ensure that fuel and power are conserved. Schedule I to the Building Regulations contains a number of sections relevant to the concerns of this chapter, most notably Part D Toxic substances and Part F Ventilation, although the Schedule acknowledges that some buildings will require greater sensitivity and more flexibility. These include historic buildings, so English Heritage have produced, for example, an Interim Guidance Note on how to balance the needs for energy conservation with those of building conservation. International regulation is also in evidence with the World Health Organisation (WHO) starting to show an interest in the indoor environment that parallels its work on outdoor air quality. The WHO list of pollutants recommended for guideline development (group 1) and those with insufficient scientific evidence and corresponding systematic reviews (group 2) are as follows: ∑

Group 1 pollutants: 1.1 formaldehyde, 1.2 benzene, 1.3 naphthalene, 1.4 nitrogen dioxide (NO2), 1.5 carbon monoxide (CO), 1.6 radon, 1.7 particulate matter (PM2.5 and PM10), 1.8 halogenated compounds (tetrachloroethylene, tricholoethylene, etc.), 1.9 polycyclic aromatic hydrocarbons (PAH), especially benzo-a-pyrene (BaP). ∑ Group 2 pollutants – current evidence uncertain or insufficient for guidelines: 2.1 toluene, 2.2 styrene, 2.3 xylenes, 2.4 acetaldehyde, 2.5 hexane, 2.6 nitric oxide (NO), 2.7 ozone (O3), 2.8 phthalates, 2.9 biocides, pesticides, 2.10 flame retardants, 2.11 glycol ethers, 2.12 asbestos, 2.13 carbon dioxide (CO2), 2.14 limonene, pinene, 2.15 total volatile organic compounds (TVOC). In this context we can see a focus on a widening range of organic compounds driven especially through concerns over ‘tight buildings’ or ‘sick building syndrome’. Many of the larger organic molecules are not familiar from an engineering perspective, although useful textbooks in this area include Indoor Air Pollution edited by Peter Pluschke (2004), which treats the organic chemicals within the context of indoor air and for reference the classic data source on environmental organic compounds is Karel Verschueren’s Handbook of Environmental Data on Organic Chemicals (1996). A good


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listing of the compounds found in residential and commercial buildings is found in the 2003 Lawrence Berkeley National Laboratory report: Volatile Organic Compounds in Indoor Air: A Review of Concentrations Measured in North America Since 1990 (http://eetd.lbl.gov/ied/pdf/LBNL-51715.pdf) by Hodgson and Levin. The health effects of these compounds are not easy to interpret at the low concentrations found, but typically some such as aldehydes are irritants, others have odour and some are potential carcinogens. The difficulties are that the health outcomes at low concentration may arise from reactions or synergistic effects. Our increasing worries about risk in pubic spaces received attention in the Commission for Architecture and the Built Environment report Living with risk – Promoting better public space design (CABE 2007), which recognised the need to carefully balance the risks in their design. The authors of the report envisaged that public spaces are in danger of becoming bland and standardised because of over-sensitivity to risk, arising from misplaced fears of a rampant compensation culture and unquestioning interpretations of health and safety aspects of indoor building regulations. A range of issues have emerged in recent years in terms of public spaces. The public has been worried about the issue of carpets and allergens, and exposure to indoor dust and fungi from damp. This has broadened to strive to minimize the negative effects to individuals from exposure to perfumes, scents and other odours within our facilities. Perhaps of widest impact has been the increasing number of countries that have developed smoke-free legislation for public places to limit exposure to the dangers of second-hand tobacco smoke.

6.2

Safety issues in occupied spaces

There are a wide range of safety issues indoors that include escape, security and falls, but this chapter will focus on fire and what might be regarded as threats to the indoor environment. There has been a rising interest in indoor air pollution in recent years, with countries such as South Korea promulgating legislation and the WHO addressing the potential in its report from October 2006 Development of WHO Guidelines for Indoor Air Quality (WHO 2006) for regulation of pollutants that affect health. There has also been increased litigation over issues such as exposure to indoor mould and much concern from the occupants of new buildings about sick building syndrome and multiple chemical sensitivity, although both these have been resistant to clear understanding or ready solutions. There is currently a view that we spend a great deal of time indoors and that this is increasing. From 1992 through 1994, the US Environmental Protection Agency used telephone interviews of participants who kept diaries to record their activities and locations. This probability-based National Human Activity



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Pattern Survey (NHAPS) was conducted with over 9000 respondents across the country (Brasche and Bischof 2005). The national results were generally consistent with an earlier Californian study, with the mean percentage of time spent indoors being 87%. This was split into 69% of time spent in a residence and 18% of the time spent in other indoor locations. Studies in Germany (Brasche and Bischof 2005) give very similar results: the overall mean time spent at home was 15.7 (65%) hours per day, consistent with the earlier German Environmental Survey (1990/92) and a small German study in 1987. Thus there is evidence of the large amount of time we spend indoors, but less convincing is that this is increasing dramatically; indoor activities may have changed along with the range of consumer products. Additionally problems could have increased indoors because of pressures to decrease ventilation as part of energy conservation. This arises because of the desire to provide adequate ventilation at minimum energy cost (Sherman 1999). Typical ventilation rates from an engineering perspective are given in Table 6.1. There has also been an increased spread of heating and ventilation systems which have the potential to negatively affect indoor air quality, such as the outbreaks of Legionella (Diederen 2008) that have been particularly widely reported. Air quality in schools has become a special problem that can often be increased through parental worry over the health of their children. Air pollution in schools has been studied frequently in recent years (e.g. Janssen et al. 2001), given the potential vulnerability of younger children to the effects of indoor air pollutants and disease. Indoor air pollution might also reduce the productivity of teachers and degrade the student learning experience (e.g. Yang et al. 2009). Modern industries, such as electronics and nanotechnology, have emphasised the importance of clean production facilities where it has become important to keep particles and pollutants from sensitive nano-fabricated materials (Clark et al. 1992, Muller et al. 1994, Lebens et al. 1996, Bhushan and Chandra 1999). Telephone switching centres have proved vulnerable to air pollutants (e.g. Shields and Weschler 1992) and photographic and optical manufacturing (Spyak and Wolfe 1992) must maintain high standards. Museums, historic properties, art galleries and major libraries focused on the traditional agents of deterioration: relative humidity, temperature and light in the past. These buildings have experienced air pollution (Brimblecombe 1990, 2008) and, more recently, dust (Lloyd et al. 2007) has been shown to be an important consideration in the management of their interiors.

6.3

Combustion, fire and combustible materials

Combustion indoors can be intentional, as in terms of heating or cooling or during fires, where very special risks are imposed on occupants and


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Materials for energy efficiency and thermal comfort in buildings Table 6.1 Air change rates recommended for some typical rooms and buildings (http://www.engineeringtoolbox.com/air-change-rate-roomd_867.html) Building/room

Air change rates (AC/h)

All spaces in general Attic spaces for cooling Auditoriums Banks Barber shops Bars Beauty shops Boiler rooms Cafeterias Churches Computer rooms Court houses Department stores Dress shops Engine rooms Factory buildings, ordinary Foundries Garages repair Garages storage Homes, night cooling Kitchens Laundries Libraries, public Medical centres Municipal buildings Museums Offices, public Offices, private Post offices Restaurants Shopping centres Supermarkets Town halls Theatres Warehouses Waiting rooms, public

min 4 12 – 15 8 – 15 4 – 10 6 – 10 20 – 30 6 – 10 15 – 20 12 – 15 8 – 15 15 – 20 4 – 10 6 – 10 6 – 10 4 – 6 2 – 4 15 – 20 20 – 30 4 – 6 10 – 18 15 – 60 10 – 15 4 8 – 12 4 – 10 12 – 15 3 4 4 – 10 8 – 12 6 – 10 4 – 10 4 – 10 8 – 15 2 4

the fire services from exposure to combustion products, during firefighting. There is a long history of exposure to indoor smoke. Seneca, Emperor Nero’s tutor, complained bitterly about exposure to the smoke of green wood from indoors fireplaces. In medieval England charcoal was often used for heating as few of the houses had chimneys. Exposure to wood smoke indoors remains a considerable problem in much of the world and



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has been the impetus for many studies that have health perspectives (Smith et al. 2000), along with pressures for simple, inexpensive but low emission cook-stoves. There are frequently poverty and gender issues here. It is women in the developing world that seem to be exposed at the highest concentrations and poor people who have less choice over the fuels they use so are disproportionately exposed to high indoor air pollution (Adonis and Gil 2001, Kavi Kumar and Viswanathan 2007). Space heating, especially the use of kerosene heaters, which was common in the past, can lead to high concentrations of air pollutants indoors (Leaderer 1982). Carbon monoxide is a particularly critical indoor pollutant, and in many countries it accounts for more than half of the accidental poisonings (Raub et al. 2008) This can be especially severe as ventilation may be deliberately limited in winter to conserve heat, so carbon monoxide from poorly vented heaters can reach high concentrations, or gas lines may leak unnoticed into rooms. These problems can disproportionately affect the poor or the elderly, even in more salubrious neighbourhoods (Brimblecombe et al. 1996). Carbon monoxide poisoning can reach epidemic proportions in severe winter storms where electrical failures can cause residents to use unconventional methods of heating or attempt to generate their own electricity. Again there are significant social issues with high frequency of problems among ethnic minority groups with poor reading ability in the language used for warning labels (Houck and Hampson 1997). Kerosene is used for cooking in some countries. In Indian households there were noticeable increases not only in indoor particulate materials, but also VOCs and PAH with naphthtalene, benzo(a)pyrene and phenanthrene all having I/O greater than 7 (Pandit et al. 2001). There have also been a series of investigations suggesting that gas cooking had the potential to increase indoor exposure to NO2. In kitchens these were associated, classically by Melia and his co-workers, with an increased risk of respiratory illnesses, especially among children (Melia et al. 1985). The problem can be improved by increasing the ventilation in kitchens, particularly the installation of hoods over stoves. Fire statistics from the United Kingdom suggest that the majority of deaths in fires result from inhalation of toxic gases that are produced in the fires. Carbon monoxide once more is the most severe hazard, although the products of thermal decomposition or combustion of combustible material produces toxic gases. These vary according to the material burnt but cover a wide range, and toxic products are also dependent on the fire conditions. The gases can be asphyxiants which cause hypoxia. Others are respiratory irritants that stimulate nerve receptors in the eyes, nose, throat and upper respiratory tract. At sufficiently high concentrations, most irritants can also penetrate deeper into the lungs, causing pulmonary irritation effects which are normally related both to concentration and to the duration of exposure.


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After exposure respiratory distress and even death can occur from a few hours up to several days after exposure due to asphyxiation from pulmonary oedema (Hartzell 2001). multiple gas exposures can be considered in terms of a fractional effective dose (FeD), which can be calculated by summing the concentrations of the fire effluents and dividing by the lethal concentrations: FeD =

m [co] 21 – [o o2 ] [Hc cN N] [Hcl] [HcHo] [Ac] + + + + + [co o 2 ]]-b 21 – LC50,O2 LC50,CN LC50,HCl LC50,HCHO LC HC HCHO 50,Ac

where the lethal concentration for 50% of the population (LC50) over a 30 min exposure time, Ac is acrolein. The first term, the effect of the CO, is enhanced by the increase in respiration rate caused by high concentrations of co2 (expressed as a step function with one set of values of constants m and b for co2 concentrations below 5% and another for those above 5% (Hull et al. 2008)). Table 6.2 presents potential exposure limits to toxic gases released during fires. Table 6.2 Potential limits to exposure from toxic gases released during fires (data adapted from Hartzell 2001 and http://www.doctorfire.com/toxicity.html) Safea ppm

Formula

Gas

CO2 C 2H 4O C 2H 4O 2 NH3 HCl CO HBr NO COS H 2S HF C 3H 4N COF2 NO2 C 3H 5O CH2O SO2 HCN C 9H 6O 2N 2 COCl2 C 4F 8

carbon dioxide acetaldehyde acetic acid ammonia hydrogen chloride carbon monoxide hydrogen bromide nitric oxide carbonyl sulfide hydrogen sulfide hydrogen fluoride acrylonitrile carbonyl fluoride nitrogen dioxide acrolein formaldehyde sulfur dioxide hydrogen cyanide toluene diisocyanate phosgene perfluoroisobutylene

100 3500b 100

Unsafea ppm

1000 35000b 1000

50

500

25 3 25 15

250 30 250 150

a

Assumed LC50 30 min >150,000 20,000 11,000 9000 3700 3000 3000 2500 2000 2000 2000 2000 750 500 300 250 – 135 100 90 6

Concept of limits on exposure of occupants as being safe or unsafe for most occupants. b Units: ppm min.



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Plastic is often a significant hazard in fire; for example, as much as 0.3 g Hcl can be produced from 1 g of polyvinylchloride (PVc) during poorly ventilated fires (Hull et al. 2008). Polyurethanes are widely used in paints and yield a wide range of combustion products, including alkenyl isocyanates, propylene through to octylene derivatives during combustion (Boutin et al. 2004). in a recent eU project, ToXFire, that aimed to set guidelines for management of fires in chemical warehouses, test fires (Andersson et al. 2005) included three compounds with S, N, c and polymers: (1) tetramethylthiuram monosulphide (TmTm) used in vulcanising processes, (2) 4-chloro-3-nitrobenzoic acid (cNBA) used in dye production, (3) chlorobenzene and polymers, (4) nylon 6,6 (nylon, also containing N) and (5) polypropylene. Hydrogen chloride and sulphur dioxide yields are high compared with the maximum theoretical yields, which is in agreement with the assumption that these compounds are end products in the case of well-ventilated fires. Hydrogen cyanide (HcN) was formed in the experiments with nylon, TmTm and cNBA. But the yields are rather low, with a maximum yield of around 2%.

6.4

Infiltration of outdoor pollutants

Air pollutants found indoors can accompany incoming air or arise from processes within buildings. The same pollutant can have both indoor and outdoor sources. Advection of pollutants through windows or doors or through active ventilation via air conditioning systems is a most obvious source of indoor pollutants. Sensitive industrial facilities or archives carefully control the quality of incoming air, filtering and sometimes scrubbing the air free of pollutant gases. once indoors many pollutants react with the indoor surfaces and this reduces their concentration. This is part of the explanation for the advice given to sensitive individuals that they should remain indoors during air pollution episodes. The reduction in pollutant concentration can be represented as an indoor–outdoor ratio of pollutant gases. This has become an important concept in the understanding and management of indoor spaces, and can be described in terms of the simple equation: Ci/Co = E/(E + vd A/V) where: C i = indoor pollutant concentration, C o = outdoor pollutant concentration, E = air exchange rate of the room (hr–1), A = surface area of interior (m 2 ), V = volume (m 3 ) of interior, ratio, and vd = average deposition velocity (m hr–1 or converted from cm s–1). The air exchange rate has already been discussed in terms of the ventilation of typical spaces (see Table 6.1). The key notions are deposition velocity and air exchange rate in determining indoor–outdoor ratios. The deposition velocity represents the way in which a gas deposits and then sticks irreversibly


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to a surface. It can be thought of in terms of a number of separate resistances such that:

vd = [ra + rb + rs]–1

where ra, rb and rs are the aerodynamic resistance, the boundary layer resistance and the surface transfer resistance, respectively. The resistance at the surface can be thought of in terms of surface reactivity. Deposition velocity has been represented in terms of the uptake coefficient or sticking coefficient by Cano-Ruiz et al. (1992), where this is the ratio, in a molecular sense, of the fraction of all pollutant molecules colliding with the surfaces that result in irreversible removal. The magnitude of the deposition velocity can increase very dramatically with humidity or even the hygroscopicity of the surface. Typical values are given in Table 6.3 and more extensive tabulations onto indoor materials appear in Grontoft and Raychaudhuri (2004). The ventilation system itself can be a source of indoor pollutants. These are derived from compounds found deposited on the filters and within the ducting, but odour can also arise from heating coils and heat exchangers. These odours arise from volatile components within the dust. They are emitted more effectively when temperature and relative humidity increase. It is the longer chain aldehydes which contribute most to the odour, with nonanal and decanal especially important. Longer chain carboxylic acids have low odour thresholds, but they are bound to the dust more effectively. However, unsaturated nonenal, which has an unusually low odour threshold, might also be an important contributor to odour. Nitrogen containing compounds, such as indoles (e.g., 1H-isoindole-1,3(2H)-dione), pyrroles, quinolines, and pyridines (methyl and dimethylpyridine), could also be released through thermo-desorption (Hyttinen et al. 2007). Additionally n-aldehydes can also Table 6.3 Deposition velocities of gases to typical indoor materials at 70% RH (for full details see Grontoft and Raychaudhuri 2004)

O 3 (cm/s)

NO2 (cm/s)

SO2 (cm/s)

Glass Dense alkaline stone Soft alkaline stone Fine concrete Brick Painted wood and surfaces Treated gypsum board Metal Wool Wallpaper Cloth >1 year old Plaster

0.0002 0.0002 0.068 0.019 0.135 0.01 0.063 0.0024 0.1 0.08 0.0088 0.044

0 0 0.025 0.01 0.076 0.0043 0.0043 0.0014 0.12 0.017 0.025 0.03

0 0.014 0.15 0.2 0.016 0.047 0.16 0.011 0.059 0.043 0.023 0.17



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be produced via reaction with ozone with surfaces (see Wang and Morrison 2006).

6.5

Indoor emissions and outgassing

Construction materials and furnishings outgas volatile compounds over time as they age or degrade. This adds to outdoor pollutants or those produced from combustion processes indoors during cooking and heating. This section will take a brief look at some of the principal components of occupied spaces that are likely to release gases to the indoor environment, but more comprehensive lists of sources (Wang et al. 2007) and greater details of the chemistry are available elsewhere (Uhde and Salthammer 2007).

6.5.1 Wood As wood ages it releases a range of materials, most notably acids and also aldehydes which derive from heat-treated woods. Formic and acetic acid are best known, but some longer chain carboxylic acids are also products from wood. Substantial emissions of acetic acid and/or furfural can also be observed for heat-treated wood (thermowood) or kiln-dried samples of various wood species (e.g. beech, rubber wood, etc.) (Uhde and Salthammer 2007). Terpenes such as a-pinene and 3-carene are commonly released from air-dried wood (Manninen et al. 2002) and can be an important source of these larger semi-volatile organic compounds indoors (Hodgson et al. 2002). Chloromethane can be found as fungi (Hymenochaetaceae) rot wood and additionally pectin reacts with chloride ion to form chloromethane (Watling and Harper 1998).

6.5.2 Floors Floors may be wooden, but also incorporate glues and varnishes, etc. Parquet floors of birch, coated with acrylic lacquer and adhesive emit n-butylacrylate and n-butanol from the hydrolysis of n-butylacrylate ester along with pentanal and hexanal from the degradation of fatty acids (Uhde and Salthammer 2007). Horn (1998) has reported high emissions of furfural and acetic acid from composite cork products and Udhe and Salthammer (2007) list carbonyl disulfide, acetic acid, furfural styrene, benzaldehyde, cyclohexanone, phenol methylbenzoate, 4-phenylcyclohexene and benzophenone from the degradation of hemicellulose, adhesives binders and photo-initiators from a new cork parquet floor. Linoleum (Jensen et al. 1995a, Jensen et al. 1995b) can be a source of volatile organic compounds as the oxidation of linolenic acid leads to unsaturated compounds like 2,4-heptadienal (Belitz and Grosch 1992).


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6.5.3 Paint, solvents and preservatives Paints and varnishes are an important source of aromatic compounds (Edwards et al. 2001). Traditional paints, based on boiled linseed oil and other natural materials have become popular in recent years because the paint is said to last significantly longer than ordinary gloss paints primarily as the vegetable resins are impervious. Additionally there is a well advertised belief that these paints have no chemical emissions. However, hexanal, nonanal and propanal are released, while Toftum et al. (2008) have shown that beech boards painted with an ‘eco’ paint emitted large amounts of the terpene limonene and lesser amounts of carvone. Furniture coatings containing alkyd or natural resins (Afshari et al. 2003, Salthammer 1995a, Salthammer 1995b) on oxidation form many types of volatile aldehydes. Typical degradation products of oleic acid are saturated aldehydes from heptanal to decanal, while linoleic acid gives mainly hexanal. Organic solvents were used in paints more extensively in the past so compounds such as toluene, isopropanol, acetone and butyl acetate were emitted, but increasingly water-based paints have been adopted. The desire to reduce the volatile compounds emitted from paints and lacquers has encouraged manufacturers to use more UV-curable formulations. These require a photo-initiator. Uhde and Salthammer (2007) investigated the importance of the photo-initiator (a blend 1-hydroxy-cyclohexyl-phenone (HCPK)/benzophenone) and its degradation products as a pollutant source to indoor air. Cyclohexanone was formed from the hydroxy–cyclohexyl radical upon cleavage of HCPK. The photo-initiator benzophenone was found along with its degradation products benzaldehyde and methyl benzoate. Some indoor materials are protected with pesticides. Naphthalene concentrations can be high indoors, particularly in warmer climates where it is frequently used as an insecticide. Pesticides can be deliberately sprayed into air by householders or evaporate from treated products. Given our sensitivity to pesticides, risks from these need to be reduced and often require more thought in our approach to cleaning (see Section 6.6.2). Some pesticides degrade within materials over time. An important example of this is the production of indoor pollutants from pentachlorophenol, commonly used to protect wood from fungi, as the compound degrades over time and gives rise to emissions of chloroanisoles (Gunschera et al. 2004).

6.5.4 Fabrics and coverings Indoors large areas of fabrics cover the floors and furnishings in addition to acting as curtains. The tendency in many countries for increased areas of floor to be covered by carpets raised concern in the late 20th century because of an increasing frequency of asthma and its potential link to dust mites that



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inhabit carpet. Homeowners were sometimes advised to remove carpet from their rooms to limit the risk to allergy and asthma patients. There has been much disagreement about this because clean, dry, well-maintained carpets can also improve air quality as they can absorb both inorganic pollutants such as ozone and SO2 and organic compounds (Won et al. 2001). Additionally, emissions from other types of flooring could impose other types of risk. Volatile organic substances released from carpets are highly variable and depend on their type. Often these are not related to the carpet surface itself, but to the foam backing. Quite exotic substances can be formed during the reaction, which is a radical-induced polymerisation that creates the styrene–butadiene copolymers (SBR) in foam. Residual monomers styrene and butadiene are removed by distillation, but odour-intensive compounds 4-phenylcyclohexene (4-PCH) can remain (Uhde and Salthammer 2007). PVC-backed carpet emitted formaldehyde, vinyl acetate, isooctane, 1,2propanediol and 2-ethyl-1-hexanol with vinyl acetate and propanediol at the highest emission rates, while a polyurethane-backed carpet emitted mostly butylated hydroxytoluene (Hodgson et al. 1993). Some materials with a high sulphur content such as wool can give off large amounts of carbonyl and hydrogen sulphide (Brimblecombe et al. 1992). Other more exotic coverings can be important in some environments. Chang et al. (1995) studied volatile organic compounds (VOCs) released from athletic running tracks which included: 2-methyl furan, butanal, methyl ethyl ketone, benzene, heptane, methyl isobutyl ketone, toluene, octane, hexanal, nonane+ethylbenzene, xylenes+styrene, propyl benzene, decane, 1,3,5-trimethyl benzene, 1,2,4-trimethyl benzene, 1,2,3-trimethyl benzene and undecane. Hexanal was the most common and the principal compound from the synthetic rubber and polyurethane tracks studied.

6.5.5 Flame retardants Flame retardants are used to make materials, especially plastics, wood and wood-based textiles flame-proof. Worldwide in 2000, usage of flame retardants was in excess of 1.08 million tons of which nearly a quarter were organophosphate flame retardants. These also include a range of phthalates (possible endocrine disruptors) and pose potential risk as they outgas and become additionally incorporated into indoor dust. Added to this, retardants can release volatile products derived from rather non-volatile precursors that yield 1-chloro-2-propanol, 2-chloro-1-propanol and 1,3-dichloro-2-propanol. These can be found in indoor air samples following hydrolysis of flame retardants such as tri(chloropropyl) phosphate and tris(dichloropropyl)phosphate, respectively (Salthammer et al. 2003). The latter requires increased attention since 1,3-dichloro-2-propanol has been acknowledged as a carcinogenic substance.


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A study by Ni et al. (2007) of wallpapers showed a significant positive correlation between emissions of tris(2-chloroisopropyl)phosphate (TCPP) and its content in wallpaper samples. Additionally a linear relationship (Arrhenius like) was found between reciprocal temperature and the logarithm rate of the TCPP emission rate at different temperatures. The observed maximum emissions of TCPP from 5 w/w% content wallpaper materials was 645 mg m–2 h–1, at 25 oC. Nevertheless, the inhalation of phthalates and particulate organic chlorine at levels reaching the allowable daily intake seems unlikely under normal living conditions. However, the evaluation of the exposure to household dust is far more difficult if we include ingestion (Wensing et al. 2003).

6.5.6 Plastics, foams and glues Glue is an especially important source of indoor air pollutants, notably formaldehyde. It has a wide range of sources in air and is a frequent oxidation product outdoors. Formaldehyde is characteristic of hospitals where releases to air will occur when it is used for sterilisation. More general indoor sources are from formaldehyde-based insulation and furniture foams, cigarette smoke and carpets, resins, wood, paints and glue, which mean indoor concentrations can be very high (see Table 6.4). It has long been known that the hydrolysis of ureids (glue based on urea-formaldehyde), due to the unavoidable presence of water, leads to hydrolysis of the N–O bond and, as a consequence, to the release of formaldehyde (Uhde and Salthammer 2007). Fibreboards can be especially problematic because of the urea-formaldehyde resins used to bond the constituent parts together. Other aldehydes and organic acids arise from woods, insulating foams, textiles and glues. Sensory irritation of some Table 6.4 Some reported occupational and environmental concentrations of formaldehyde (http://www.mflohc.mb.ca/fact_sheets_folder/formaldehyde.html) Site

Concentration (ppm)

Outdoors Rural pastures Los Angeles Basin Outdoor air in industrial cities Smog

0.0005–0.002 0.01–0.048 0.004–0.015 0.09–0.15

Indoor Conventional homes Homes with urea formaldehyde foam insulation Mobile homes Schools Offices Clothing stores Passive cigarette smoke

0.02–0.095 0.02–0.11 0.1–0.5 0.13–0.57 0.02–0.12 0.9–3.3 0.23–0.4



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shorter aldehydes (e.g., formaldehyde and acrolein) might be higher than that of the longer ones (in passing, Hyttinen et al. 2007). Because of the importance of furniture in enhancing the exposure to formaldehyde in the home, there has been increasing regulation of formaldehyde in furnishings. The UK Department for Environment, Food and Rural Affairs (DEFRA) introduced Building Regulations to prohibit the use of urea-formaldehyde foam (UFF) in buildings and to control its installation elsewhere. Broadly the regulations only permit UFF insulation between cavity walls with masonry inner and outer leaves. Here the inner leaf should afford good resistance to vapour penetration. Plastics often contain plasticisers to improve the plasticity of the product. Those present in phthalates and adipate containing plastics tend to hydrolyse over time. A typical reaction product found in indoor air samples is 2-ethyl1-hexanol from the hydrolysis of diethylhexylphthalate or butanol from the hydrolysis of butylphthalates or n-butylacrylate. Both substances have a strong odour and can lower the perceived air quality.

6.5.7 Secondary production There are also contributions to indoor air from the reaction of pollutant gases with surfaces. Such secondary emissions arise when ozone reacts with paints to give formaldehyde (Reiss et al. 1995) and on carpets (Weschler et al. 1992) or counter tops and releases a range of aldehydes, especially nonanal (Wang and Morrison 2006). We are all aware of the smell of tobacco smoke that persists long after smoking has ceased. This can be due to the sorption of organic compounds such as nicotine and their slow desorption which may take years (Van Loy et al. 2001). Some of these compounds are organic bases and subtle shifts in the acidity of wall coverings could affect the rate of release.

6.6

Occupant activity

6.6.1 Cooking Obviously kitchens expose occupants to combustion products, and in many countries this exposure may be largest for women and children. Beyond combustion products, the preparation of food can involve the release of ethanol or acetic acid (vinegar). The cooking process produces organic materials such as n-alkanes (C25–C36), saturated and unsaturated monocarboxylic acids (C16–C24), saturated and unsaturated dicarboxylic acids (C6–C14), nonanal, lactones, levoglucosan, along with aromatic compounds, particularly toluene and the xylenes (Edwards et al. 2001; Zhao et al. 2007) that can be found as films on


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window glass (Liu et al. 2003). Oil droplets are also forced into the air along with particles of ingredients. We are all aware of the irritancy of capsaicin in kitchen air when chilli peppers are being fried. Frying itself with modern Teflon-coated pans releases a range of compounds such as polyfluoro- and/ or polychlorofluoro- (C3–C14) carboxylic acids, a pyrolysis product of polytetrafluoroethylene (Teflon). Although there are few cases of effects on humans, their impact on birds is well known (Boucher et al. 2000).

6.6.2 Cleaning and fragrances Cleaning products and fragrances have an impact on the composition of indoor air. In addition to abrasion, cleaning releases a range of volatile compounds and other particles to indoor air (Wolkoff et al. 1998). These include surfactants (from detergents), complexing agents as water softeners (e.g. EDTA), disinfectants and waxes. Typical volatile compounds found in cleaning agents include alkanes and alkenes and various halogenated derivatives, alcohols, ethers and carboxyl compounds. There have been relatively few studies that assess the risk balance involved in cleaning within buildings. In domestic interiors, fragrances in cleaning products represent an additional source of the terpenes, a-pinene and limonene (Edwards et al. 2001) along with synthetic aromatic nitro-musks and polycyclic musks, scenting agents in cosmetics, perfumes and cleaning products, that are widely dispersed in the environment. These have largely replaced natural musks, but increasing concern about their environmental impact may have led to lower production (Weschler 2009). Terpentine oils are produced in large amounts (~0.3 Mt/a) and limonene is increasingly seen as a safe fragrance, cleaner and solvent and is globally produced in very large amounts (~70kt/a). Little is known of the risks through inhalation exposure to d-limonene, although it is an irritant. However enhanced risks may arise from its oxidation products and indoor air chemistry. Increasing numbers of people express a sensitivity towards perfumes and scents. There is also a growing concern about exposure to the volatile materials from household products such as air fresheners. There was more than a 30% increase in babies that suffered diarrhoea in homes where air fresheners were used every day, compared with homes where they were used once a week, while their mothers had more headaches and depression (Farrow et al. 2003).

6.6.3 Ozone generation Indoor activities are contributors to the generation of ozone gas. It is sometimes argued that ozone can arise from electrical equipment and photocopying



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machines, but the amounts are quite small. Ozone is produced in larger amounts from ozone generators designed to remove odours and to destroy organic compounds, though there is increased awareness that these should not be used for occupied spaces. Elevated concentrations of ozone may be used for deodorising hotel rooms or homes affected by fires and to disinfect mould-afflicted buildings. Here ozonation can lead to the formation and emission of carbonyls (Poppendieck et al. 2007).

6.7

Transformations within the interior

Indoor air is chemically active. As with secondary pollutants produced by reactions in outdoor air that are characteristic of the Los Angeles smog, indoor air has its own transformations (Weschler and Shields 1997). The transformations of compounds in indoor air can be a function of light. In particular, this occurs as the result of the decrease in the amount of light as air moves indoors. This occurs in large rooms with weakly absorbing walls (metal, polymer or glass). NO2 can be generated as the ‘equilibrium’ shifts to the right (Brimblecombe et al. 1999):

NO + O3 ´ NO2 + O2 + hu

In buildings with a high surface to volume ratio (i.e. small rooms), surface absorption can result in the ozone and to a lesser extent nitrogen oxides being absorbed by walls. However, surface chemistry can transform NO2, so the situation may be different:

2NO2(ads) + H2O Æ HONO + HNO3

There is also an active organic chemistry within interior spaces. Oxidation reactions in indoor air have been studied because of the potential that the oxidation products of volatile organic matter may be more harmful to health than the precursors. Oxidation products include free radicals, secondary ozonides, epoxides, aldehydes, ketones, acids, diacids and dicarbonyls (Weschler 2009). Ozone reactions with the terpenes typically found in rooms produce particles. Such chemical transformations of limonene and a-pinene reactions may be responsible for much of the formaldehyde, almost all of the p-tolualdehyde and a substantial fraction of the particle mass generated indoors (Weschler and Shields 1999, Fan et al. 2003). This can be an important source as they are components of air fresheners and ozone is advected through open widows or more critically arises where ozone generators are used. The particles produced in these reactions are in the submicron range and thus a potential health risk as they reach deep into the lung.


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6.8


Particles in buildings that impact on environmental health and safety

A wide variety of particles are found indoors and these range from the large particles that trouble households as dust, often containing fibres as much as a millimetre in length through to much smaller particles. Soot arises from incomplete combustion and is common indoors, normally being associated with wax candles, incense, mosquito coils, gas stoves or smoking, and many of the particles are in the submicron range (Afshari et al. 2005).

6.8.1 Non-viable particles Concrete is known to yield small alkaline particles (Toishi and Kenjo 1967, 1975) and plaster indoors is likely to give fine gypsum as it degrades (Camuffo et al. 1999). Mechanical abrasion and cleaning is a source, especially vacuum cleaners which produce fine particles. The degradation of materials containing asbestos has been a special problem. Asbestos was widely used in the building industry because of their resistance in fires. Asbestos minerals are highly fibrous and exposure can lead to tumours of the respiratory system and lung cancer, so asbestos is well regulated and within the UK. the three previous sets of regulations covering the prohibition of asbestos, the control of asbestos at work and asbestos licensing now fall within the Control of Asbestos Regulations 2006. It is important to distinguish between ‘friable’ and ‘non-friable’ asbestos, because it is the friable material that produces dangerous fibres. Friable materials are those that contain more than 1% asbestos and can be reduced to powder by the human hand.

6.8.2 Spores and bacteria In addition to non-viable particles there are also numerous fungal spores, bacteria and fragments of biological material. The production of these has become particularly associated with dampness in buildings which encourages the growth of fungi. Although the future climate of Britain may be drier, warmer conditions could enhance fungal growth. Fungal infections are often associated with microbial volatile organic compounds (MVOCs) which include alcohols, ketones, aldehydes, aromatic hydrocarbons, amines and terpenes (Kuske et al. 2005). The best known indoor bacterial infection is from Legionella, which is associated with the growth of the bacteria in water particularly associated with cooling systems or spas (Diederen 2008). It can even be a problem with domestic electric showers, because although domestic hot water systems are often 60 oC and kill legionella, electric showers are cold water fed. Biologically derived particles can also incorporate allergens. Of particular



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concern are those from the dust mite which are associated with asthma and allergy. Allergens are also associated with pets and although pet ownership has increased over time, a Mintel report in June 2005 shows recent declines as people have less time to care for pets and keep them company, particularly in a period where children have many other activities. Dog ownership in Britain fell by 26% between 1985 and 2004, but the outcome in terms of susceptibility to allergens is not easy to predict.

6.9

Materials and toxicity

Modern materials and lifestyles have imposed new risks within occupied spaces and the choice of construction and furnishing materials has become especially difficult. We have to balance the risks imposed by flame retardants, cleaning agents or pesticides against the problems that arise if they have not been used. There is much advice on making such choices, but the reasoning behind such advice is not always obvious. There is a general tendency to regard natural materials as preferable to artificial, on the basis that they have been in use for a long time. Use over a long time is often a good guide as we learn correct approaches to the materials, but it does not seem to make them inherently safe.

6.9.1 Regulation There is a widening regulatory framework that controls the quality of indoor air within occupational, public and domestic spaces. Best known in the UK are the the Building Regulations where Part C concerns resistance to contaminants, Part D to toxic substances and Part F ventilation. Formaldehyde from a number of sources has been of concern and the UK Department for Environment, Food and Rural Affairs pressed for Building Regulations Part D to prohibit the use of UFF in unsuitable buildings and to control its installation and use elsewhere. In parallel there is regulatory concern over formaldehyde from medium density fibreboard, such that in the US emissions must limit yield from MDF to 0.3 ppm and in Germany the exposure limit is 0.l ppm, while Britain’s Health and Safety Executive has issued a number of hazard assessments (e.g. Medium Density Fibreboard – Hazard Assessment Document, EH75/1). Despite the regulatory progress, high levels of formaldehyde are still found when buildings are refurbished, as illustrated by the renovation or construction of schools in Korea where the indoor concentrations of HCHO were higher in schools less than a year old (Yang et al. 2009). Materials such as lead in paint or asbestos have been revealed as problematic, but the range of problem materials is more extensive than this. Asbestos is now well covered by the Control of Asbestos Regulations 2006.


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The use of lead pigments in primer paints on some prefabricated domestic wooden windows continued through to the 1980s, but the UK implemented the provisions of an EU Directive (89/677/EEC) and restricts the use of lead paint. The Regulations allow the manufacture and use of lead, but in controlled circumstances for the redecoration of Grade I and II* listed historic buildings, though the general sale of lead paint in the UK is prohibited. Nevertheless, during the redecoration of older houses, paintwork containing lead pigment can be mobilised, so must be treated as a potential health risk. Furthermore, if it is flaking, likely to be chewed by children or animals, additional risks are imposed.

6.10

Advanced material requirements

Those who wish to create buildings with low environmental risk suggest avoiding materials that are likely to produce persistent organic pollutants. These have the potential to accumulate in fatty tissues concentrating as they move up the food chain and to act as endocrine disruptors. Chlorinated building materials, notably the plastics are likely to be associated with chlorinated dibenzodioxins and furans during their manufacture or when they burn. They also advise against brominated flame retardants, particularly polybrominated diphenyl ether and various perfluorocompounds such as Teflon and Scotchguard. The latter has been used to protect furnishings and fabrics. The key ingredient was perfluorooctane sulfonate, although this has now been reformulated with the less persistent perfluorobutane sulfonate. It is suggested that heavy metals are to be avoided, mercury in switches, lead flashing and solder and copper. The choices advocated for interior materials typically focus on the use of natural substances. This approach to the use of flooring materials can potentially make a big change to indoor safety. Natural linoleum is made from linseed, cork, tree resin, limestone and jute. It is biodegradable and anti-microbial. Natural cork flooring is produced from the bark of the cork oak tree and often layered with UV cured acrylic. It is highly durable, comfortable, sound and thermally insulating. In the case of carpeting and furnishing fabrics, wool is often seen as a non-toxic alternative to synthetic fibres which can outgas, but wool releases sulfides (Brimblecombe et al. 1992). Bamboo is seen as a durable, anti-microbial and water resistant material that can be used for floors and walls. As a plywood, it can be used for kitchen cutting boards, plates, bowls, utensils, countertops and walls and furniture. Paints and finishes are preferred where they contain low or no VOCs, no fungicides or biocides and natural pigments or they are from naturallyderived materials such as citrus peel extracts, essential oils, seed oils, tree resins, inert mineral fillers, tree and bee waxes. The careful choice of materials in improving the indoor environment



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makes sense, but it must be recognised that even the materials in the recommendations above are not without problems. In particular, a focus on natural resins are a problem as they can produce significant concentrations of highly reactive terpenes as indoor VOCs and the UV curing process can release VOCs (Uhde and Salthammer 2007).

6.11

Future trends

As emphasised in the recent article by Weschler (2009), there have been enormous changes in the pollutants we encounter in air over the last fifty years. These will no doubt continue. It is also hard to appreciate the magnitude of potential social change given what we have witnessed in the last fifty years. Social changes have an impact that is difficult to predict. If we look at the European heat-wave of 2003, it is possible to see that changing family structures affected the stress/death among elderly people in that very hot summer, possibly from indoor ozone. Our lifestyles have also changed with increasing use of computers and entertainment within the home rather than outside. It is hard to predict how our indoor lives will change in the future (time inside, electronics, food, etc.). Furthermore, climate change will impose novel requirements on heating and ventilation systems. There is no doubt that we will see an increasing regulation of consumer products and building materials. It is likely that regulations will develop to establish a wider range of criteria for indoor air quality. Since 2004 South Korea has guidelines for formaldehyde (210 mg m–3), benzene (30 mg m–3), toluene (1000 mg m–3), ethylbenzene (360 mg m–3), xylene (700 mg m–3) and styrene (300 mg m–3) in public buildings as part of the Indoor Air Quality Management Act (Kim et al. 2006). In other countries, including China, Singapore, Canada, United Kingdom and Germany, air quality guidelines have been developed also for the indoor environment, but they are highly variable in terms of the compounds and factors considered. Standards relating to the emissions from materials are increasingly common but in some countries, such as China, rapid development has led to a deterioration of indoor air quality because of the use of new materials, e.g. the extensive use of chipboard with high emissions of formaldehyde. In the future there may be entirely new materials such as nano-dusts from emerging nanotechnology. In parallel we can expect an increase in monitoring of indoor pollutants. Carbon monoxide is a particularly critical indoor pollutant, as it accounts for so many deaths. This has meant a great interest in monitors, which are increasingly common in rooms to detect leaks. Changes in the way we ventilate spaces, the range of materials and modern activities pose new dangers within our buildings. The changes are not all for the worse, but we have to recognise the types of risks our


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choices impose. Flame retardants are dangerous and cleaning agents are risky, yet these substances can save lives or reduce the chance of infection. Modern approaches should favour the adoption of materials and processes that lead to the broadest reduction of risk and are compatible with notions of sustainability.

6.12

References

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7

Life cycle assessment and environmental profiling of building materials

K. S t e e l e, Arup: Façades and Materials, UK

Abstract: Sustainability is a complex challenge facing the world today. It requires a holistic approach to thinking which brings together social, economic and environmental dimensions. The built environment, with its vast consumption of energy and resources, and resultant emissions has an important role to play in meeting this challenge. It follows that the manufacture and supply of building materials must contribute to this. Within this context, life cycle thinking and the examination of materials’ embodied impact must go hand in hand with a broader agenda including healthy and safe materials, responsible sourcing and good resource management. This chapter examines these issues taking a focused look at embodied impacts and the use of life cycle assessment (LCA) as an important tool with which to address the environmental sustainability of our building fabric. Key words: construction products and materials, life cycle assessment, LCA, embodied impact, healthy and safe materials, responsible sourcing, resource management, sustainability, environment, ISO, CEN and BSI standardisation.

7.1

Materials sustainability

Sustainability in the context of modern development is a term that transcends the original definition of the word. It is therefore briefly important to examine what it means. From a qualitative perspective, sustainability is, by definition, something that has the ability for continuance, in essence, the ability to keep itself going. This is a simple concept to grasp. However, within the context of our developing society it has developed into a hugely complex debate and agenda. The reason for this is that the term has become synonymous with the very existence of humanity, and with our (logical and rightful) desire for continuance. To sustain life on earth, the global environment must be sustainable, for, as Sarah Parkin (2000) bluntly puts it, ‘if the environment cannot support life, then we are dead’. For hundreds of millions of years, the world has successfully supported and sustained life (and more recently, human life). Unfortunately, evidence now indicates that the global environment could be losing its qualities of sustainability. Indeed, it is now widely recognised that humanity’s existence as we know it is threatened. 175 © Woodhead Publishing Limited, 2010

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This has prompted concern that the world will develop into an unattractive and perhaps uninhabitable place in which to live. The underlying blame can be attributed to human activities and their burden on the environment. It follows that human activities are unsustainable; they are causing potentially irreversible damage and it is now widely recognised that this problem must be addressed. This is a complex and global challenge that requires a reexamination of how we do things in all walks of life. The built environment, with its vast consumption of energy and resources and resultant emissions, in construction, operation, and at end-of-life has an important role to play. This is well recognised and the drive is now on to design, construct and manage our built environment in a way that is sustainable. The manufacture and supply of materials has an important contribution to make to this objective and the sustainability of building materials and construction products has become an issue against which we should judge our building fabric. But what does this mean and how do we allow for this complex agenda in materials selection, specification and procurement? The start point is to recognise that sustainable decision making requires that environmental, social and economic dimensions are brought together in a holistic approach considering multiple criteria within a life cycle context. Therefore this enables technical performance of a material to be examined alongside its impacts. Within this context it should be recognised that both the benefits and impacts a material has occur across all three environmental, social or economic dimensions. Slowly the industry is establishing tools and methods for this new framework of assessment. This chapter takes a focused look at one of these tools called life cycle assessment (LCA), as well as introducing the concepts of healthy materials, responsible sourcing and resource management and how they are expected to play an increased role in the next period in the assessment of sustainable materials.

7.2

A life cycle approach to selecting building materials

Commonly referred to as ‘cradle-to-grave’ or ‘cradle-to-cradle’ assessment, life cycle assessment, or LCA, examines the life cycle of a product, system or process and evaluates the environmental impact it has over its life or a defined study period. In this regard LCA follows two key principles important to sustainable decision making. These are a ‘systems’ approach to assessment, i.e. it assesses life cycle performance; and secondly evaluation across multicriteria, i.e. performance is measured across multiple environmental impact categories (Fig. 7.1). Both are fundamental to making environmentally sustainable decisions.


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Inputs: energy, water, materials, land

Raw materials

Production

Distribution and retail Consumer use

End of life Outputs: greenhouse gases, air emissions, effluent, solid waste Result: climate change, resource depletion, air, water and soil pollution Example environmental impact categories is not definitive. ∑ Climate change ∑ Ozone layer depletion ∑ Human toxicity ∑ Freshwater aquatic ecotoxicity ∑ Terrestrial ecotoxicity ∑ Photochemical oxidation ∑ Acidification

often assessed in LCA; this list ∑ ∑ ∑ ∑ ∑ ∑ ∑

Eutrophication Fossil fuel depletion Solid waste Radioactivity Mineral resource depletion Water use or extraction Land use

7.1 Life cycle assessment has two key strengths as an environmental sustainability assessment tool. These are that it can follow a cradleto-grave or even a cradle-to-cradle assessment approach, i.e. it works on a ‘life cycle’ basis; and secondly it can study a system against a broad selection of environmental impacts, which can include headline issues such as climate change (i.e. as commonly recognised by ‘embodied carbon’ or ‘embodied CO2’); but also embodied waste, embodied water, etc. Life cycle thinking and multi-criteria are fundamental to making environmentally sustainable decisions.

7.3

A brief history of life cycle assessment (LCA)

The history of LCA is well documented (Hunt and Franklin, 1996); Boustead, 1996). As a concept, it has its roots in the late 1960s and early 1970s when the idea was first envisioned to quantify the environmental consequences of Coca-Cola beverage packaging options over a life cycle in terms of energy, transport, waste, materials, etc. Despite this, it was not until the 1990s that a recognised methodology for LCA emerged. This was largely delivered through a body of work supported by The Society of Environment Toxicology and Chemistry (SETAC).


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Following these initiatives international standards for LCA started to emerge in the mid-1990s through work organised by the International Organisation for Standardisation (ISO). The subsequent BS EN ISO 14040 Series entitled ‘Environmental management – life cycle assessment’ represents the first complete and internationally accepted standardisation of LCA technique and methodology. Despite these developments and the fact that the concept of LCA is more than 40 years old and hence is relatively mature, its use to model and communicate the environmental impact of different assessed scenarios until recently remained relatively unusual. This is perhaps due to its complexity, and data intensive nature which makes any programme of assessment a time- and capital-intensive exercise. All this is starting to change, and in the last five years LCA has started to move into the mainstream with many construction industry stakeholders taking an interest. Before we move on to consider the Standards framing this context, it is first worth briefly exploring ‘environmental labelling’ and where LCA sits within this.

7.4

Environmental labelling

The ISO 14000 series is the family of standards which have been developed to address environmental management. Their scope is broad and encompasses many different strands, but generally speaking two key branches can be identified. These are standards which are organisation oriented, and those which are product oriented. The organisation orientated standards provide guidance for establishing, maintaining and evaluating environmental management systems (EMS). By contrast, the product oriented standards are primarily concerned with determining the environmental aspects and impacts of products and have been developed for the application of environmental labels and declarations associated with them. The framework for product environmental labelling and declaration is standardised in BS EN ISO 14020. This is a general principles document which further identifies three ‘Types’ of label/declaration/claim. A summary of this framework is provided in Table 7.1. Each declaration ‘Type’ is supported by a separate ISO Standard which has been developed to further harmonise practice. BS EN ISO 14020 and its complementary standards are intended to help an organisation gather the information needed to support planning for, and decision making on, its product/service and to communicate specific environmental information about that product/service to its stakeholders. It is important to recognise that LCA information might form content in any of the three identified label or declaration Types, but that with Type III labelling, an LCA approach must be used. Of the three Types of environmental label, it is Type III and its use of LCA which lends itself most readily to



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Table 7.1 Environmental labelling and declaration according to BS EN ISO 14020 Type I Third Party Environmental labelling BS EN ISO 14024:2001

Label: a voluntary, multi-criteria based third party programme/standard that awards a licence which authorises the use of environmental labels on products indicating overall environmental preference of a product within a particular product category based on life cycle considerations. These are commonly referred to as ‘ecolabels’ (e.g. environmental labels like Blue Angel, Nordic Swan and European Eco-Label)

Type II Self-declared environmental claims BS EN ISO 14021:2001

Claim: a self-declared environmental claim is a statement, symbol or graphic that indicates an environmental aspect of a product (e.g. self-declaration claiming a material or product such as a brick or concrete block to be recyclable, or alternatively to incorporate recycled content)

Type III Third Party verified Environmental Product Declaration (EPD) BS EN ISO 14025:2006

Declaration: a set of quantified environmental data consisting of pre-set parameters (so-called ‘nutritional label’) based on LCA according to the BS EN ISO 14040 series of standards, with at least a minimum set of parameters for the product group (e.g. EPD with a mandatory third party validation: IBU or BRE Environmental Profiles).

considering environmental sustainability of materials. This is because it can take account of life cycle impacts and assessment across multiple issues. The chapter now looks at LCA and the steps for undertaking an assessment.

7.5

Life cycle assessment (LCA) of building materials

LCA can provide a holistic and comprehensive method for assessing environmental performance because it can apply a life cycle-based approach to investigation. It can be used to identify where environmental impacts are arising within a system’s life cycle, and offer a process for examining opportunities for improving performance. In this context, LCA is best described as a form of ‘systems analysis’. In construction, the system could be a material manufacture process; the fabrication of a building; a building element such as an external wall; an entire building or civil asset over its life; or even a neighbourhood consisting of all of the above. This flexibility means that LCA can be applied to specific built assets or service requirements such as waste management practice, transport scenarios or energy provision.


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7.5.1 LCA methodologies and product category rules (PCR) An LCA methodology, or more specifically a PCR, is a record of the specific rules which govern how an LCA should be undertaken for one or more ‘product categories’. Commonly, a PCR is developed for a defined product category which is a ‘group of products that can fulfil equivalent functions’. The PCR will be developed with a specific goal and scope in mind. It should be followed to ensure that the LCA is fair, consistently applied, and that the results can be used comparatively. The PCR will define: ∑ the predetermined LCA environmental impact categories ∑ rules on reporting additional environmental information ∑ calculation process, including requirements for data quality, verification procedure, allocation method, declared and functional units, inventory analysis method, cut off rules, boundary conditions, etc. ∑ general requirements for reporting and communicating the LCA. Importantly, the PCR will also state the three-step process for undertaking an LCA including characterisation, normalisation and weighting procedure.

7.5.2 Characterisation In very simple terms LCA is about modelling system flows. Called ‘inventory flows’, these are the environmental interventions that take place between the study system, and the environment around it. They consist of all the inputs and outputs to the study system and include extraction of raw materials and fuels, requirement for heat and water consumption, and emissions to air, discharges to water and emissions to land. The linkage of these inventory flows to environmental impacts happens through a process that ‘characterises’ inventory flows (also known as inventories or interventions), into predefined categories of environmental impact (e.g. CO2, methane, CFCs and N2O all contribute to global warming potential). An important component of characterisation is the use of characterisation factors which recognise that emissions have different contribution levels to impact categories (Fig. 7.2). In Acid rain

Sulphur dioxide

Carbon dioxide

1

Global warming

21 Methane

Summer smog

7.2 Example of LCA characterisation; methane has 21 times more global warming potential than carbon dioxide. This means a characterisation factor of 21 is applied.



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this step of an LCA it is possible to determine the magnitude of environmental impacts attributed to a system.

7.5.3 Normalisation Normalisation is the calculation of the magnitude of the impact category characterised data as set against a relative reference value. Normalisation can be used to bring a degree of relativity to LCA findings. Typically this is done by dividing the characterised impact figure values by comparable impact category figures, but determined for a reference geographic area per head of population over a defined time period. This figure could be used to normalise characterised impact data for global warming potential (climate change). An outcome of normalisation is better understanding of the relative magnitude of each impact result of the studied system, but also preparation of the LCA findings for an additional procedure of weighting.

7.5.4 Weighting Weighting, also known as valuation, is a process of assigning a degree of significance to the normalised impact category data. This is done by multiplying the normalised data by numerical factors (i.e. from a total of 100%). These factors are typically based on value choices, i.e. an opinion on which environmental impact category is more important than another. Weighted impact category results can be left disaggregated or added together to a single ‘score’ of environmental performance. It is important to recognise that weighting steps are based on value choices which may not be scientifically based. BRE conducted a weightings exercise for its LCA environmental profile methodology in 2007 (Aizlewood et al., 2007; Hamilton et al., 2007). The weightings derived from this are shown in Table 7.2.

7.5.5 LCA data/life cycle inventory (LCI) information The input/output data defined in LCA models is called life cycle inventory (LCI) data. There are many commercially available LCI datasets which can be accessed for LCA modelling. Key databases include: ∑ Ecoinvent: produced for the Swiss Government  www.ecoinvent.ch ∑ Boustead: produced by a UK-based LCA consultancy  www.boustead-consulting.co.uk ∑ GaBi: produced by PE Consulting  www.gabi-software.com


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Materials for energy efficiency and thermal comfort in buildings Table 7.2 Weighting factors for the 2008 Environmental Profiles Methodology

∑ ∑ ∑ ∑ ∑

Environmental Issue

Weighting (%)

Climate change Water extraction Mineral resource extraction Stratospheric ozone depletion Human toxicity Ecotoxicity to water Nuclear waste Ecotoxicity to land Waste disposal Fossil fuel depletion Eutrophication Photochemical ozone creation Acidification

21.6 11.7 9.8 9.1 8.6 8.6 8.2 8.0 7.7 3.3 3.0 0.20 0.05

IDEMAT: produced by TU Delft www.idemat.nl IVAM: produced by the University of Amsterdam  www.ivam.nl Franklin: produced by Franklin Associates in the USA  www.fal.com ETH-ESU: produced by ESU services in Switzerland  www.esu-services.ch BUWAL: produced by the Swiss Government  www.bafu.admin.ch 

In addition there are numerous sector specific, and regionally specific, initiatives which have developed to gather LCI or assist with the harmonisation of LCA. From an EU perspective, the Platform project on LCA delivered through the European Commission Joint Research Centre: Institute for Environment and Sustainability (http://lca.jrc.ec.europa.eu/lcainfohub/directory.vm) is a significant initiative. It has the twin objectives of developing an international reference life cycle database as well as an international resources directory and discussion forum for LCA. It also provides links to more widely available LCA data and tools: ∑ ∑

Data: http://lca.jrc.ec.europa.eu/lcainfohub/databaseList.vm Tools: http://lca.jrc.ec.europa.eu/lcainfohub/toolList.vm

7.6

Life cycle assessment (LCA) standardisation

Undertaking environmental life cycle modelling using LCA is a complex process which provides the practitioner with great flexibility to define and



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set specific methodological rules. The disadvantage with this flexibility is the potential to create LCA models which are not comparable; or in worst case scenarios, develop LCA which are inherently biased, but which appear at face value entirely sound. To help harmonise LCA application and set common rules, the International Standards Organisation has developed a series of standards for the use of LCA. In addition, the European Committee for Standardisation (CEN) is coordinating the development of a construction sector set of LCA standards. A brief summary to the ISO and CEN work is now provided.

7.6.1 International Standards Organisation (ISO) ISO has been active in the development of LCA standards since the mid1990s. There now exist a range of standards for the generic application of LCA, through to its specific use in the construction sector. ISO 14040 series: Environmental management – life cycle assessment The ISO 14040 series were originally published in 1997 to harmonise the application of LCA. There are currently two1 standard documents2: ∑ ∑

BS EN ISO assessment – BS EN ISO assessment –

14040:2006, Environmental management – life cycle principles and framework. 14044:2006, Environmental management – life cycle requirements and guidelines.

The difference between the two documents is that ISO 14040 provides a general introduction to the principles of LCA and LCI; by contrast, ISO 14044 sets out specific requirements. Both documents adopt a similar structure organising content under a consistent set of key issues including: ∑ the goal and scope definition of the LCA ∑ the life cycle inventory analysis phase ∑ the life cycle impact assessment (LCIA) phase ∑ the life cycle interpretation phase ∑ reporting and critical review of the LCA ∑ limitations of the LCA 1 There are actually three documents but ‘DD ISO/TS 14048:2002 Environmental management – life cycle assessment – data documentation format’ is only a draft for development (DD) technical specification (TS) which outlines formatting requirements for LCA data documentation. 2 ISO 14040:2006 and ISO 14044:2006 supersede EN ISO 14040:1997, EN ISO 14041:1998, EN ISO 14042:2000, EN ISO 14043:2000.


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∑ ∑


relationship between the LCA phases conditions for use of value choices and optional elements

ISO 14025: Type III environmental declarations and programmes As the use of LCA has evolved, and its reporting has become more widespread, specific labelling programmes which model production systems and label products have been established. These programmes are known as Type III Environmental Product Declaration (EPD) labelling schemes. The requirements for their establishment, management, delivery and reporting are set out in BS EN ISO 14025 (2006). LCA product data can be a useful source of environmental information to the building designer and Type III labelling schemes are a potential source for this information. ISO 14025 supports this through providing a set of ‘principles and procedures’ for establishing such labelling programmes and how their operators should use the LCA ISO 14040 series. In this regard, ISO 14025 sets out guidance for developing the PCR, programme operator responsibilities, ensuring comparability, verification procedures, transparency, reporting requirements; to name a few issues. ISO and LCA in construction Although LCA is now widely used across many industrialised systems, very few are supported by standards. The construction industry is an exception as it benefits from a suite of international standards dealing with sustainability in building and construction that include: ∑ ∑

ISO 15392 Sustainability in building construction – general principles ISO 21932 Buildings and constructed assets – sustainability in building construction – terminology ∑ ISO/TS 21929-1 Sustainability in building construction – sustainability indicators – Part 1: Framework for development of indicators for buildings ∑ ISO/TS 21931-1 Sustainability in building construction – framework for methods of assessment for environmental performance of construction works – Part 1: Buildings. Figure 7.3 shows the relationship of these different documents and how they relate to ISO 21930, a standard that sets out the principles for using LCA for the environmental declaration of building products. This means it provides a useful additional focus from the ISO 14040 series, by addressing specific issues of relevance to construction such as the service life of the building products as seen over a building’s life cycle. ISO 21930 is designed


Life cycle assessment and environmental profiling Environmental aspects

Economic aspects

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Social aspects

ISO/FDIS 15392 : Sustainability in building construction – General principles ISO/TR 21932 : Terminology Methodical basics ISO/TS 21929-1 : Sustainability Indicators – Part 1 : Framework for development of indicators for buildings

Buildings

ISO/TS 21931-1 : Framework for methods of assessment of environmental performance of construction works – Part 1: Buildings

Building products

ISO 21930 : Environmental declaration of building products

7.3 Suite of related international standards for sustainability in building construction and construction works (BS EN ISO 21930, 2007). Permission to reproduce extracts from BS EN ISO 21930 (2007) is granted by BSI. British Standards can be obtained in PDF or hard copy formats from the BSI online shop: www.bsigroup.com/Shop or by contacting BSI Customer Services for hard copies only: Tel: +44 (0)20 8996 9001, Email: [email protected].

to be used in combination with ISO 14025 as the basis for establishing Type III environmental declaration programmes. Inherent in LCA modelling is the consideration of the study system ‘life cycle’. For buildings and building components this is particularly challenging because they can exhibit long service lives, sometimes extending over tens or hundreds of years. To bring consistency to this complex area, LCA practitioners are increasingly using the ISO 15686 series of standards on ‘service life planning of constructed assets’. The relationship of these standards and ISO LCA frameworks is shown in Fig. 7.4. In this context perhaps the most important standard for LCA is BS ISO 15686-8:2008 Buildings and constructed assets – service-life planning Part 8: Reference service life and service-life estimation. This standard provides guidance on how to select or determine a construction component ‘reference service life’ (RSL) and how to use this for the purposes of calculating an ‘estimated service life’ (ESL). As a defined time period, ESL can be used to represent the service life period of a construction component and when it will reach its end-of-life in an LCA model.


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Reference service life and service-life estimation (ISO 15686-8)

Service life prediction procedures (ISO 15686-2)

General principles (ISO 15686-1)

Process modelling and stochastical data

Performance audlts and reviews (ISO 15686-3)

Performance data (ISO 15686-4)

Service life planning of buildings and constructed assets (ISO 15686)

Life cycle assessment (ISO 14040)

Planned and reactive maintenance

Life cycle costs (LCC) (ISO 15686-5)

Facilities management

Demolltion and reuse

7.4 Inputs and influences on service life planning of buildings (adapted from BS EN ISO 15686-1, 2000). Permission to reproduce extracts from BS EN ISO 15686-1 (2000) is granted by BSI. British Standards can be obtained in PDF or hard copy formats from the BSI online shop: www.bsigroup.com/Shop or by contacting BSI Customer Services for hard copies only: Tel: +44 (0)20 8996 9001, Email: [email protected].

ISO and comparability A key observation of the ISO work is that despite providing a useful framework for modelling, the standards still provide significant room for interpretation by the user (they are designed in this way due to the international and multidisciplinary context in which they will be applied). This means that although studies conducted within their standardised framework should follow a logical path and offer a degree of consistency, they still offer significant opportunity: ∑ ∑

to adopt entirely valid, but differing approaches to introduce bias through manipulation of the LCA methodology towards a particular outcome ∑ for Type III labelling schemes to apply LCA in different ways.

In conclusion, LCA conducted by different practitioners within the ISO



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standards are unlikely to be directly comparable (and this includes construction focused standards).

7.6.2 European committee for standardisation (CEN) Many ‘built environment/construction works’ EPD schemes now exist in Europe following the ISO standards. Most of these incorporate common elements but due to the flexibility of the ISO standards, the PCR they follow and the EPD that they produce are different from country to country. To avoid barriers to trade of goods and services, European harmonisation is necessary. This agenda became the subject of a CEN Mandate, under instruction from the EU Enterprise Directorate. The Mandate was written to develop a harmonised approach for ‘sustainability of construction works’ of which LCA would be an important part. The mandate was accepted in late 2004 and work on the standards began in 2005. In response to the mandate, Technical Committee TC350 was established by CEN to develop ‘horizontal standardised methods for the assessment of the integrated environmental performance of buildings’. Although the original mandate extended only to ‘environmental performance of buildings’, the work has subsequently been broadened to consider socio-economic aspects as well. TC350 consists of five Working Groups under the leadership of a single Task Group; the work programme is split horizontally into framework, building and product levels; and vertically into environmental, social and economic dimensions. The initiative will deliver European Standards (EN), Technical Reports (TR) and Technical Specifications (TS). Through TC350 the goal of the Commission is to provide a method for the voluntary delivery of environmental information that supports the construction of sustainable works including new and existing buildings.

7.7

UK context

LCA is increasingly being used in the UK construction sector. Material suppliers use it to assess the environmental impact of their supply chains and production systems; and building designers and constructors are applying it to understand the impact of their constructed assets. Clients are increasingly becoming familiar with the data LCA can present and are considering it in their procurement decisions. Building ratings systems including BREEAM and the Code for Sustainable Homes also indirectly apply LCA through their use of the Green Guide to Specification as a compliance tool. Even CEEQUAL the civil engineering assessment methodology provides reward for the application of LCA to measure embodied impacts of civil works. There is no British Standard or


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commonly ‘regulated’ approach. The work most widely applied has been that conducted by BRE.

7.7.1 UK BRE environmental profiles methodology The Environmental Profiles methodology is an LCA approach, i.e. a PCR developed for applying LCA to built environment scenarios. The methodology is designed and scoped to provide a ‘level playing field’ approach for all construction materials, i.e. an approach which is not biased to any particular type of material. The Environmental Profiles methodology allows direct comparison of the environmental impacts of functionally equivalent products, building specifications and buildings or even infrastructure. BRE developed the first edition of the Environmental Profiles methodology and its associated database in 1999 (Howard et al., 1999). A research programme through 2006–8 has culminated in its update, and the publication of a second edition of the Environmental Profiles methodology. The methodology provides the UK with an approach to the environmental life cycle assessment of all types of construction materials. It was developed with the UK material supply sector through close coordination with the Construction Products Association and its members. The Environmental Profiles methodology reports using the 13 environmental impact categories summarised in Table 7.2. It also applies a weightings step which enables a single score of environmental impact to be derived called the Ecopoint. The methodology has enabled BRE to develop three key LCAbased initiatives. These are now summarised.

7.7.2 Green guide to specification The Green Guide to Specification is now in its fourth edition. It is a specification aiding tool aimed at providing the building professional with easy-to-use guidance on how to make the best environmental choice when selecting construction materials and components. The fourth edition is available online and provides environmental ratings for more than 1200 specifications across various types of building. It is underpinned by the Environmental Profiles methodology and a LCI data collection exercise across a broad range of UK material suppliers. The environmental impacts for discrete specifications are presented as Ecopoint scores which have been converted into A+, A, B, C, D and E ratings. The online tool is expected to grow in functionality as BRE continue to develop it.

7.7.3 Envest This is a BRE commercial building software tool that combines LCA and whole life costing (WLC). It allows both environmental and financial © Woodhead Publishing Limited, 2010


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tradeoffs to be considered in the design process. The user assigns their building design parameters (height, number of storeys, glazing area, etc.) and material specification choices (external wall, roof covering, etc.) into the tool. Envest then assesses the environmental impact (with LCA) and cost (with WLC) of the design. The tool reports on environmental impact using the Environmental Profile impact categories and Ecopoints, and costs in pounds sterling.

7.7.4 The environmental profiles certification scheme This is a Type III EPD certification scheme managed by BRE Global (the certification company of the BRE group). The scheme uses the Environmental Profiles methodology to produce EPD of manufacturer specific construction products and publishes Type III EDP declarations on behalf of the manufacturer. Each EPD is based on verified LCI for the manufactured product which has been audited by a certification officer through visiting the manufacture facility and reviewing production documentation. EPD are then listed in the GreenBookLive and also used to generate Green Guide to Specification ratings.

7.8

Other issues

The sustainability of a material or product is more than how it performs in a life cycle assessment. This section looks at three additional aspects and how they contribute to considering the sustainability of a material.

7.8.1 Healthy and safe materials Materials and their constituent components can contribute to a range of human health problems including carcinogenicity, endocrine disruption, and skin and mucous membrane irritation and sensitisation (to name but a few). From a safety perspective, material specification can have implications towards personal injury, accident, trauma or loss for stakeholders. Both health and safety aspects need to be considered throughout the building life cycle including extraction of basic material ingredients, the manufacture of these into a product, installation/construction, the maintenance requirement it demands in service and the implications for deconstruction and disposal at end-of-life. Therefore different materials and the way a design uses them will exhibit different health or safety levels. Generally government legislation exists to govern material impacts across these. However, this topic is important because many materials originate in places where human health and safety are not well regulated, or where the legislative frameworks might be considered


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too tolerant. Beyond legislation, various frameworks for considering these issues exist and are formalising.

7.8.2 Resource management Resource management recognises that within a finite global system, there are limits to be respected by human activities. These will vary significantly from material to material, but resource management indicators can be used to determine performance including: ∑ ∑ ∑ ∑ ∑ ∑

abiotic depletion: depletion of non-renewable material resources renewability (or even rapidly renewable) recycled content recyclability efficient design and dematerialisation waste management practice.

Different strategies exist for driving good resource management and efficiency. The most common is that presented within the waste hierarchy of prevention, minimisation, reuse, recycling, energy recovery and disposal. LCA has the capacity to include a range of resource management based indicators and generally the issue should be considered in life cycle terms. On a practical level in the UK construction Site Waste Management Plans offer a mechanism to reduce waste arising on site and dispose of it efficiently when it arises.

7.8.3 Responsible sourcing Responsible sourcing is an approach of supply chain management, responsible manufacture and product stewardship, and encompasses social, economic and environmental dimensions. The concept is broad and many different variations of the concept exist. Within the construction products sector, the aspect is most mature within the timber industry with sustainable timber supply schemes like the Forest Stewardship Council (FSC) and the Programme for the Endorsement of Forest Certification schemes (PEFC) leading the way. The reality, however, is that responsible sourcing should be an objective of all material supply sectors. A company demonstrating good responsible sourcing practices is likely to be contributing to all the materials sustainability issues outlined in this chapter including addressing embodied impact, working to good heath and safety standards and ensuring good resource management practice. Sector schemes to drive corporate social responsibility of manufacturing practice, exercise good supply chain management, and product stewardship,



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are starting to evolve. These commonly require that manufacturers address the following objectives: ∑ quality ∑ health and safety ∑ supply chain management ∑ environmental management ∑ specific environmental and social impacts of their operations.

7.9

References

Aizlewood C, Edwards S, Hamilton L, Shiers D, Steele K (2007), Environmental weightings: their use in the environmental assessment of construction products. Information Paper IP 4/07, Bracknell, IHS BRE Press. Boustead I (1996), LCA – how it came about. The beginning in the UK. International Journal of LCA, 1(3), 147–150. BS EN ISO 14020 (2002), Environmental labels and declarations – general principles. Geneva, International Standards Organisation. BS EN ISO 14021 (2001), Environmental labels and declarations – self-declared environmental claims (Type II environmental labelling). Geneva, International Standards Organisation. BS EN ISO 14024 (2001), Environmental labels and declarations – Type I environmental labelling – principles and procedures. Geneva, International Standards Organisation. BS EN ISO 14025 (2006), Environmental labels and declarations – Type III environmental declarations – principles and procedures. Geneva, International Standards Organisation. BS EN ISO 14040 (2006), Environmental management – life cycle assessment – principles and framework. Geneva, International Standards Organisation. BS EN ISO 14044 (2006), Environmental management – life cycle assessment – requirements and guidelines. Geneva, International Standards Organisation. BS EN ISO 15686-1 (2000), Buildings and constructed assets – service life planning – Part 1: General principles. Geneva, International Standards Organisation. BS EN ISO 21930 (2007), Sustainability in building construction – environmental declaration of building products. Geneva, International Standards Organisation. BS ISO 15392 (2008), Sustainability in building construction – general principles. Geneva, International Standards Organisation. BS ISO 15686-8 (2008), Buildings and constructed assets – service-life planning Part 8: Reference service life and service-life estimation. Geneva, International Standards Organisation. BS ISO/TS 21931-1, Sustainability in building construction – framework for methods of assessment for environmental performance of construction works – Part 1: Buildings. Geneva, International Standards Organisation. BS ISO 21932, Buildings and constructed assets – sustainability in building construction – terminology. Geneva, International Standards Organisation. Hamilton L, Edwards S, Aizlewood C, Shiers D, Thistlethwaite P, Steele K (2007), Creating environmental weightings for construction products: Results of a study, BR 493. Bracknell, IHS BRE Press.


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Howard N, Edwards S, Anderson J (1999), BRE Methodology for Environmental Profiles of construction materials, components and buildings, BR370. Bracknell, IHS BRE Press. Hunt R G, Franklin W E (1996), LCA – how it came about – personal reflections on the origin and development of LCA in the USA. International Journal of LCA, 1(1), 1–4. ISO/TS 21929-1, Sustainability in building construction – sustainability indicators – Part 1: Framework for development of indicators for buildings. Geneva, International Standards Organisation. Parkin S (2000), Sustainable development: the concept and practical challenge. Proceedings of ICE, Civil Engineering, 138, November, 3–8.


8

Inorganic mineral materials for insulation in buildings R. G e l l e r t, FIW München, Germany

Abstract: Insulating products made from natural and/or synthetic mineral raw materials dominate the insulation market in Europe. Owing to their overall cost/performance ratio or their special product properties, mineral wool products for building insulation have gained the largest market share. Products such as mineral wool, cellular glass and expanded perlite are specified by harmonized European standards, while the niche-type products have either a manufacturer-specific European or national technical approval to prove to the consumer the fulfilment of regulatory requirements for the particular application. Key words: mineral wool, cellular glass, foamed glass, calcium silicate foam, ceramic fibres, aerogel, vacuum insulation, expanded perlite, exfoliated vermiculite, expanded clay.

8.1

Introduction

Thermal insulation is used to keep buildings cooler in summer and warmer in winter by reducing the flow of heat through the exterior surfaces of the building. The choice of insulation products will be guided by the application for which it is to be used, and the amount of insulation required will depend on the climate of the location, latitude and altitude at which the building is constructed. Buildings are responsible for 40% of Europe’s energy use and the largest share of energy in buildings is heating [1]. It is thought that up to 50% of buildings in Europe are uninsulated; however, thermal insulation can reduce the heat lost from buildings and therefore save energy and money. European legislation is driving increased installation of insulation through harmonization of building standards up to the levels of the most ambitious Member States. The European Energy Performance of Buildings Directive (EPBD) [1] was adopted in 2002 and is concerned with promoting energy efficiency in buildings across Europe using cost effective measures, whilst at the same time harmonizing standards across Europe to those of the more ambitious Member States. The EPB Directive centres around four key strands: This chapter is dedicated to Dr Walter F. Cammerer on the occasion of his 90th birthday.

193 © Woodhead Publishing Limited, 2010

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∑

providing a methodology for calculating the energy performance of buildings, taking into account local climatic conditions; ∑ applying energy performance standards to both new build and existing building stock; ∑ providing a certification scheme for all buildings; ∑ regular assessments of any heating and cooling equipment installed.

These components had to be ratified by Member States by 4 January 2006, but most opted to defer this for three years. Unimpressed by the progress of Member States, the EU launched a public consultation on the recasting of the EPBD in 2008. This recast seeks to, amongst other things, broaden the scope of the EPBD by scrapping the 1000 m2 useful floor space threshold for buildings requiring to meet minimum energy efficiency standards; requiring Member States to apply ‘effective, proportionate and dissuasive’ penalties for non-compliance; and stipulating that Member States must draw up national roadmaps by 2011 for the development of low and zero carbon buildings. This legislation has a significant impact on the levels of insulation required in buildings and is likely to increase demand for insulation products in the future. As an example, in Germany an ‘Energy Savings Ordinance’ (‘EnEV’) is in effect. Amendments to the EnEV were enacted in the summer of 2008 and gave notice of the following major changes affecting building insulation (reference draft to EnEV 2009): ∑ tightening of energy requirements on building by an average of 30% ∑ extension of individual reconditioning obligations for equipment and buildings. Table 8.1 compares the highest values for the thermal transmission coefficient (Umax) stated in EnEV 2007 and the draft from 2009 for building envelopes on changes to building condition. The previously applicable version of EnEV specified the limit values to be observed for primary energy consumption of the building as a whole (envelope plus installations) and the specific transmission heat loss for the building (structural elements). Since the U-value (thermal transmission coefficient in W/(m2•K)) is included in the calculation of heat loss, it becomes Table 8.1 Highest thermal transmission coefficient values in accordance with the German EnEV for renovation of individual building components in existing buildings Building structural elements

EnEV 2007 EnEV 2009 (Umax W/(m2• K))

Tightening (%)

External walls (external insulation) External windows, French windows Slabs, roofs and inclined roofs Cellar slabs (cold-sided cladding)

0.35 1.70 0.30 0.40

31 24 20 25

0.24 1.30 0.24 0.30



195

an important parameter for (un)insulated structural elements. The U-value is a measure for the transmission heat flow through a single or multi-layered material layer which separates two temperature areas (here: ‘external’ and ‘internal’). The inverse value of the thermal transmission coefficient is the thermal resistance, R (in (m2•K)/W). Both variables are important key figures for the characterisation of insulation effectiveness. The higher the R-value (in other words, the smaller the U-value), the better the material heat insulation properties. Insulation manufacturers will have to regulate their products to meet the (basic) requirements of the EnEV for thermal insulation in buildings. Thermal insulation materials are subject to heat transmission theories, as are all other building materials. Better heat insulation properties of insulation materials in comparison with other building materials result primarily from the trapped still air (or gases) inside the insulation material. The material of the insulator itself is mostly totally unsuitable or insufficient for thermal insulation on its own. For this reason the main aim in the development of heat insulation materials is to enclose a sufficient quantity of still air or other gases in small pores which are well distributed across the heat insulation material, in addition to using a base material which has as good heat insulation properties as possible. The heat transmission properties or thermal conductivity of insulation material is mainly affected by the following four variables: ∑

heat transmission of the insulation material framework (cellular framework or fibrous proportion) ∑ heat transmission of cell gases or gases in the interstices ∑ heat radiation in gas-filled interstices ∑ heat convection due to movement of gas particles. Good thermal insulation properties are achieved if an insulation material prevents heat transmission due to these four variables as much as possible. Insulation materials are categorized based on their internal structure in order to differentiate the influence of these variables on the insulation material. Since other physical boundary conditions are present in closed-cell insulation materials than in fibrous insulation materials or filled materials, one has to take this into account when judging the performance based on structural parameters of an insulating material. The thermal conductivity of the insulation material is almost independent both of the internal structure of the insulation material and of the material of the selected solid if the pores are small and well distributed. An increasing solid material proportion, accompanied by an increasing bulk density, results in an increase of the material thermal conductivity; a lower solid material proportion reduces the proportion of thermal conductivity attributable to the material framework for the overall thermal conductivity but does, however,


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increase the thermal transmission due to radiation due to the increasing proportion of pores [2]. the ‘resulting’ material characteristic value for insulation materials is the thermal conductivity l (in W/m•K). this is linked to the thermal resistance R by the layer thickness d: R = d (in (m m 2 •K K)/W)) l Insulating materials can be separated into organic and inorganic groups based on their raw materials. Furthermore, each group is separated into natural and synthetic materials. Depending on the structure, the results are fibrous materials, foams or granulates. The organic natural raw materials group offers the widest range of products. the european market is split into these two main product categories according to the chemical and physical structure of the insulation materials: inorganic mineral materials and organic materials (see table 8.2). For instance, in 1999 building thermal insulation accounted for 65% of the total UK thermal insulation market, compared with 62% in 1995 and this was expected to increase by 8% between 2000 and 2004. the largest share of the market (52%) was represented by wall insulation products, 41% of which consisted of cavity wall insulation. loft insulation had the second largest share, accounting for 25% of the market in 1999. According to a recent study, inorganic fibrous materials will experience a 5% growth in the next decade. It should be noted that these figures are for the UK only and therefore cannot be taken as representative of other eU countries, but indicate their magnitude. the situation in Germany is illustrated in Table 8.3. Table 8.2 Mineral materials employed for the manufacture of insulating materials and products for building applications Inorganic insulating materials Synthetic

Natural

Mineral wool: ∑ glass wool ∑ rock wool Cellular glass Foamed glass Calcium silicate foam Ceramic fibres Aerogel Pyrogenic silicic acid/ (Vacuum insulation panels) Slag wool Gypsum foam

Expanded perlite Exfoliated vermiculite/expanded mica Expanded clay Pumice Insulating clay bricks



197

Table 8.3 Market share of insulating materials and products in Germany [3] Product/Year Market share (%) ———————————————————— MF Organic insulating Others materials

Total Market (Mio. m3)

1989 1994 1999 2000 2003 2004 2005

16 28 34 34 27 27 24

59 60 58 58 58 55 56

40 39 36 37 38 39 40

MF = Organic insulating materials = Others =

8.2

1 1 6 5 4 6 4

mineral fibre products expanded polystyrene (EPS), extruded polystyrene foam (XPS), polyurethane foam (PUR/PIR) including products from natural organic and inorganic raw materials

Regulatory requirements

Complementing the EPBD, the European Construction Products Directive (CPD) is aimed at creating a single market for construction products through the use of ‘CE marking’. It outlines key requirements relating to materials intended for construction, which is defined in the Directive as products that are manufactured to form a permanent part of structures. The materials must meet fundamental requirements including mechanical resistance and stability, safety in case of fire, hygiene, health and the evironment, safety in use, protection against noise and energy economy and heat retention. The Directive mandates that standardization organizations such as CEN develop standards in consultation with industry. A list of these standards can be found on the European Commission’s website. Where harmonized standards are not available, existing national standards may still be used. Since insulation materials are usually regulated building products, their properties (specifications) are described in technical codes of practice (standards, building authority certification). Now that the building products law based on the European building product guidelines has been passed, harmonized European standards have been produced, or are in the process of being drawn up, for standard market insulation materials. Their transfer into the national regulations means that they are now given a national EN designation. A different procedure applies to the European technical approvals (ETAs) for insulation materials which have not yet been standardized, but ETAs also have to be accepted nationally. The harmonized product standards (hEN) are supported by cross-sectional


198


standards such as testing methods, calculations or safeguarding of standard observance. The harmonized specification standards enable insulation material manufacturers to provide a mostly open description of their products using standard features; these are mostly expressed in classes, levels or nominal values (e.g. for thermal conductivity). If the manufacturer declares conformity with the appropriate standard he can give his products the CE conformity marking and place it on the market. An additional voluntary product certification is based on codes of practice which determine the process by which the conformity verification has been produced in the context of the certification programme. This enables manufacturers to receive a protected marking in accordance with the rules of a certification programme by specific authorized organizations (e.g. quality protection organizations, associations or testing laboratories). Voluntary product certification has a long tradition in many Member States. For insulation materials, such markings are often issued in combination with the Institute supervision previously required by the building authorities and an agreement verification process. The following information about an insulation material is provided on the label or in the manufacturer’s declaration – the first five being compulsory, others depending on the individual product specification standard: ∑ ∑ ∑ ∑ ∑ ∑

thermal conductivity/thermal transmission resistance dimensions and tolerances mechanical characteristics behaviour in water (vapour) fire behaviour (Euro class) soundproofing characteristics, e.g. dynamic stiffness or length-related air flow restistance

Table 8.4 lists the existing standards, preliminary standards or approvals for the insulating materials and products described in this chapter. As the standards for the factory made products were written according to a ‘model standard’ and published as a ‘package’, the three standards listed in Table 8.4 all have an identical structure. In the ‘heart of the standard’ one will find the requirements, the test methods, the designation code, the rules for the attestation of conformity and the rules for marking and labelling. For insulation products which are produced on-site, the relevant standards – still in the preliminary stage – are written in two parts: ∑ ∑

one part addressing the (basic) raw material the second part describing the ‘conversion’ from a loose-fill material into a thermal performance product on site.

Based on properties to be declared for all applications and also – if declared


Table 8.4 European technical specification standardsa and approvalsb for insulating materials based on mineral materials [4], [5], [6] Abbreviation

Technical specification Factory made

In-situ application

Building equipment


1 Mineral wool MW EN 13162: 2008 pr EN 14064-1 pr EN 14303 EN 13500: 2003c pr EN 14064-2 2 Cellular glass CG EN 13167: 2008 pr EN 14305 3 Foamed glass National technical approval 4 Calcium silicate foam CS ETA pr EN 14306 (and similar mineral foams) (ETAs) 5 Ceramic fibres, foams Industrial application mainly 6 Aerogel National technical approval 7 Vacuum insulation panel VIPd National technical approval 8 Expanded perlite EP EN 13169: 2008 EN 14316-1: 2004 pr EN 15501 (EPB = boards) EN 14316-2: 2007 9 Exfoliated vermiculite EV EN 14317-1: 2004 pr EN 15501 EN 14317-2: 2007 10 Expanded clay EN 14063-1: 2004 pr EN 14063-2 a

Published standards and standards under development are listed by CEN (www.cen.en/CENORM); EOTA provides details on technical approvals (www.eota.be); c EN 13500 specifies MW-products for the external thermal insulation composite system application; d VIP with pyrogenic silicic acid as core material. b


No. Product

199

200


by the manufacturer – for a specific application one can summarize important standardized performance criteria as in Table 8.5. Table 8.5 Standardized properties, relevant test methods and designations Property

Test method

Symbol

Thermal resistance EN 12667 or RD EN 12939 lD Length and width EN 822 l and b Thickness EN 823 d Squareness EN 824 Flatness EN 825 Dimensional stability EN 1603 De under normal laboratory conditions Bending strength EN 12089 sB Reaction to fire EN 13501-1 Apparent density EN 1602 ra Dimensional stability EN 1604 De under specified temperature Dimensional stability EN 1604 De under specified temperature and humidity conditions Deformation under EN 1605 e compressive load Compressive stress at EN 826 s10 10% deformation Tensile strength EN 1607 smt perpendicular to the faces Tensile strength EN 1608 st parallel to faces Point load EN 12430 Compressive creep EN 1606 ect Shear strength EN 12090 t Water absorption, EN 1609 short term Water absorption, EN 12087 Wlp long term Water vapour EN 12086 m transmission/resistance factor Dynamic stiffness EN 29052-1 s’ Thickness EN 12431 dL Thickness dB Compressibility c Sound absorption EN ISO ap 354:1993/A1 aw Air flow resistance EN 29053 Organic content EN 13820

Dimension 2

m •K/W W/(m•K) mm mm mm/m mm %

Designation R, l L and W T S P or Smax DS (N)

kPa BS (Euroclasses) A, B, C, D, E, F kg/m3 % DS(T)

%

DS(T, H)

%

DLT

kPa

CS(10)

kPa

TR

kPa N % kPa kg/m2

PL(P) CC( )sc

kg/m2

WL(P)

m2 • h• Pa/mg

Z or MU

MN/m3 mm mm —

SD

kPa•s/m3 %

AF Moc


WS

CP AP/AW


8.3

201

Building-related properties

Since differentiation must be made between European and national applications under the harmonized codes of practices in the philosophy anchored in the building products guidelines, the client, planner or architect must take note of the requirements contained in the (national) list of technical building regulations for thermal insulation (sound and fire protection as well, if necessary). In this case the technical regulations of interest for thermal insulation are contained (in Germany) in DIN V 4108: (application-based requirements on insulation materials). ∑ ∑

Part 4:2004-07 Thermal insulation and energy economy in buildings – Part 4: Hygrothermal design values. Part 10:2004-09 Thermal insulation and enrgy economy in buildings – application-related requirements for thermal insulation materials – Part 10: Factory made products.

Examples of the most common applications of thermal insulation products in buildings, excluding building equipment are given in Table 8.6. Because of its importance as the most important building-related property, in the European system for handling the thermal conductivity lD (or the thermal resistance RD respectively) of insulation products, one has to follow an elaborate (statistical) scheme, which includes the following details: ∑ The declared thermal conductivity lD is determined on the basis of statistically evaluated measurements (l90/90) – to be organized by the manufacturer. Details are given in the respective product standard (in the normative Annex A); lD is declared in steps of 0.001 W/(m•K). ∑ Design values for the particular building application are defined nationally – due to different building codes and/or the climate in question. An overview of the basic building-related properties is given in Table 8.7, some specific properties of interest here in Table 8.8. Besides the ‘core properties’ which determine the performance of an insulating material, some products have good acoustic or specific fire performance characteristics that the manufacturer wishes to declare – or has to declare for a certain application – already when placed on the market, i.e. the reaction to fire classification. The latter has now been based on the European (classification) standard EN 13501-1 and the following Euroclasses with respect to the combustibility of building materials (reaction to fire) have been agreed upon: ∑ A1 + A2 No contribution to a fire/non-combustible ∑ B Very limited contribution to a fire ∑ heat propagation ∑ flame propagation


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Table 8.6 Examples of the most common applications of thermal insulation products in buildings – excluding building equipment [7] Application Roof Pitched Unloaded insulation between rafters, fully supported roof Insulation separating rafters and outer covering Insulation separating supporting construction and outer covering Insulation beneath rafters Flat roof

Insulation between rafters or beams Inverted, insulation above roofing membrane including roof gardens and parking decks On steel deck, insulation beneath roofing membrane Accessible to light or heavy traffic or loads from roof garden (soil layer, plants, etc.) and parking decks (concrete pavers or slabs), insulation beneath roofing membrane Accessible only to maintenance personal, insulation beneath roofing membrane

Wall

Masonry or concrete wall, external insulation covered by rendering Timber stud construction, outside insulation and rendering directly supported by the studs Timber stud construction, insulation at the internal side with rendering Masonry or concrete wall, fully supported internal insulation supporting light protective internal facing (e.g. gypsum board) Masonry or concrete wall, internal insulation supporting light protecting facing, partly supported by studs Masonry or concrete wall, internal insulation with heavy selfsupported protective internal facing (e.g. tiles at roomside) Timber or metal stud construction with boards covering, insulation between the studs Cavity wall construction, insulation between the leaves, cavity ventilated Cavity wall construction, cavity fully filled with insulation, outer leaf not watertight Timber or metal stud construction with boards covering, insulation supported by boards: or masonry or concrete wall, supporting the insulation with ventilated exterior covering Wall under ground, external insulation behind waterproof membrane with mechanical protection Wall under ground, external insulation with direct contact to the ground Cellar or crawlspace hall, internal insulation with or without covering

Ceiling/floor Insulation over the supporting construction or between the beams Insulation under load distributing flooring, fully supported Insulation under the construction Foundation

Concrete, insulation under the slab with direct contact to the ground Concrete, insulation supported by the slab, above waterproof membrane, beneath load distributing flooring Concrete, insulation under the slab above waterproof membrane Frost insulation in or against the ground



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Table 8.7 Typical building application related properties (where given) [8–10] Material

Property

Thermal Density conductivity l D1 p (mW/(m • K)) (kg/m3)

Specific heat capacity c (J/(kg • K))

Mineral wool 32–50 10–400 600–1000 Cellular glass 38–60 100–220 800–1100 Foamed glass 70–92 150–300 800–1000 Calcium silicate foam 45–65 115–390 1000 Ceramic fibres, -foam 30–70 100–240 1040 Aerogel 15–21 30–100 700–1150 150–300 n.a. VIP (SiO2 as core material) 4–8 (Pyrogenic silicic acid) (18–21) (300) (1000) Expanded perlite (board) 40–70 90–400 1000 Exfoliated vermiculite 60–70 60–180 800–1000 Expanded clay 85–160 260–500 1000 Insulating clay bricks 70–140 500–750 1000

Water vapour diffusion resistance index m 1–4 Water vapour impermeable 1–5 3–20 n.a. n.a. Water vapour impermeable 3–5 3–4 2–8 5–10

1. Declared thermal conductivity by the manufacturer. Table 8.8 Selected specific properties based on manufacturers’ declarations (where appropriate and given) [8] Material/Product

Specific property

Compressive strength (kPa)

Tensile strength perpendicular to faces (kPa)

Maximum service temperature short-term long-term (°C)

(°C)

Mineral wool: with binder 3.5–80 250 100–200 Glass wool: without binder 600 500 Rock wool: without binder 1000 600–750 Cellular glass 500–1700 700 430 Foamed glass 120–140 600–700 Calcium silicate foam 350–> 1000 1050–1100 Ceramic fibres, -foam > 1000 1100–1430 Aerogel Pyrogenic silicic acid* Expanded perlite board 150–300 > 40 250 800 Exfoliated vermiculite 100–450 n.a. 700–1100 Expanded clay ViP 45–120 *Core material of vacuum insulation panels (VIP)


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∑ C ∑ D ∑ E ∑ F

Materials for energy efficiency and thermal comfort in buildings ∑

smoke propagation Limited – but some – contribution to a fire Not negligible contribution to a fire Poor fire reaction properties ∑ acceptable ignitability ∑ limited flame propagation No performance determined – no data available

Table 8.9 summarizes those properties. There are six main applications for mineral insulation products in the building sector (general definitions are given here, details with respect to individual products are listed in Table 8.10): 1. Cavity wall. A cavity wall is defined as an external wall made of two layers with a small air gap or ‘cavity’ between them. By filling the gap between the two walls with an insulating material, the amount of heat escaping through the walls can be reduced. 2. Solid wall. A solid wall is defined as an external wall without a cavity. Solid walls lose more heat than cavity walls of the same material. However, insulation can be applied to the inside and/or the outside of the wall to reduce heat loss. Insulation boards are typically used for these purposes. Solid walls also cover stone walls which have a random

Table 8.9 Properties related to acoustic and fire performance (manufacturers’ declarations – where given) [8] Material/Product

Acoustics

Dynamic stiffness s¢ (MN/m3)

Reaction to fire Euroclasses Sound impedance acc. to per unit length EN 13501-1 r 2 (kPas/m )

Mineral wool – glass wool – rock wool 7–35 6–43 Cellular glass Foamed glass Calcium silicate foam (mineral foam) Ceramic fibres, -foam Aerogel Pyrogenic silicic acid Expanded perlite board Exfoliated vermiculite Expanded clay VIP (SiO2 as core material)


A1, A2 A1 A1 (non combustible) A1 (A2) (non-combustble) A1 (non-combustible) A2-s1,d0; B-s1,d0, C-s1,d0; D-s1,d0 A1 (non-combustible) E


205

Table 8.10 Application matrix (based on the German DIN 4108–10 and ISO TR 9774) [7], [10]

∑

∑

∑

∑ 

∑

∑

External insulation of upper floors or roofs, protected from the weather, insulation below roof covering

∑

∑



External insulation of upper floors or roofs, protected from the weather, insulation below waterproofing

∑

∑

External insulation of roofs, exposed to the weather (inverted roof)

Insulation between rafters, double-skin roofs, accessible but non-trafficked topmost floor

∑

Internal insulation of upper floors (underside) or roofs, insulation below rafters/ structure, suspended ceiling, etc.

∑

∑

Internal insulation of upper or ground floors (top side) below screed, without sound insulation requirements

∑

∑ 

Internal insulation of upper or ground floors (top side) below screed, with sound insulation requirements

∑

Wall

External insulation of walls, ∑ behind cladding External insulation of walls, behind waterproofing

∑

External insulation of walls, behind render

∑

∑

Cavity insulation of double- leaf walls

∑

∑ 







  ∑



∑ 

∑

∑

 ∑

∑

∑ 


 ∑

∑

∑

∑



Expanded clay

Expanded perlite board



Floor and roof

Exfoliated vermiculite

VIP

∑ 

Ceramic fibres, – foam

∑

Calcium silicate foam

Foamed glass

Cellular glass

Aerogel

Products from mineral raw materials

Mineral wool

Application

206


Table 8.10 Continued

Insulation to timber-frame and timber-panel constructions

∑

Internal insulation of walls

∑

Insulation between partition walls with sound insulation requirements

∑

Insulation to separating walls

∑

Basement

External thermal insulation of walls in contact with soil (on outside of waterproofing)

∑ 

External thermal insulation below ground floor slab in contact with soil (below waterproofing)

∑ 

∑



∑

∑

∑

∑

Expanded clay

Expanded perlite board

∑ 

Exfoliated vermiculite

VIP

 ∑

Aerogel

∑ 

Ceramic fibres, – foam

∑

Calcium silicate foam



Foamed glass

Cellular glass

Products from mineral raw materials

Mineral wool

Application

∑ with standardized properties  with building authority approval for the product for this application  rarely used as thermal insulating material

arrangement of cut or more usually irregular broken stone blocks bonded with a cementious mortar. 3. Loft (attic) insulation. Loft insulation is defined as material applied to the floor of the loft, between and over joists. This is most often loosefill insulation, which is blown onto the loft floor or batts and rolls of insulation are used. 4. Floor insulation. Floor insulation is defined as material that is installed between the beams or joists under a raised floor or, in the case of concrete floors, insulation board installed under floor finishes, such as carpet or under a floating floor. 5. Roof insulation. Here insulation is placed on the underside of the roof instead of on the loft floor. Spray foam (rarely) or insulating boards are most often used for this purpose. Roof insulation between rafters is a major application area for, e.g., mineral wool.



207

6. Insulation of pipework and ducts as part of building equipment. To preserve the temperature of water and air flowing through pipework and ducts, thermal insulation is used. Insulation of pipework and ducts is defined as lagging or insulation used around pipes or ducts. Products for this purpose are often moulded to the shape of the pipes. Other types of insulation include prefabricated structural wall and roof panels, e.g. sandwich panels, stressed skin panels, and steel frame systems. The properties outlined above in the various tables – in short the performance characteristics – have led the products made from mineral materials to be used in certain applications which can best be visualized in a matrix (Table 8.10). Based on the European technical harmonization, the EU Member States are responsible for developing rules for the specific use of insulating materials in construction works, e.g. in roofs or walls – as outlined in Table 8.10 for Germany as an example.

8.4

Ecological and health aspects

Since industrialization the energy intensity of the production of building materials has increased due to the shift from locally produced raw materials and human energy to high temperature manufacturing processes that consume large amounts of fuel energy and use components and materials that have been transported across the world. This increase in the pre-use energy consumption of materials is accompanied, however, by the advent of insulating materials that can significantly reduce the operational energy use of a building. There is general agreement across the construction industry that installing insulation materials in new and existing buildings is one of the best ways to reduce the environmental impact of buildings. In this respect, any insulation material can be deemed preferable to no insulation. There is a range of insulating materials each with their own benefits in terms of environmental performance. The life cycle of a thermal insulation product consists of a number of key phases, which are summarized as follows: ∑

∑

Extraction and processing of raw materials. Mining operations and refining ores for the manufacturing process will result in environmental issues and impacts; these include high energy use, physical disturbance of the landscape and pollution from toxic emissions, which may affect land, water and air. There is scope to use recycled content within many insulation products, which would reduce the impacts at this stage. Insulation manufacture. The manufacturing processes for the different categories of insulation vary; however, all have significant environmental


208

∑

∑

∑

∑

∑


impacts, including energy consumption, the use of materials with hazardous properties and the production of hazardous wastes. Packaging. The insulation is cut to size and wrapped in packaging to allow transportation. This packaging has environmental implications through its manufacture and, if oil-based polymers are used, it could result in additional non-biodegradable waste if not reused or recycled properly. Transport to retail unit. The main environmental impact is energy consumption (fuel use) and associated air quality and emissions by the vehicles. It should be noted that insulation products that require smaller volumes of material and/or are lighter (lower density) to achieve the desired level of thermal resistance might have lower transport emissions. This is due to fewer vehicles being required and/or less energy to transport the material. Installation. Transportation from the retail unit to the building where the insulation is to be used has an impact through fuel use and vehicle emissions. The efficiency of the installation also affects the degree of wastage and the efficiency of the insulation (gaps in the insulation can reduce its effectiveness). Also, heavier insulation products may require additional equipment and fixtures to secure them in place compared to lighter (less dense) alternatives. Use and maintenance. There is very little in-use impact. However, the maintenance of insulation materials can affect their effectiveness. For example, dampness and compression can reduce the effectiveness of some insulation products as the presence of water or reduced thickness through compaction will increase the amount of heat transferred through the material in comparison to properly dry air-filled insulation. Insulation products with closed cell structures and high compressive strengths are less vulnerable to these factors. End-of-life. Reducing further environmental impact will depend on the management and handling of insulation products at the end of their useful life; mismanagement can result in increased impacts on the environment, for example hazardous waste being disposed of to landfill or the emission of toxic particulates into the atmosphere. Disposal options for insulation depend on the type of material used: some will have to be disposed of as hazardous waste.

For each of the phases listed there will be an element of energy input. There will also be emissions to air, land and water and some degree of waste, which will either be suitable for reuse or recycling or have to be sent for disposal. The so-called ‘eco-balance’ takes into consideration all stages of the life cycle, i.e. acquisition, production, use and end-of-life plus transportation for combining the different stages. For a specific use-phase Table 8.11 gives a comparison of the most important ecological parameters. © Woodhead Publishing Limited, 2010


Material/Product

Ecological parameter a

b

Material property c

Calorific value PE-Input GWP APd (MJ/kg) (MJ/m3) (kg CO2-eq./m2) (kg SO2-eq./kg)

Density Thickness ~ (kg/m3) (m)

Mineral wool: – glass wool 0.0 102 5.335 0.04 23 0.11 – rock wool 0.0 400 27.979 0.19 160 0.13 Cellular glass 0.0 467 16.873 0.104 115 0.13 Calcium 0.0 6400 504.1 0.69 200 0.19 silicate foam (non-renewable) 260 (renewable) a

Calorific value: Can be gained in a later combustion process. PE-Input: Primary energy input covering all energy used to manufacture the product. c GWP: Global warming potential. d AP: Acidification potential. b

Lambda lD (W/(m•K)) 0.035 0.040 0.040 0.060


Table 8.11 Ecological performance based on standardized conditions (U-value = 0.3 W/(m2•K); PE-investment = 0.22 W/(m2•K)) [3]

209

210


Provided that in the near future data and parameters for all insulation materials are generated on a standardized basis, the end-user can choose their product both on the technical and the ecological performance checklist. However, one should keep in mind that no matter which approved insulating product is chosen, the ‘return-on-energy investment’ is – depending on the climate – usually only one heating period. Under Mandate 350 of the EU Commission the relevant CEN working group aims to establish a methodology for calculating the use of non-renewable energy (measured in mega joules) for different stages of the product life cycle. The Mandate will lay down the methodology to allow manufacturers to calculate these values and list the use of non-renewable energy and many other environmental/sustainability values in an environmental product declaration (EPD). The EPD will allow contracting authorities to benchmark products based on a comparison of the information included in the EPD. So far those product characteristics that have been described reflect the more technical performance, i.e. mechanical strength, fire protection, sound insulation and, of course, thermal insulation. The ‘Essential Requirement 3 (ER 3)’ of the European CPD demands that construction works should not endanger users through any of the following: ∑ ∑ ∑ ∑ ∑ ∑

the giving-off of toxic gas the presence of dangerous particles or gases in the air the emission of dangerous radiation the pollution or poisoning of the water or soil the faulty elimination of wastewater, smoke, solid or liquid wastes the presence of damp in parts of the works or on surfaces within the works.

When suitable quality control and professional installation has been applied, it is normally possible to say that there are no health or usage influences through the use of thermal insulation materials. It is, however, necessary to state at this point that there is a possibility of general odour annoyance, formaldehyde gas emissions, (natural) radioactive radiation, dust loading and the danger of pesticide residues. The reusability of an insulation material is mainly dependent on whether it can be dismantled homogeneously and true-to-type. Often this is possible with relatively few problems for filled materials or insulation layers which are only fixed mechanically; the reuse of glued or rendered products is almost impossible. If use of soiled insulation material residues is not possible as an additive mixture or for soil loosening, the material has to be taken to a land-fill or used energetically (as far as is possible and feasible) as an incineration material. Now that national and European politics is becoming more environmentally aware, consideration of the potential harm to health of insulation materials with



211

regard to manufacture, use and disposal is becoming more and more the centre of focus in standardization activities.

8.5

Individual product profiles

As listed in Table 8.2, there are various natural and synthetic (raw) materials employed in the manufacture of mineral insulation products. While most of the products listed play a more or less significant role in terms of market share for thermal building applications, two of the products mentioned, namely slag wool and gypsum foam, are of no importance for the applications addressed in this chapter. Some information about these products can be found in reference [8]. This leaves ten product types that will be profiled in more detail in this section. Table 8.12 lists their usual forms of supply to the marketplace.

8.5.1 Mineral wool Glass wool (Fig. 8.1) is made from sand, limestone and soda ash with a high proportion of recycled glass, plus other minerals. These are melted, spun into fibres and mixed with organic resins before curing into products. The mineral raw materials are melted at approximately 1400 to 1500 °C. An aqueous binder is sprayed onto the fibres during the spinning process. The fibres cool down fast and stiffen glassily. The binder is then hardened in a tunnel stove at roughly 250 °C through which the products get their structural stability. Afterwards they can be cut into boards, slabs or batts. The typical composition of glass wool is roughly 70% recycled glass, 0.5 to 7% binder (on a phenolic resin basis) and 0.5% mineral oil to avoid dusting. The other Table 8.12 Usual forms of supply of products made from mineral raw materials Material/Product

Forms of supply

Boards, batts Loose fill

Caulking materials

Mineral wool ∑ ∑ Cellular glass ∑ Foamed glass ∑ Calcium silicate foam ∑ Ceramic fibres, -foam ∑ Aerogel ∑ ∑ Vacuum insulation ∑ Expanded perlite ∑ ∑ Exfoliated vermiculite ∑ ∑ Expanded clay ∑


Sandwich ∑

∑ ∑

212


8.1 Glass wool MW (Source: FIW München).

components are derived from the usual glass manufacturing, i.e. quartz and limestone. Recently mineral wool products have been introduced to the market that contain additives (binders) on a renewable organic raw material basis, affecting the optical appearance and improving the ecological balance. Stone or rock wool (Fig. 8.2a) is based on natural minerals, e.g. volcanic rock, typically basalt, plus recycled post-production waste materials; the components are melted, spun into fibres and then mixed with a binder and an impregnation oil. There is a wide variety of products ranging from loose pelleted materials suitable for cavity wall insulation, to rolls and light boards for loft insulation and dense slabs used for light load bearing application in floors and roofs. Pre-formed pipe insulation and wired matting are used for industrial applications. Rock or stone wool thus usually contains 30% recycled raw material, i.e. waste glass, 0.5 to 7% binder (phenolic resin polymer) and 0.5% mineral oil, the larger portion composed of the natural raw materials mentioned above. ‘Hybrid products’ combining both raw material basis and technologies



213

(a)

(b)

8.2 (a) Rock wool MW, (b) Composite board WW and MW (Source: FIW München).

are now offered to the market thus combining the high melting point of rock or stone wool with the elasticity of glass wool. Mineral wool stands out due to its universal material qualities for a wide range of applications. It offers protection from cold and warmth, absorbs sound and offers a good fire protection. In addition, lightweight mineral wool


214


shows good handling properties, is physically quite safe for the health and very resistant to ageing. Furthermore, mineral wool is a highly standardized insulation material. The conformity with standards is supervised by recognized independent institutes (‘notified bodies’) and the products usually carry an (additional) quality mark next to the CE marking, the latter being based on EN 13162. Because mineral wool can be delivered as a non-combustible insulation material, it offers an important contribution to the preventive structural fire protection. Depending on the implementation, it meets most building requirements for fire resistance. Mineral wool products are offered by manufacturers in a large variety of implementation options, e.g. with additional functions like protection against noise. A good insulation in connection with warmth storing mass in the inside area offers very good thermal protection in the summer. Since mineral wool can be processed with ease from the cellar up to the roof even by DIY enthusiasts in their own home; it has gained wide acceptance in the DIY market. The simple handling of mineral wool is further supported by a versatile choice of ‘system products’ for different problem solutions (Fig. 8.2b). The processing of mineral wool, e.g. at façades (ETICS) should, however, be carried out by professional craftsmen. As mentioned before, mineral wool products carry a quality mark and comply with certain (national) safety standards. Before the products are marked with a quality mark, the mineral wool manufacturer must undergo quality examinations with respect to the fibre composition and fibre dimensions. These proofs are provided by independent testing institutes. Even after awarding a mark, e.g. in Germany by RAL, the production of the manufacturers is under regular third party control. Mineral wool can in principle be recycled and utilized again. For example, (production) waste can be supplied again to the mineral wool production process and will be reutilised in new mineral wool. Mineral wool can be disposed of like any other building rubble. The regulations of single individual operators or regional specifications have yet to be taken into account if necessary. Returning to the health and safety aspect and taking Germany as an example, the so-called ‘chemical ban ordinance’ – more correctly ‘Ordinance for the Change of Chemical Legal Ordinances’ addresses, among other things, the fibre issue. Since June 2000 it states a ban on the circulation, production and use of bio-persistent artificial mineral fibres for heat protection and sound absorption in buildings, and for the technical insulation. At the same time, it includes an exemption regulation for non-bio-persistent, thus biologically soluble fibres which, if in compliance with the three ordinance certificate criteria, are exempted and consequently permitted by the ordinance. The three compliance exoneration certificate criteria are defined clearly; they are derived from the German Hazardous Substances Ordinance and are at



215

the same time the criteria to be obeyed by mineral wool products with a quality mark. Mineral wool products awarded with such a quality mark are not subject to the prohibition of the chemical ban ordinance. For particular applications, the fibrous structure has been adapted to the specific application needs: in the external wall sector (ETICS), products have been optimized and these new developments provide the market with insulation panels with ‘dual-layer characteristics’: the normal lamellar layers provide the good insulation properties, the high compaction of lamellae on the outer side creates an extremely hard surface layer which makes the application of the reinforcing compound and the rendering easier. Another development, a thermal-bridge-free insulating layer can be applied with fabric rolls for cavity masonry (cavity wall insulation).

8.5.2 Cellular glass Cellular glass (CG) (Fig. 8.3) is an inorganic thermal insulating material with a closed cell structure. It is mainly made of glass (usually with a high proportion of recycled and waste glass added) plus calcium fluoride, sodium carbonate, iron oxide, manganese oxide, sodium sulfate and sodium nitrate melted at 1400 °C to form glass, then – using carbon as a blowing agent – baked in a high temperature tunnel furnace, annealed in a long tunnel where the temperature is slowly decreased and finally cut into boards or slabs. It well preserves the chemical stability of inorganic glass. As a result, CG has the following features: fairly low density, low thermal conductivity, water

8.3 Cellular glass CG (Source: FIW München).


216


and water vapour impermeability, no water absorption, incombustibility, protection from mould, high mechanical strength but easy to cut and able to bear all kinds of chemical erosions except hydrofluoric acid. Cellular glass, non-toxic in itself, has an excellent durability and a good thermal insulating performance over a wide temperature range (from cryogenic temperature to high temperature). At the same time, it can protect from humidity, fire and erosion. It is praised as ‘permanent thermal insulating material with no need of renewal’, because it is not only safe but also durable in extreme conditions of low and cryogenic temperature, underground, open air, flammability, moisture sensitivity and even chemical erosion. Consequently, it is widely applied in permanent projects as heat and cold insulating material in industry. For this application the manufacturers supply the industry with specially designed parts, e.g. in shapes of various diameters for pipe insulation. In building application the boards or slabs can be used as insulating material in floors, walls and roofs. Specially cut (‘tapered’) products can be used to construct insulated tapered roof systems. Owing to its brittleness the boards or slabs are usually completely coated in bitumen so they can be laid down evenly. Due to the higher price compared with other standard insulating materials, CG is typically found in applications with high compressive or moisture loads, e.g. roof top car parks or below-ground load-bearing uses. A special load-bearing application is the use of ‘cellular glass bricks’ at the base of single-leaf external walls, at the base of the inner leaf of double-leaf walls on suspended floors over outside air and over the basement. This will help to eliminate thermal bridges. Depending on the intended use, manufacturers also supply the market with a variety of faced cellular glass boards; the one, or two, facings may be roofing felt, metal foil or paper cardboard or plastic foil. The core of these composites may consist of either one board, a part of a board or a number of boards bounded edge-to-edge in the factory; bitumen may be a suitable adhesive which may be used to bond the joints and the facings. This in turn will, of course, affect the reaction-to-fire rating (Euroclassification).

8.5.3 Foamed glass Foamed glass (Fig. 8.4) is formed from a reaction between glass and carbon at high temperature resulting in CO2 being trapped in minute bubbles in the glass, giving a cellular structure. It contains no petrochemical binders or preservatives. It is blown with carbon dioxide and has sulphur added. The composition (by volume) of foamed glass is usually: 5% glass, 95% gas (comprising 99% CO2, 0.7% H2S, 0.3% N2). Also waste glass can be recycled into foamed glass yielding a product with excellent mechanical and decent insulation properties. The product is water vapour impermeable and



217

8.4 Foamed glass (Source: FIW München).

therefore suitable for insulating areas like below-ground structures in contact with soil and water. Because of this inertness foamed glass has an excellent durability and does not rot or grow mould and is resistant to rats or insect attack. This makes this material ideally suited as load-bearing insulation below ground requiring national technical approvals. Depending on the particular application, the foamed glass can be supplied in different grain sizes (from 2 mm up to 50 mm). It is not suitable for acoustic applications or between timbers (too rigid).

8.5.4 Calcium silicate foam and mineral foam Calcium silicate (CS) foam boards (Fig. 8.5) made with slightly different raw materials are also known as mineral foam or mineral insulating boards. The raw materials for CS are calcium and silicon dioxide plus an aggregate of 3–6% cellulose. These are mixed with water to form a slurry and this produces calcium silicate hydrate. The cellulose content improves flexibility and edge stability. The mixture is poured into moulds and then autoclaved. The result is a fine-pore, open-cell, rigid foam which is subsequently cut into boards and treated with special additives to give it hydrophobic properties. The high service temperature and the good load-bearing property gives the foam its ideal application area in the industrial sector, and with an acceptable price/performance ratio. The use of calcium silicate foam for the building sector, especially the internal insulation of walls is relatively new. Based on extensive research, it


218


8.5 Calcium silicate foam CS (Source: FIW München).

was proven that, due to the high capillary action and hydrothermal material properties, the foam can be used to improve the moisture protection – especially when energetically renovating old buildings where the outside façade is unsuited for insulation measures. Additionally, the high pH of 10 inhibits the growth of fungus.

8.5.5 Ceramic fibres Standard ceramic fibres (Fig. 8.6) are made from inorganic materials, primarily aluminium oxide (Al2O3) and silicon dioxide (SiO2), leading to very high thermostability. Under normal circumstances standard ceramic fibres can be applied at temperatures up to 1260 °C. By adding small amounts of, e.g., zirconium oxide (ZrO2) the application temperature can be increased to approx. 1600 °C. Another type of fibre (basis: SiO2–CaO) is a synthetic fibre made from alkaline earth silicates. The fibre is soluble in the human body and consequently completely harmless from a health point of view. Such a fibre can normally be applied at temperatures up to approximately 1100 °C. Ceramic fibre products are used in areas where good insulating properties at high temperatures are wanted – for packing and for expansion joints. If there is no mechanical wear, ceramic fibres can be used as the only refractory material in contact with high temperature processes, e.g. in kilns for heat treatment of metal, glass, etc.



219

8.6 Ceramic fibres (Source: FIW München).

Ceramic fibre is manufactured with different qualities depending on the production method: the spun and the blown fibre. A spun fibre yields the strongest, but also the most expensive product. In most fields of application the two types of fibre function in the same way, and therefore the spun fibre is the most commonly used type. Another commonly used type of ceramic fibre is blankets that are manufactured by placing the spun fibre on a conveyor belt. The belt speed and the thickness of the fibre layer determine the two most important quality parameters: density and thickness. The fibre blankets are frequently impregnated, felted, burnt and cut in order to achieve the well-known standard products. The standard fibre blankets are available in the density range 100 to 160 kg/m3 and in thicknesses from 13–50 mm. Ceramic fibres are also available as massive boards, specially designed modules and wet blankets that are impregnated with a liquid making it possible to form the fibre products into the wanted shape that will remain after heating. Because of their unique (high temperature) properties ceramic fibres are rarely used in conventional thermal insulation applications in buildings.

8.5.6 Aerogel Aerogel (Fig. 8.7) is an extremely lightweight, highly porous, solid material. Metal oxides or polymers are among the various raw materials possible.


220


8.7 Aerogel (Source: FIW München).

However, aerogel is primarily manufactured from silicates (SiO2) which convert to a gel upon addition of a catalyst. This gel is subsequently dried under extreme conditions to a non-brittle consistency. This sol–gel process was developed as long ago as 1930 and over the course of time the mix of raw materials has been varied and improved. In the meantime, pore sizes range down to 20–40 nm. With a thermal conductivity of 0.017–0.021 W/(m•K), aerogel exhibits excellent thermal insulation properties; it is also good as sound insulation and remains stable in temperatures up to approximately 1200 °C. Aerogels have a density of 60–80 kg/m3. They are resistant to moisture and mould growth and do not discolour even after long exposure to ultraviolet radiation. Since more than 40% of heat lost from an average building is through the walls, aerogel particles can be a solution for cavity wall insulation, especially in small spaces or historical buildings where a change in the building’s appearance is not acceptable. Aerogel cavity wall insulation is permanent and maintenance free. This in turn can justify the higher price compared with standard insulation materials.



221

A new commercial development incorporates aerogels into a non-woven fabric or a fibrous blanket; this is especially useful in applications where a thin product is required that can be easily cut, rolled and shaped on the job site, i.e. as a thermal break in places that otherwise conduct heat such as over studs and around windows.

8.5.7 Pyrogenic silicic acid and vacuum insulation panels Fumed or pyrogenic silicic acid is produced by burning silicon tetrachloride in a hydrogen flame. In conjunction with a stabilizer, this produces a microporous insulating material. In order to minimize the release of radiant heat, opacifiers such as titanium oxide (TiO2) or, for high-temperature applications, zirconium oxide (ZrO2), are added. This mixture, together with reinforcing fibres, is compressed to form boards with densities of up to 300 kg/m3. Depending on the density, this results in rigid or flexible boards with a microporous structure that significantly impedes the migration of gas molecules and hence reduces the thermal conductivity to 0.020 W/m•K. These boards can be used as a core material in the manufacture of vacuum insulation panels (VIPs) (Fig. 8.8a and b). VIPs ‘transform’ the principle of the thermos bottle into a flat panel. The core material like fumed silicic acid is wrapped in a high gas barrier film or foil and then evacuated and sealed. While VIPs have been in use in high performance packaging for some time, only recently have VIPs been especially developed for the (rough) building application. The core of the panel consists of amorphous silicon dioxide powder and an inorganic opacifier; the panel is sealed under vacuum into a high barrier film or foil which can additionally be covered for protection by a special fleece [9]. During assembly at the construction site care has to be taken in handling and in tightly fitting the panels together avoiding thermal leaks due to the seams of the film covering the panels. Of course, there is no cutting option on the job site. Due to the low thermal conductivity, the drastically reduced thickness (usually 10–40 mm) allows new solutions in building applications. Figure 8.8(c) shows an exterior wall insulation system utilizing VIPs and a protective layer of polyurethane. The higher costs of the system were more than offset by the gained indoor living space (Munich, 2005). National approvals have recently been granted, e.g. by the German Institute for Building Technology. In the approval, ageing and edge losses of the panel are taken into account by fixing the design thermal conductivity at 8–11 mW/(m•K) although the initial value might be as low as 4–5 mW/ (m•K). A special manufacturing process allows for the design of cylindrically shaped products that are useful for building equipment and pipe insulation (Fig. 8.8(b)).


222


(a)

(b)

(c)

8.8 (a) Vacuum insulation panel VIP, (b) Vacuum insulation cylinder, (c) External wall insulation with VIP (Source: va-Q-tec, Würzberg, Germany). © Woodhead Publishing Limited, 2010


223

8.5.8 Expanded perlite Perlite is not a trade name but a generic term for naturally occurring siliceous rock. The distinguishing feature which sets perlite apart from other volcanic glasses is that, when heated to a suitable point in its softening range, it expands to four to twenty times its original volume (Fig. 8.9). This expansion is due

(a)

(b)

8.9 (a) Expanded perlite EP, (b) Expanded perlite board EPB (Source: FIW München). © Woodhead Publishing Limited, 2010

224


to the presence of 2–6% water in the crude perlite rock. When quickly heated to above 900 °C, the crude rock expands as the water vaporizes and creates countless small cells which account for the light weight and other exceptional physical properties of expanded perlite. This expansion process also creates one of perlite’s most distinguishing characteristics: its white colour. While the crude rock may range from transparent light gray to glossy black, the colour of expanded perlite ranges from snowy white to grayish white. Since perlite is a form of natural glass, it is classified as chemically inert and has a pH of approximately 7. Because of perlite’s outstanding insulating characteristics and light weight, it is widely used as a loose-fill insulation in masonry construction. In this application, free-flowing perlite loose-fill masonry insulation is poured into the cavities of part of the structure where it completely fills all cores, crevices, mortar areas and holes. Lightweight brick masonry blocks with factory filled cores are available, too. In addition to providing thermal insulation, perlite enhances fire ratings, reduces noise transmission and it is rot, vermin and termite resistant. Perlite is also ideal for insulating low temperature and cryogenic vessels. When perlite is used as an aggregate in concrete, a lightweight, fire resistant, insulating concrete is produced that is ideal for roof decks and other applications. Perlite can also be used as an aggregate in Portland cement and gypsum plasters for exterior applications and for the fire protection of beams and columns. Other construction applications include under-floor insulation, chimney linings, paint texturing, gypsum boards, ceiling tiles, and roof insulation boards. The insulation boards (EPBs) (Fig. 8.9b) are usually manufactured in a process which blends the expanded perlite particles with reinforcing fibres, special binders and hydrophobic additives. For the internal insulation of walls and ceilings, a special fibre-free board has been developed, which not only reduces the heat loss but also helps to control the indoor air humidity. Composite boards, e.g. EPB in combination with mineral wool, are also offered to the market.

8.5.9 Exfoliated vermiculite Vermiculite is the geological name given to a group of hydrated laminar minerals which are aluminium iron magnesium silicates resembling Mica in appearance. Minute layers of water are trapped between the plates, and when the particles are subjected to heat (> 1000 °C) they have the unusual property of exfoliating, or expanding, due to the inter-laminar generation of steam (Fig. 8.10). A typical chemical composition is given in Table 8.13. In its exfoliated state, vermiculite has the following important qualities: low density, moderate low thermal conductivity, high temperature resistance, high absorbency, high specific surface area and cation exchange property. It is also non-toxic and sterile.



225

8.10 Exfoliated vermiculite EV (Source: Techno-Physik Eng. Essen, Germany). Table 8.13 Typical chemical composition of exfoliated vermiculite Chemical

% by weight

SiO2 38–46 TiO2 1–3 AI2O3 10–16 Fe2O3 5–13 MgO 16–35 CaO 1–5 K2O 1–6 H2O 3–16

Exfoliated vermiculite will readily hold liquids within the inter-laminar voids of particles. It will also hold a certain amount between the individual particles. Water retained as a percentage of the air dry weight will be approximately 240%. Typical building-related applications are loft insulation and cavity fill. Exfoliated vermiculite is available as aggregate (EVA), coated (EVC), bitumen-coated (EVB), hydrophobic (EVH) and in a pre-mixed form (EVM).

8.5.10 Expanded clay Expanded clay pellets (Fig. 8.11), most commonly known under the brand names LECA (acronym of light expanded clay aggregate) or LIAPOR (porous


226


8.11 Expanded clay (Source: FIW München).

lias clay), also known as Hydroton and under the non-proprietary terms fired clay pebble, grow rocks, expanded clay (pellets) or hydrocorns, are small globes of burnt and expanded clay, used in construction and farming, and especially in hydroponics. After being thoroughly prepared, the raw clay is burnt at a temperature of about 1200 °C in a rotary kiln. During this process the organic components of the clay, which are evenly dispersed, combust. The spheres expand and expanded clay with very fine pores comes into existence. The weight, size and strength can be controlled exactly. The pellets are available in different sizes such as 4–8 mm in diameter. The dry density of lightweight expanded clay aggregates is 300 to 500 kg/m3. Expanded clay pellets offer decent insulation properties, they are lightweight and heatproof. Due to their low weight and good shear strength the product is used for lightweight fill in civil engineering structures to reduce settlement and reduce pressure on structures. The still preliminary European standard prEN 15732 describes the specifications needed for this application. Expanded clay is very open to diffusion and can absorb a considerable amount of moisture. It is an incombustible material and is also highly resistant to chemicals. Rodents and insects find no nutrients in expanded clay; however, fungi can grow on it if moisture is constantly present. When used as a thermal insulation material, expanded clay is mainly used to fill voids and, for example, contributes to improving the sound insulation of suspended floor constructions. Expanded clay is also used as an aggregate in the manufacture of masonry units and precast concrete elements, and in



227

a small grain size for improving the insulation properties of mortar, render and plaster.

8.5.11 Insulating clay bricks (Fig. 8.12) The brickmaking industry has been offering special masonry units with a thermal conductivity of < 0.1 W/(m•K). Their insulating effect is therefore close to that of some thermal insulating materials and thus should be mentioned in this chapter. The clay content of the bricks has been reduced by optimizing the arrangement of the hole pattern to such an extent that the load-bearing capacity for normal structural requirements is still sufficient but at the same time the thermal insulating properties are considerably better than before. Additionally, the holes can be filled with an insulating material. Expanded perlite (EP) or special mineral granulate are used as a filling material; at a density of 650 kg/m3 the thermal conductivity can be as low as 0.09 W/ (m•K) for the design value. In order to reduce the mortar’s influence on the thermal conductivity of the wall, these products are constructed exclusively as precision bricks laid in thin-bed mortar.

8.6

Summary

Insulating products made from natural or synthetic mineral raw materials dominate the insulation market in Europe due to their overall cost/performance

8.12 Insulating clay brick with EP (Source: FIW München).


228


ratio or their special product properties, mineral wool products for building insulation having gained the largest market share. The versatility of the raw materials and the different production processes allow for a wide range of combinations of product properties: from commodity type insulation materials to highly specialized materials for niche-type applications in the building and the industrial sector. Very often it is the combination of thermal, fire and acoustic performance that make the materials described in this chapter the ‘product of choice’. For trading in the European market, products like mineral wool, cellular glass and expanded perlite are specified by harmonized CEN standards, while the niche-type products have either a manufacturer-specific European or national technical approval to prove to the consumer the fulfilment of regulatory requirements for the particular application.

8.7

References

1. http://eur-lex.europa.eu/ 2. Gibson, L.J. and Ashby, M.F.: Cellular Solids: Structure and Properties. Pergamon, Oxford (1988). 3. Danner, H.: Comparison of ecological insulating materials, Munich, www.muenchen. de/bauzentrum (2008). 4. EN 13162: (2008) Thermal insulation products for buildings – factory made mineral wool (MW) products – specification. 5. EN 13167: (2008) Thermal insulation products for buildings – factory made cellular glass (CG) products – specification. 6. EN 13169: (2008) Thermal insulation products for buildings – factory made products of expanded perlite (EPB) – specification. 7. ISO TR 9774: (2004) Thermal insulation for building applications – guidelines for selecting properties. 8. Pfundstein, M., Gellert, R., Spitzner, M.H. and Rudolphi, A.: ‘Insulating materials’, Edition Detail, Institut für internationale Architektur – Dokumentation GmbH & Co. KG, Munich, Germany (2007); English version: distributed by Birkhäuser – Publishers for Architecture, Basel, Switzerland (2008). 9. DIN 4108-4: (2004-07) Thermal insulation and energy economy in buildings – Part 4: Hygrothermal design values. 10. DIN V 4108-10: (2004-06) Thermal insulation and energy economy in buildings – application-related requirements for thermal insulation materials – Part 10: Factory made products.


9

Natural fibre and fibre composite materials for insulation in buildings

R. G e l l e r t, FIW München, Germany

Abstract: Insulating materials from natural organic raw materials have gained a small (4–6%) but growing market share in Europe in the last fifteen years. On the one hand, this consumer acceptance is based on the fulfilment of regulatory requirements, i.e. conformity with recently published European standards and/or the issue of European Technical Approvals (ETA). On the other hand, the growing awareness of the public towards ecological performance of building materials has given niche-type products from natural sources a certain boost, and increasingly recently-erected – often called ‘green buildings’ or ‘eco-buildings’ – can be found throughout Europe. Key words: cellulose fibres, cereal granulate, coconut fibres, cotton, flax, hemp, cork, reeds, sea grass, sheep’s wool, straw bales, wood chippings, wood fibres, wood wool.

9.1

Introduction

Insulating materials and products are usually classified into two major groups depending on the raw materials used for their production, i.e. ∑ ∑

inorganic or mineral insulating materials organic insulating materials.

The latter is further subdivided into ∑ ∑

synthetic organic and natural organic insulating materials

Although the so-called natural products may contain varying amounts of additives such as fire retardants, binders or substances to improve mechanical and moisture repellent properties, they are usually offered as ‘natural’ to the market. Products based on the raw materials in Table 9.1 can be found in the market for this particular application. Some of the materials listed in Table 9.1 are relatively rare or no longer commonly used, in particular peat and giant Chinese silver grass. Generally This chapter is dedicated to Dr Walter F. Cammerer on the occasion of his 90th birthday.


230


Table 9.1 Natural organic raw materials employed for the manufacture of insulating materials and products Insulating materials Inorganic

Organic

Synthetic

Natural raw materials

Cellulose fibres Cereal granulate Coconut fibres Cotton Flax (Giant Chinese silvergrass) Hemp Cork (Peat) Reeds Sea grass Sheep’s wool Straw bales Wood chippings Wood fibres Wood wool

Table 9.2 Market share of insulating materials and products in Germany [1] Product/ year

Market share (%) MF

EPS

XPS

PUR/PIR

Others

1989 1994 1999 2000 2003 2004 2005

59 60 58 58 58 55 56

32 32 28 28 28 28 29

3 3 4 4 5 6 6

5 4 4 5 5 5 5

1 1 6 5 4 6 4

MF = EPS = XPS = PUR/PIR = Others =

mineral fibre products. expanded polystyrene. extruded polystyrene foam. polyurethane foam/polyisocyanurate foam. including products from natural organic raw materials.

Total market (Mio. m3) 16 28 34 34 27 27 24

speaking due to the increasing cost for energy, the sale – and use – of insulating materials and products for the housing sector has risen in the European market during the last two decades. Table 9.2 illustrates the market growth since 1989 for Germany – with a slump of 20% in recent years – but the market recovered in 2006 and 2007. As can be noted the market shares have hardly changed over the years due to the individual price/performance characteristics of each product (group) compared with the others. Products


Natural fibre and fibre composite materials for insulation

231

based on natural organic raw materials have gained some market share in recent years – most likely not only based on their performance but also based on their (more) ecological image. Within this group one can find the market share distribution in Germany shown in (Table 9.3). Most of the aforementioned products are supplied to the market in one of the four forms of supply listed in Table 9.4.

9.2

Regulatory requirements

In order to bring transparency into the European market for manufacturers and designers, and to guarantee legal security for all those involved, national regulations have been subject to a process of harmonization at the European level. The current transitional phase is characterized by the coexistence of European and national regulations, the validity of which must be carefully considered. Table 9.3 Market share of insulating materials from natural organic sources (Germany 2004) [1] Product

Market share (%)

Cellulose (LFCI) Wood fibre (WF) Wood wool (WW) Flax/Hemp Sheep’s wool Others

32 28 20 9 4 7

Table 9.4 Forms of supply of products made from natural organic raw materials Material/Product

Forms of supply

Boards, batts Loose-fill

Blown or caulking materials

Wood-fibre ∑ ∑ Wood wool (incl. composites) ∑ Insulation cork board ∑ ∑ Cotton ∑ ∑ Flax ∑ ∑ Cereal granulate ∑ Hemp ∑ ∑ Coconut fibres ∑ ∑ Sheep’s wool ∑ Reeds ∑ ∑ Cellulose fibres ∑ ∑ Sea grass ∑ ∑ Wood chippings ∑ Straw bales ∑


Sandwich

∑

232


It was in 1989 that the ‘Council Directive 89/106/EEC of 21 December 1988 on the approximation of laws, regulations and administrative provisions of the Member States relating to construction products (89/106/EEC)’ was published in the Official Journal of the European Communities (OJ). This so-called Construction Products Directive (CPD) states that construction products – and the technical specifications that describe them – must be of such a form that their use does not have a detrimental effect on the levels of safety and protection that apply to the construction works. Such levels can vary, depending on the location of the works, their use or other circumstances. The levels of safety and protection are divided into six primary requirements listed in Annex I of the CPD: 1. Mechanical resistance and stability 2. Safety in case of fire 3. Hygiene, health and the environment 4. Safety in use 5. Protection against noise 6. Energy economy and heat retention. For the implementation of the CPD, the European Commission has provided detailed explanations and commentaries of individual parts in the form of interpretive documents and Guidance Papers; the Member States and the CEN (Comité Européen de Normalisation – European Committee for Standardisation) have to take into account these non-binding documents in their national legislation. The above-mentioned technical specifications that describe construction products, e.g. thermal insulating materials and products, will normally be either harmonized standards or European Technical Approvals. ∑

∑

Harmonized standards (hEN) [2–4] are technical rules drawn up on the basis of mandates of the Commission of the European Communities by the European standardization organizations with respect to the essential requirements; they are implemented in corresponding national standards (e.g. as DIN EN or BS EN). Normally, national and state governments contribute to the drafting of harmonized standards within the scope of the participation of interested parties in order to incorporate into European standardization the latest technical requirements. European Technical Approvals (ETA) are favourable technical assessments of fitness for an intended use issued to a manufacturer for his construction products by certain Approval Bodies according to this law or the legislation adopted by other Member States of the European Union or other contracting states to the agreement on the European Economic Area (EEA).

Binding legislation on the one hand and standards and approvals on the



233

other hand have created an extensive set of rules and regulations covering the manufacture of insulating materials and their use in construction. The standards and approvals are by definition not mandatory, but do become so when statutory instruments make reference to them. Table 9.5 lists the existing standards, preliminary standards or approvals for the insulating materials and products described in this chapter. As the standards for the factory made products were written according to a ‘model standard’ and published as a ‘package’, the three standards listed in Table 9.5 all have an identical structure: in the ‘heart of the standard’ one will find the requirements, the test methods, the designation code, the rules for the attestation of conformity and the rules for marking and labelling. Due to the ‘in-situ-situation’ the standard – still in the preliminary stage – for cellulose fibre is written in two parts: one addressing the (basic) raw material, the second part describing the ‘conversion’ from a loose-fill material into a thermal performance product on site. Based on properties to be declared for all applications and also – if declared by the manufacturers – for specific application one can summarize important standardized performance criteria in a table (Table 9.6). As shown in Table 9.5, most products under consideration here are placed on the market via a European Technical Approval (ETA) – issued specifically to the particular manufacturer. An ETA can be granted when any of the following conditions apply: Table 9.5 European technical specifications for insulating materials based on natural organic raw materials [2–4] No. Product

Abbreviation

Technical specification

Factory made In-situ application 1 Wood wool and WW EN 13168: composites WW-c 2001-05a 2 Wood fibre WF EN 13171: 2001-05a 3 Cork board ICB EN 13170: 2001-05a 4 Cellulose fibres LFCI Insulating materials based on natural fibres (except 1–3) ETAb

Loose-fill materials based on natural fibres (except 4)

All others

a

ETAb

pr EN 15101

ETAb ETAb

Recent additions and corrigenda have been published (www.cen.eu). Individual companies are holders of approvals (www.eota.be).

b


234


Table 9.6 Standardized properties, relevant test methods and designations Property

Test method Symbol Dimension 2

Thermal resistance EN 12667 or RD m •K/W EN 12939 l D W/(m•K) Length and width EN 822 l and b mm Thickness EN 823 d mm Squareness EN 824 mm/m Flatness EN 825 mm Dimensional stability under EN 1603 De % normal laboratory conditions Bending strength EN 12089 sB kPa Reaction to fire EN 13501-1 (Euroclasses) Moisture content EN 12105 % (w/w) Apparent density EN 1602 ra kg/m3 Dimensional stability under EN 1604 De % specified temperature Dimensional stability under EN 1604 De % specified temperature and humidity conditions Deformation under EN 1605 e % compressive load Compressive stress at 10% EN 826 s10 kPa deformation Tensile strength EN 1607 smt kPa perpendicular to the faces Point load EN 12430 N Compressive creep EN 1606 ect % Shear strength EN 12090 t kPa Water absorption, short term EN 1609 kg/m2 Water vapour transmission/ EN 12086 m m2•h•Pa/mg resistance factor Dynamic stiffness EN 29052-1 s¢ MN/m3 Thickness EN 12431 dL mm Thickness dB Compressibility dL–dB c mm Sound absorption EN ISO ap — 354:1993/A1 aw Air flow resistance EN 29053 kPa•s/m3 Chloride content EN 13168-C.1 %

Designation R, l L and W T S P or Smax DS (N) BS A, B, C, D, E, F H DS(T) DS(T, H)

DLT CS(10) TR PL(P) CC( )sc WS Z or MU SD

CP AP/AW AF Cl

∑ ∑

no relevant harmonized standard for the product exists no mandate for such a standard has been given by the European Commission ∑ the European Commission considers that a standard cannot be developed (yet) ∑ a product deviates significantly from the relevant harmonized standards. In conjunction with an ‘Attestation of Conformity procedure’ (which is intended



235

to ensure that the product specification set out in an ETA is maintained by the manufacturer), ETAs allow manufacturers to place the CE marking on their product(s). Although in certain circumstances it may be possible for an ETA to be issued on the basis of a common assessment procedure agreed among EOTA members (according to the art. 9.2 of the Council Directive 89/106/EEC), in most cases an ETA for a product will be granted to a manufacturer based on the assessment principles set out in an ETA Guideline for the relevant product sector. When a European Technical Approval has been issued it is valid in all EEA countries, for a period of five years, renewable thereafter. The European Organization for Technical Approvals (EOTA) lists ETA documents, the type of products and the manufacturer on its home page (www.eota.be). Both routes lead to the CE marking on the insulating products.

9.3

Building-related properties

Because of its importance as the most important building-related property, in the European system for declaring the thermal conductivity lD (or the thermal resistance RD respectively) the following details can be given: ∑ The declared thermal conductivity lD is determined on the basis of statistically evaluated measurements (l90/90) – to be organized by the manufacturer. Details are given in the respective product standard (usually in a normative Annex); lD is declared in steps of 0.001 W/m • K. ∑ Design values for the particular building application are defined nationally – due to different building codes and/or the climate in question. An overview of the basic building-related properties is given in Table 9.7, some specific properties of interest here in Table 9.8. Besides the ‘core properties’ which determine the performance of an insulating material, some products have good acoustic or specific fire performance characteristics that the manufacturer wishes to declare – or has to declare for a certain application – already when placed onto the market, i.e. the reaction to fire classification. The latter has now been based on the European (classification) standard EN 13501 and the following Euroclasses with respect to the combustibility of building materials (reaction to fire) have been agreed upon: ∑ A1 + A2 No contribution to a fire/non-combustible ∑ B Very limited contribution to a fire ∑ heat propagation ∑ flame propagation ∑ smoke propagation


236


Table 9.7 Typical building application related properties [5–7] Material

Property

Thermal conductivity l Da (mW/(m•K))

Woodwool (WW) Wood fibres (WF) Cork board (ICB) Cellulose fibres (LFCI) Hemp (mats) Sheep’s wool Cotton Flax Cereal granulate Reeds Coconut fibres Sea grass Wood chippings Straw bales

90–110 350–600 40–55 160–250 42–50 100–220 40–45 35–60 40–50 24–42 40–45 18–30 40 20–60 40 20–80 50 105–115 55–90 120–225 40–50 70–120 43–50 75 45–55 70–140 38–72 90–110

a

Density r (kg/m3)

Specific heat capacity C (J/(kg•K))

Water vapour diffusion resistance m

2100 2100 1800 2200 1600 1700 1300 1600 1950 1200 1600 n.a. 2100 n.a.

2–5 5–10 5–10 1–1.5 1–2 1–5 1–2 1–2 2–3 2–5 1–2 n.a. 1–2 n.a.

Declared thermal conductivity by the manufacturer.

Table 9.8 Selected specific properties based on manufacturers’ declarations (where given) [5] Material/Product

Specific property

Compressive Tensile strength stress at 10% perpendicular deformationa to facesb (kPa) (kPa)

Maximum service temperature Short- longterm term (°C) (°C)

Woodwool (WW) 150–200 2.5–50 180 110 Wood fibres (WF) 40–200 2.5–50 n.a. 110 Cork board (ICB)c 100–200 30–50 180–200 110–120 Cellulose fibres (LFCI) — n.a. 60 Cellulose fibres (boards) 2.5 Hemp (mats) Sheep’s wool 500 130–150 Cotton < 400 100 Flax Cereal granulate Reeds Coconut fibres Sea grass Wood chippings Straw bales 120 100 a

EN 826 EN 1607 c ICB not bitumenised b



∑ C ∑ D ∑ E ∑ F

237

Limited – but some – contribution to a fire Not negligible contribution to a fire Poor fire reaction properties ∑ acceptable ignitability ∑ limited flame propagation No performance determined – no data available

Table 9.9 summarizes those properties. The properties outlined above in the various tables – in short the performance characteristics – have led the products made from natural raw materials to be used in certain applications which can best be visualized in a matrix (Table 9.10). Based on the European technical harmonization, the EU Member States are responsible for developing rules for the specific use of insulating materials in construction works, e.g. in roofs or walls – as outlined in Table 9.10 for Germany as an example.

9.4

Ecological and health aspects

So far those product characteristics that have been described reflect the more technical performance, i.e. mechanical strength, fire protection, sound insulation and, of course, thermal insulation. Table 9.9 Properties related to acoustic and fire performance (manufacturers’ declarations – where given) [5] Material/Product Euroclasses

Acoustics Dynamic Sound impedance stiffnees per unit length s¢ r (MN/m3) (kPas/m2)

Woodwool (WW) 4–8 9–100 Wood fibres (WF) 4–8 9–100 Cork board (ICB) Cellulose fibres (LFCI) — 3.6–20 Cellulose fibres (boards) 3–7 43–76 Hemp (mats) — > 6 Sheep’s wool Cotton Flax — > 2 Cereal granulate Reeds Coconut fibres Sea grass Wood chippings Straw bales


Reaction to fire acc. to EN 13501-1 B-s1, d0 E E E E B-s2, d0 to C-s2, d0



















































Straw bales





Wood chippings





Sea grass



Coconut fibres



Reeds 



Cereal granulate



Flax





Cotton



Sheep’s wool

Cellulose fibres



Hemp

Insulation cork board (ICB)





Wall External insulation of walls, behind cladding         External insulation of walls, behind waterproofing





Floor and roof External insulation of upper floors or roofs, protected from the weather, insulation below roof covering External insulation of upper floors or roofs, protected from the weather, insulation below waterproofing External insulation of roofs, exposed to the weather (inverted roof) Insulation between rafters, double-skin roofs,  accessible but non-trafficked topmost floor Internal insulation of upper floors (underside)  or roofs, insulation below rafters/structure, suspended ceiling, etc. Internal insulation of upper or ground floors  (top side) below screed, without sound insulation requirements Internal insulation of upper or ground floors (top side) below screed, with sound insulation requirements

Wood fibres (WF)

Products from natural raw materials Wood wool (WW)

Application

238

Table 9.10 Application matrix (based on the German DIN V 4108–10) [5], [7]


Basement External thermal insulation of walls in contact with soil (on outside of waterproofing) External thermal insulation below ground floor slab in contact with soil (below waterproofing)  with standardized properties  with building authority approval for the product for this application  rarely used as thermal insulating material

 


External insulation of walls, behind render     Cavity insulation of double-leaf walls     Insulation to timber-frame and timber-panel             constructions Internal insulation of walls          Insulation between partition walls with sound  insulation requirements Insulation to interior walls         

239

240


The ‘Essential Requirement 3 (ER 3)’ of the European CPD demands that construction works should not endanger users through any of the following: ∑ ∑ ∑ ∑ ∑ ∑

the giving-off of toxic gas the presence of dangerous particles or gases in the air the emission of dangerous radiation the pollution or poisoning of the water or soil the faulty elimination of wastewater, smoke, solid or liquid wastes the presence of damp in parts of the works or on surfaces within the works.

Given the origin of the products described in this chapter, they will most likely contribute to a healthy ‘building environment’. In order to fulfil all mandatory building-related performance criteria, additives have to be used to meet these requirements. ∑

Reaction to fire: since in many countries at least Euroclass E has to be fulfilled by construction products, flame retardants are being added; in use are usually aluminium hydroxide, ammonium phosphate, -sulfate, borax, boric acid and others. ∑ Mechanical performance: in order to improve the dimensional stability artifical (polyester) fibres are added. ∑ Resistance against biological attack: the proof of the resistance of the product is usually based on an EOTA test procedure (‘factory-made thermal insulation material and/or acoustic insulation material made of vegetable or animal fibres’ Edition June 2003, Rev. 1 June 2005). The (possible) growth of fungus has to be judged based on EN ISO 846: 1997-06, Table 4 and should yield the classification ‘0’. The so-called ‘eco-balance’ takes into consideration all stages of the life cycle, i.e. acquisition, production, use and end-of-life plus transportation for combining the different stages. For a specific use-phase Table 9.11 gives a comparison of the most important ecological parameters. Provided that in the near future data and parameters for all insulation materials are generated on a standardized basis, the end-user can choose their product both on the technical and the ecological performance checklist. However, one should keep in mind that no matter which approved insulating product is chosen, the ‘return-on-energy investment’ is – depending on the climate – usually one heating period.

9.5

Individual product profiles

9.5.1 Woodwool (composite) boards (WW) (Fig. 9.1) The boards or slabs are made from long shavings of softwood and are bonded by the use of an inorganic cement agent. In the composite woodwool © Woodhead Publishing Limited, 2010

Ecological parameter b

Material property


Material/Product

PE-Input GWP APd Calorific value (MJ/kg) (MJ/m3) (kg CO2-eq./m2) (kg SO2-eq./kg)

Density Thickness ~ (kg/m3) (m)

Lambda lD (W/m•K)

Woodwool (WW) Wood fibres (WF) Cork board (ICB) Cellulose fibres (LFCI) Cellulose fibres (boards) Hemp (mats) Sheep’s wool Flax Wood chippings

17.0 18.0 16.7 24.7 17.0 16.0 20.4 12.3 18.0

520 160 120 40 80 25 30 30 80

0.090 0.045 0.045 0.040 0.040 0.045 0.040 0.040 0.045

a

a

728 447 230 38 142 53 48 116 47

c

122.877 –9.110 –23.744 –4.570 7.390 –1.950 –0.076 1.405 –11.844

0.422 0.024 0.051 0.005 0.056 0.024 0.011 0.005 0.014

Calorific value: Can be gained in a later combustion process. PE-Input: Primary energy input covering all energy used to manufacture the product. c GWP: Global warming potential. d AP: Acidification potential. b

0.28 0.14 0.14 0.13 0.13 0.14 0.13 0.13 0.14


Table 9.11 Ecological performance based on standardized conditions (U-value = 0.3 W/m2K; PE-investment = 0.22 W/m2K) [1]

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9.1 Woodwool WW (Source: FIW München).

slabs the woodwool is bonded with a mineral binder; on one or both face(s) another insulating materials, e.g. mineral wool or a foamed rigid cellular plastic can be bonded.

9.5.2 Wood fibre (WF) (Fig. 9.2) Insulation products are manufactured from wood fibres with or without the addition of bonding agents and/or additives; some products are only mechanically bonded. Depending on the bonding agent the product(s) can be supplied either as mats, batts, felt, rolls or lamella rolls. In the latter case strips of wood fibre boards are bonded to a flexible facing and supplied in a bond. This will lead to a continuous insulating layer when the product is unrolled.

9.5.3 Cork-board (ICB) (Fig. 9.3) The protective layer of the cork oak tree (Quercus suber L.) is periodically removed from its trunk and branches to provide the raw material for cork products. The cork is granulated by grinding and/or milling. From this the pre-formed product is made by expansion and bonding exclusively with its own natural binder by heating under pressure.



9.2 Wood fibre WF (Source: FIW München).

9.3 Cork-board ICB (Source: FIW München).


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9.5.4 Cellulose fibres (LFCI) (Fig. 9.4) Loose-fill cellulose insulation (LFCI) is a fibre or fibrous or granulated insulation material derived from paper, paper stock and/or wood, leaf or stalk strings with or without binders. The product is blown, injected or applied with or without moisture. The insulation system usually consists of a particular type of loose-fill cellulose used in conjunction with a blowing machine with a blowing hose and nozzle attached to a hole which is cut or formed into a masonry cavity or frame construction through which the cellulose is blown.

9.5.5 Hemp (Fig. 9.5) Insulating products are either made from the fibres of the hemp plant (Cannabis sativa) or the shives (Fig. 9.6). Supporting fibres (usually from polyester) are used to give the mats mechanical support; starch is sometimes used to bind small layers of hemp fibres. Additives (salts of boric acid) are used to give the product its flame retardant property. Shives can be impregnated and then either formed into boards or used as loose-fill material. The proper ratio of hemp shives and lime has been combined into a bicomposite building material with excellent thermal and acoustic properties.

9.4 Cellulose fibres LFCI (Source: FIW München).



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9.5 Hemp mats (Source: FIW München).

9.6 Hemp shives (Source: FIW München).

9.5.6 Sheep’s wool (Fig. 9.7) Raw or recycled wool from sheep is run through a cleaning process and then treated with additives such as the salts from boric acid or urea derivatives to give it flame retardant and moth repellent properties. Often supporting polymeric fibres (polyester) are added.


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9.7 Sheep’s wool (Source: FNR, Gülzow, Germany).

9.5.7 Cotton Similar to sheep’s wool, cotton can be converted directly into an insulating material, using the same additives to give the rolls or batts the needed properties for the building application. Rarely loose flakes of cotton are used as blown insulation material.

9.5.8 Flax (Fig. 9.8) Only the short fibres of the flax plant are used to manufacture a fleece; additives similar to the ones used for hemp mats are used to give the product the needed performance characteristics.

9.5.9 Cereal granulate (Fig. 9.9) An extrusion process is used to obtain this loose-fill insulation material from fine rye grains, rye pulp and other additives – especially to introduce the needed resistance to mould, insects and rodents. The product is used in particular when cavity walls in wooden constructions have to be insulated.

9.5.10 Reeds (Fig. 9.10) The stalks of reed plants are positioned in a great number, pressed slighty and then bound together with the help of iron wire or nylon threads. Further



9.8 Flax (Source: FNR, Gülzow, Germany).

9.9 Cereal granulate (Source: FIW München).


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9.10 Reeds (Source: FIW München).

9.11 Coconut fibres (Source: FIW München).

additives are not used. However, the product is only used in regions where there is a tradition of employing this material.

9.5.11 Coconut fibres (Fig. 9.11) The fibres of the coconut shell are used to produce this insulating material; for 1 m3 of product, roughly 700 to 1600 coconuts are needed. Additives are employed to give the product flame retardant properties.



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9.5.12 Wood chippings (Fig. 9.12) This insulating material is usually part of an industrial process where prefabricated wooden houses are being manufactured. The waste chippings are impregnated with additives to give them a fire retardant property – and then blown as loose-fill into, e.g., cavity walls; some compaction is required.

9.5.13 Straw bales (Fig. 9.13) As a by-product of agricultural production this raw material is readily available. After cutting the straw to the right length and reducing the moisture content, the product can be used as a loose-fill material or – similar to reed – converted into batts of rather thick proportions.

9.6

Reference buildings

Although many of the above-mentioned materials have been used for generations in different climate conditions – often as traditional building material – only recently can reference objects be cited where the use is extensively documented, especially under energy-saving criteria. The ‘Agency for Renewable Raw Materials’ in Germany has recently published a list of reference objects – although currently only available in German [8]. Three examples are described in more detail in this section –

9.12 Wood chippings (Source: FIW München).


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9.13 Straw bales (Source: FIW München).

more extensive literature, e.g. describing the use of straw bales, can be found elsewhere [9].

9.6.1 Example 1: ‘Werlte Climate Centre’ (Fig. 9.14) This example describes the reconstruction of former barracks into a model house for renewable energy and renewable resources within the perimeters of the ‘Perspective 50Plus’ project and was funded by the Federal Ministry for Labor and Social Affairs (BMAS) of Germany. From 1968 to 2005 the former company building was part of the ‘Hümmling barracks’. The reconstruction to a model house took place within the parameters of an employment project with simultaneous qualification for elderly unemployed. The building, with a usable area of approx. 2000 m 2, was redeveloped while taking aspects of energy saving into consideration and using renewable resources. Heating has since changed to vegetable oil (communal heating/power station), and also supplies the neighbouring office building. The climate centre became a reference facility with role model character. Several isolating façade systems are an example of the energetic refurbishment of an old building with renewable resources. These are unique



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9.14 Werlte Climate Centre (Source: www.3-m.info, Emsland, Germany).

nationwide, equipped with over 50 sensors and supplying continual data regarding the temperature and humidity. The roof is provided half with insulation between the rafters and insulation of the loft. Good insulation and a healthy room climate are also prominent in the selection of windows and flooring of wood, linoleum and tiles. Approx. 99 tons of CO2 and 1600 litres of heating oil are saved every year as a result of improved insulation. The climate centre was launched in 2008. The responsible bodies are the administrative district of Emsland and the community of Werlte (Germany). With its exhibition areas and the information it provides, the climate centre has become a forum for companies, consumers and research. Several exhibition areas present modern bio energy technology and regenerative energy efficiency. More than 30 exhibitors present their products and services here. Construction date: 2006–2008 Energy consumption: After redevelopment: a saving of approx. 99 tons of CO2 and 1600 litres of heating oil per year. Exterior wall 10 different façade systems with insulation construction: materials from renewable resources (wood fibre, cellulose and meadow grass) are available for practice testing and data gathering. Example for façade 1 (from inside to outside): 1.5 cm plaster


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Floor construction:

Roof construction: Surface treatment: Heating method: Solar industrial water heating: Photovoltaic: Rainwater use:

24.0 cm lime sand bricks 12.0 cm wood fiber boards 2.0 cm building slab HD plastered Insulation of the roof with different variations Partition 1: Insulation of the loft with piled insulation from sea grass and cellulose. Construction: OSB sheathing (19 mm thick) lie on upright cardboard sleeves (40 cm high) as the loadbearing component. Cavities and sleeves are filled with the insulation material. Partition 2: Under rafters insulation with various products: 1. Sector: 40 cm cellulose, 2. Sector: 40 cm meadow grass, 3. Sector: 40 cm perlite (the rafter thickness is predetermined by the old building) Walls and floors: Installation of wood floors, linoleum and tiles and the application of ecological paints. Adjacent heating network (communal heating/power station) with vegetable oil (test facility with pellet and woodchip heating for demonstration purposes). no yes no

9.6.2 Example 2: Kindergarten in Döbeln Directly next to the Jakobikirche in Döbeln, the Evangelist-Lutheran church parish built a new kindergarten. Similar to a convent, the St. Florian kindergarten forms a constructional unit with the church; a cloister with an inner courtyard joins the buildings together. After the 2002 Mulde flood the replacement new building had become necessary. The modern colourful wood construction stands self-confidently next to the 19th century church; both mutually enhance each other in their effect. You enter the single story kindergarten from the street, then your eyes continually wander from the foyer through the large glass façade to the northerly situated church. The carmine red glazed loam wall with built-in cloakrooms separates the hallway from the southerly situated group rooms. Via large doors with glass side sections, the children enter their play rooms with their own wooden galleries and bathrooms. The 68 day nursery and kindergarten children can go directly into the garden from their four group rooms. Everywhere you can detect the wonderful smell of wood and loam; this is thanks to the construction method



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with healthy building materials and the controlled ventilation. This system is located visibly behind a glass wall over the northerly situated adjoining rooms. The concept of the ecological passive house was very well adopted by the building owners and the children. Construction date: Size:

April 2004–March 2005 554 m 2 (3172 m 3 ), kindergarten for 68 children Energy consumption: 15 kWh/(m2a) Total costs: 71.3 m Exterior wall construction: 2 cm loam rendering on 70 stem reed panels 2 cm OSB sheathing as bracing wood panel 35 cm cellulose insulation between wood double T-sections 4 cm soft wood fibre boards 3 cm rear ventilation 2.4 cm horizontal larch wood boarding Floor/ceiling construction: 2.4 cm industrial parquet 5 cm cement screed 12 cm heat insulation 25 cm floor boards 12 cm perimeter insulation from XPS Roof construction: 2 cm OSB sheathing as bracing panel 35 cm cellulose insulation between wood double T-sections 4 cm soft wood fibre boards 12 cm rear ventilation space with ventilating rafters 3 cm planed and edged plank boards 0.5 cm roof sealing 6 cm green roof Wall and floor surfaces All interior walls are masonry loam walls treatment: with loam rendering on 70 stem reed panels and loam rendering after that glazed with plant pigments Energetic concept: Heating method: Passive house, no individual heating method since it is attached to the church Solar industrial water heating: yes Photovoltaic: no Rainwater usage: yes Purification plant: no


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9.6.3 Example 3: ‘Passive house’ in Esslingen The property is situated in a new construction area on the outskirts of Esslingen/Neckar (Germany) with a very rigid development plan. Ground area, construction window, number of stories and flat roofs are the guidelines. Flanked by the access road on the north side, the property falls away slightly towards the south. The construction company wanted a four person residential family dwelling that should be constructed as a passive house from ecological and sustainable materials. The building complies with the development plan and was built as a simple cube. The dwelling’s rooms face to the south and their large window areas contribute towards an optimal usage of solar energy. The north and east façades are very limited in their openings, a narrow staircase window serves as a connecting component between the floors. An approved penthouse floor as well as filter and transition zones between inner and outer rooms can be added on at a later date. Enclosures walls of the lower floor are constructed with water impermeable concrete due to the risk of water damage from the slopes and backwater. The staircase walls act as a system boundary between heated and unheated areas and are constructed with a highly insulated 30 cm strong porous concrete with an additional 18 cm of XPS. Heat bridges in the concrete ceiling are consequently prevented by the insertion of Isokorbs. The walls, ceiling and roof are constructed in wood frame design and are prepared complete in the joiner’s workshop. Straw bales as insulation material were built into the stud cavities and plated, the preconstruction was then transferred to the construction site by a truck crane. In this way the shell could be constructed on site and made rainproof within two days. Thus deviating from the supervisory construction approval, the approval was obtained individually. In regard to the building materials, attention is paid to a sensible and sustainable selection. In particular, regarding this, the straw bale insulation contributes significantly. The straw bales are delivered directly to the joiner’s workshop from the producer by tractor and are therefore, in production, to be appraised with the least primary energy costs. The roof is to be ‘greened’ with a 35 l/m2 storage capable structure. Surplus water is collected in a 3 m3 cistern. The building is to be designed and constructed as a passive house, that is to say, heat loss via the shell surfaces is minimized. The solar energy supplied by the large surface glazing is retained in the house and is transferred via a ventilation system with heat recovery to fresh air. Heating, ventilation and hot water preparation is undertaken by a passive house compact appliance that provides energy with an extracted air heat pump. Construction date: Size: Energy consumption:

2008/2009 15 kWh/(m2a) heating requirements



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Exterior wall construction: 12 mm plaster fibre board 50 mm installation layer, soft wood fibre boards 15 mm OSB sheathing 400 mm insulated studs (stud cavities filled with straw bale insulation) DWD 16 mm, UK 24 mm Larch form boards 24 mm Floor/ceiling construction: 16 mm parquet 50 mm tile floor finish 300 mm EPS 030 insulation Roof construction: 12 mm plaster fibre board 24 mm formwork with replaceable faces Variable vapour barrier 450 mm insulated studs (stud cavities filled with wood fibre injected insulation) Wall and floor surfaces treatment: Energetic concept: Heating method: heating and cooling via compact appliance, air fan heating Photovoltaic: no Rainwater usage: yes Purification plant: no

9.7

Summary

Insulating materials from natural organic raw materials have gained a small (4–6%) but growing market share in Europe in the last fifteen years. On the one hand, this consumer acceptance is based on the fulfilment of regulatory requirements, i.e. conformity with recently published European standards and/or the issue of European Technical Approvals. On the other hand, the growing awareness of the public towards ecological performance of building materials has given niche-type products from natural sources a certain boost, and increasingly recently-erected – often called ‘green buildings’ or ‘ecobuildings’ – can be found throughout Europe. Since most of these products are fairly new to the market, a careful monitoring of the physical performance, especially with respect to building properties, is advisable.

9.8

References

1. Danner, H., Comparison of ecological insulating materials, Munich, www.muenchen. de/bauzentrum (2008). © Woodhead Publishing Limited, 2010

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2. EN 13168: 2001-05 Thermal insulation products for buildings – factory made wood wool (WW) products – specification. 3. EN 13170: 2001-05 Thermal insulation products for buildings – factory made products of expanded cork (ICB) – specification. 4. EN 13171: 2001-05 Thermal insulating products for buildings – factory made wood fibre (WF) products – specification. 5. Pfundstein, M., Gellert, R., Spitzner, M.H. and Rudolphi, A., ‘Insulating materials’, Edition Detail, Institut für internationale Architektur – Dokumentation GmbH & Co. KG, Munich, Germany (2007); English version: distributed by Birkhäuser – Publishers for Architecture, Basel, Switzerland (2008). 6. DIN V 4108-4: 2004-07 Thermal insulation and energy economy in buildings – Part 4: Hygrothermal design values. 7. DIN V 4108: 2004-06 Thermal insulation and energy economy in buildings – application-related requirements for thermal insulation materials – Part 10: Factory made products 8. www.natur-baustoffe.info/index (in German only) 9. Jones, B., Building with Straw Bales: A Practical Guide for the UK and Ireland, Green Books (2009).


10

Polymeric foam materials for insulation in buildings D. F e l d m a n, Concordia University, Canada

Abstract: This chapter discusses polymeric foams used mainly for building insulation with a view to saving energy. It deals first with a brief foam history, the necessary materials for foam production, the polymers and the foaming (blowing) agents, and the foaming mechanism. It continues with the type of processing polymers for foam production and underlines the thermoplastic and thermosetting foams manufactured for the construction industry. It ends with foam main properties and future trends in the field of polymeric insulation materials. Key words: polymers, foam processing, thermoplastics and thermosettings, properties, future trends.

10.1

Introduction

Synthetic polymers are the raw materials used to produce plastics, synthetic fibers, elastomers, adhesives, sealants, etc. They are also important in the foam industry. Polymer foams that are used mainly for thermal and acoustical insulation of buildings are also known as cellular polymers or cellular plastics; they are multiphase material systems (composites) that consist of a polymer matrix and a fluid phase which is usually a gas. Thermal insulation has and will have a major role in energy savings in buildings. The history of the science and technology of synthetic foams can be traced from the late 1920s with the latex foam. In the 1930s phenol-formaldehyde and urea-formaldehyde (1933) foams were developed. The first patents for cellular polystyrene (PS) were obtained in Sweden (1931) and the USA (1935). The rigid polyurethane (PU) foams were developed in Germany during the early 1940s. Polyisocyanurate (PIR) (1966), polyethylene (PE), poly(vinyl chloride) PVC, polycarbonate (PC), polyethylene terephthalate (PET), ABS (acrylonitrile–butadiene–styrene) terpolymer, modified polyphenilene oxide and polyimide were produced later. Polymer foams have entered the construction industry as a direct replacement of conventional thermal insulating materials such as asbestos, glass fiber, mineral fiber, cellulosic fiber, perlite, vermiculite, etc. If necessary, some foams can contain additional reinforcement, such as fiber reinforced ones, or syntactic foams that have glass, ceramic or polymer microspheres. The 257 © Woodhead Publishing Limited, 2010

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US market in 2006 was more than 4 million tons for polymer foams; PU occupies the largest market share (53%) in terms of amount utilised, while PS is the second (26%) (Lee et al., 2005).

10.2

Foams classification, materials and foaming mechanism

Foams are made of thermoplastic or thermosetting polymers. They have been classified following different criteria such as: ∑

The kind of polymer from which the matrix is made (PU, PS, PVC, etc.). ∑ Cellular morphology (open or closed cells). The two morphologies can coexist so that a polymer foam is not always completely open or closed cell. From the point of view of the cell size polymer foams are classified as macrocellular (>100 mm), microcellular (1–100 mm), ultramicrocellular (0.1–1 mm) or nanocellular (0.1–100 nm) (Lee et al., 2005). The microcellular structure can be achieved by rapid depressurization of the polymer–blowing agent system, to allow the cells nucleation and growth as in the batch process after saturating the polymer with blowing agents (Zhang et al., 2007). ∑ Mechanical behavior (flexible or rigid); the rigid foams can be subdivided into non-structural, applied as thermal insulation, or structural which require high stiffness, strength and impact resistance. ∑ Density: low (10–30 kg/m3); medium (50–350 kg/m3), high (350–900 kg/m3).

10.2.1 Blowing agents Physical or chemical blowing agents are used in the manufacture of foams. The physical ones are compressed gases and volatile liquids such as N2, CO2, hydrocarbons, ketones and alcohols. The chemical ones are generally solid organic or minerals which decompose in a certain temperature range. A pressure-quench technique is used to obtain foams by using supercritical CO2 (scCO2) (Goel and Beckman, 1994). N2, air, CO2 or a mixture of air and helium are examples of gaseous blowing agents. Among these N2 and air are preferred since they are inert, non-toxic, non-flammable and have a low diffusivity with respect to the majority of polymers. Although N2 can be used as a blowing agent in the polymer foaming process, most publications on this topic address the foaming of polymers using CO2 because this also affects some polymer properties thereby enhancing polymer processability (Michaeli & Heinz, 2002). However, both N2 and CO2 are considered to be sustainable alternatives for the replacement



259

of the currently used blowing agents (Jacobs et al., 2008). The foaming of PS and cellulose acetate, for instance, has been studied using a technique based on the saturation of the polymer by scCO2 and rapid decompression. The diameter of the resulting microcellular structures has been controlled manipulating the process parameters (Reverchon and Cardea, 2007). Volatile liquids with boiling points below 110 °C like aliphatic hydrocarbons (C5–C7) are useful physical blowing agents. Ideal agents are the halogenated aliphatic hydrocarbons which, in addition to the above-mentioned characteristics, have a very low thermal conductivity. However, increases in chlorofluorocarbon (CFC) concentrations in the upper atmosphere would lead to long-term damage to the ozone layer. Global attention drawn to this led to the signing of the Montreal protocol in 1987. CFCs will be banned by 2010 according to this protocol. Recent measurements of the atmospheric chlorine content indicate that the equivalent effective chlorine content in the northern troposphere has been decreasing. In the rigid foam applications the alternative blowing agents are hydrochlorofluorocarbons (HCFC) with low ozone depletion potential (ODP), hydrofluorocarbons (HFC) and hydrocarbons with zero ODP. Comparing the ODP of CFC-11 (CFCl3) or CFC-12 (CF2Cl2) having ODP = 1.0 with HCFC123 (CF3CHCl2) which has 0.02–0.06 ODP and HCFC-124 (CF3CHClF) with ODP = 0.02–0.04, it is clear why the former are being replaced (Vachon, 2001). Blending small quantities of CO2 with HFC-245fa turned out to be beneficial for foaming and decreased the foam density further than using HFC-245fa alone (Vachon, 2005). The development of the next generation (zero ODP) blowing agents has been underway for several years (Wu et al., 1999; Bogdan et al., 1999; Zipfel et al., 1999; Andrady, 2003). The most important chemical blowing agents are ammonium bicarbonate (decomposition temperature 60 °C), sodium bicarbonate (decomposition temperature interval 100–140 °C), and sodium borohydrate (decomposition temperature 300 °C). Water, in some cases, can be also used as a blowing agent (Niyogi et al., 1999; Rizvi et al., 2000). Most of the known organic blowing agents belong to: azo and diazo compounds, N-nitroso compounds, sulfonylhydrazides, azides, triazines, triazols, tetrazols, sulfonyl semicarbazides, urea derivatives, guanidine derivatives and esters (Shutov, 1991; Seymour, 1991).

10.2.2 Foaming mechanism The foaming mechanism and the process of cellular structure formation can be mechanical, physical or chemical. In the first case the bubbles are created by stirring a gas into the mass of matrix components or by forcing the gas into the melted polymer matrix. In the physical process, known as nucleation, heating produces the evaporation of a low boiling liquid, thus forming the


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bubbles. In chemical foaming, the blowing agent reacts under the influence of heat, releasing a gas, which forms the voids in the melted polymer beads obtained through suspension polymerization (Michaeli et al., 1992). The blowing agent expands the molten polymer initiating cells that produce the foam. The main requirements for the blowing agent are the amount, solubility and diffusivity (Rosato, 1997). The stability of the foam is conditioned by the solubility and diffusivity of the gas in the matrix. The growth of the cell depends on the pressure difference across the cell membrane between the inside of the cell and the surrounding medium. Such pressure differences can be generated by lowering the external pressure (decompression) or by increasing the internal pressure in the cells (pressure generation). The cells that grow produce the foam. A complete dissolution of the blowing agent in the polymer is essential for producing foams with uniform cells. An incomplete dissolution leads to heterogeneous structure and surface defects. Processing aspects of some polymers like PS were correlated to off-line solubility and diffusivity of HFC-134a (Gedron et al., 2002). New phase formation known as nucleation can originate from the selfstructural adjustment or from ‘foreign’ seeds known as nucleants or nucleation agents. The bubbles are said to form by a nucleation process (Saunders and Klempner, 2004). Foaming basically involves bubble nucleation and bubble growth (phase separation) to make a foamed product that can be defined as visible gas cells dispersed in a denser continuum matrix (Lee, 2000). Foam nucleation, foam growth and cell coalescence are the three major events in the foaming process. Almost all foam technologies use nucleating agents in one form or another. The main role of a nucleant is to provide surfaces on which bubbles can organize and grow. There are two types of nucleants. Passive nucleants (mineral or organic) provide nucleating microvoids around which microbubbles can form. In addition to sites for bubble nucleation, active nucleants provide blowing gas to promote microbubble formation (Throne, 2004). The required amount of nucleating agent is determined by the polymer melt viscosity and the foaming technique and varies between 0.05 and 1%. The bubble growth is influenced by the concentration-dependent blowing agent diffusion coefficient, the transient cooling of the expanding polymer, the influence of the blowing agent on its viscosity and the escape of the blowing agent from the surface of the foam (Ramesh, 2000, Lee et al., 2007). The presence of sufficient blowing agent is very important to support efficient cell growth, and a certain solubility is necessary to achieve low foam density. Both solubility and diffusivity of the blowing agent in the melted matrix are key parameters in foam processing because they are the limiting factors in determining the density and also in regulating the phase separation dynamics during bubble growth (Talibouet, 2005). Diffusivity affects nucleation and bubble growth. It depends on several



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factors including molecular size and the solubility of the diffusing gas in the polymer. The diffusion constants for CO2, O2 and N2 are higher than that of CFCl3 (Backus, 1991). A high diffusivity favors a rapid nucleation and fine cell structure formation. Smaller molecular blowing agents such as CO2 are more suited to generating foams with high cell density and high foam density, while fluorocarbons and hydrocarbons having larger molecules are convenient for the production of low-density foams. (Nawaby and Zhang, 2005). Initially, cells of fresh foams contain the higher amount of blowing agent and the lower amount of air components. During the service life, the cell gas composition changes because of air ingress, solubility of the blowing agent in the polymer and loss of it to the environment. Ultimately (during ageing) most the blowing agent is replaced by air and the foam insulating efficiency attains its final steady level (Bomberg and Kumaran, 1995). The presence of nanoclays led to foams with different thermal and rheological properties that affect nucleation process and growth phenomena of gas bubbles. X-ray diffraction was performed to evaluate the dispersion of the silicate layers in the poly–e-caprolactam matrix. Rheological tests were performed in shear at 80 °C in order to analyse the effect of composition on the complex viscosity and their elastic and dissipative components (Iannace et al., 2003). The PS/1% Montmorillonite-layered silicate (MLS) nanocomposites universally exhibited Tg values about 15 °C lower than those of pure PS. This suggests that the dispersion of the MLS in the PS at this concentration locally disturbed the entanglements of the macromolecules enough to ease the material to soften at lower temperature. The DSC endothermic peak was not affected by the presence of MLS (Strauss and D’Souza, 2004). The nanoparticles can significantly increase the melt viscosity (Lee et al., 2005). The role of melt viscosity in the nucleation process was explored (Feng and Bertelo, 2004). Rheological studies on uniaxial elongation flow were conducted for PP clay nanocomposites with higher amounts of clay; this was about one order higher in magnitude that that of PP with lower amounts of clay (Nam et al., 2002). The extremely fine dimensions and large surface area of nanoparticles provide much more intimate contact between the particles, polymer matrix and gas. Furthermore, a significantly higher effective particle concentration can be achieved at a low nominal particle concentration. Both could lead to improved nucleation efficiency. The effect of particle concentration on foam nucleation was investigated for different polymer foams. The surface chemistry of clay nanoparticles not only affects the particle dispersion but also has a tremendous effect on the nucleation efficiency in a polymer-clay-blowing agent system. Some modified nanoclays can covalently bond polymers like PS or poly(methyl methacrylate) on the clay surface via in situ polymerization (Lee et al., 2005). Light scattering and high speed X-ray scattering have been used for studying the nucleation and bubble growth (Francis et al., 2006). Scanning electron


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microscopy application in the case of PP/clay nanocomposites confirmed the formation of high cell density and cell sizes in the range of 30–120 mm. On the other hand transmission electron microscopy for the same system showed biaxial flow-induced alignment of clay particles along the cell boundary (Nam et al., 2002). The formation of nuclei in a viscous liquid followed by cell growth is a very complex mechanism, which is influenced by many parameters, such as temperature, viscosity, blowing agent concentration, depressurization rate and pressure drop. Despite significant research efforts, it will probably take a considerable time before the complete process is fully understood (Jacobs et al., 2008).

10.3

Processing technologies

The performance of the foam depends to a great extent on the type of base polymer, the type of blowing agent and the means of processing. Depending on the type of processing there are various combinations of polymer-blowing agent. As previously mentioned the last one expands in the melted polymer initiating cells that grow to produce the foam. Polymer foams are produced batch-wise or continuously. The principal processes are extrusion, injection molding and compression molding (Rosato, 1997). Thermoplastic foams are frequently obtained by extrusion. Extrusion and injection molding can be used to turn a great variety of thermoplastic polymers in foams. However, the production of thermosetting foams like PU can be controlled because the polymer can exhibit a great variety of states and properties.

10.3.1 Batch process Most commercial moldable foam bead items are produced by a batch process using a volatile organic blowing agent. In the case of polyolefin foam manufacturing, the polymer pellets are impregnated with the blowing agent in an autoclave. Expansion is accomplished either by an abrupt discharge of the beads into the atmosphere or by heating the beads with steam (Park, 1991). In a batch process foams are produced by charging a polymer in a pressure vessel (autoclave) with the blowing agent, usually N2 or CO2, at constant pressure until a sorption equilibrium is reached. The polymer can be expanded then either by rapid decompression of a heated pressure vessel or by submitting the solid saturated material to a higher temperature environment (Vachon, 2005). Foaming of PC has been carried out mainly using the batch process at lower temperature than in the case of extrusion. As a result the foams’, properties are different, for instance the relaxation time is longer (Gedron, 2005). Using this process an intercalated PC/clay foam nanocomposite having



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different amounts of clay has been obtained using scCO2 as a blowing agent (Ito et al., 2006). The foam batch processing of neat polylactide (PLA) and two different types of PLA-based nanocomposites was carried out using scCO2 as a foaming agent (Yu et al., 2006). Polyurethane/montmorillonite nanocomposites were obtained with organically modified layered silicates (organoclays) by in situ polymerization and foams were prepared by a batch process. The presence of clay results in an increase in cell density and a reduction of cell size compared to pure polyurethane foam (Cao et al., 2005).

10.3.2 Extrusion The vast majority of thermoplastic rigid foams for construction insulation are produced by extrusion that permits the fabrication of sheets and blocks of varying cross-sections. The primary purpose of the extruder is to take room-temperature polymer resin in the form of pellets, beads or powders and convert the resin to a molten plastic at sufficiently high pressure to allow the high viscous melt to be forced through a nozzle into a mold in the case of injection-type processes, or through a die in the case of blow molding or continuous extrusion. This process involves several steps: dissolution of the blowing agent, the melting (plasticization) of the polymer, the nucleation of the bubbles upon pressure release, their growth, and cell stabilization. The gas that is formed by a decomposition reaction of the chemical blowing agent dissolves in the polymer melt because of the pressure level in the extruder. When leaving the die the pressure drops to atmospheric level, the melt becomes oversaturated with gas and bubbles are formed (Wahlen, 2006). The operating conditions are pressure, temperature and shear rate. If they are not adequate for the selected blowing agent, premature foaming occurs and a poor quality foam is produced. In the extrusion process for producing a cellular foam, such as PS, a solution of the blowing agent is introduced in the molten polymer (formed in an extruder under pressure); this is forced through an orifice onto a moving belt at ambient temperature and pressure. The polymer simultaneously expands and cools under conditions that give enough strength to maintain dimensional stability at the time corresponding to optimum expansion. The cooling of the polymer to a temperature below its glass transition temperature (Tg) provides its stabilization. Cooling is effected by vaporization of the blowing agent, gas expansion and heat loss to the environment. In the case of PS processing, this technology leads to foams having a density in the range of 27–53 kg/ m3 and even higher (Suh, 1991). The basic extrusion processing techniques utilize one of: ∑

a single screw with large diameter extruder


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∑ twin screw extruder ∑ a single screw extruder with a cooling system ∑ two extruders in tandem. Of the four, the last process is the most successful and widely used. The first extruder is used for melting, mixing, gas injection (usually N2) and feeding the extrudate to the secondary extruder. This is used for cooling and additional mixing before it exits a circular or annular die, where the foaming action is initiated. The secondary extruder is set up to ensure a uniform temperature before the exit of the melt, thus a uniform cell structure is obtained (Rosato, 1997). The shaping die is attached to the exit of the second extruder (Throne, 2004). A proper die design plays a crucial role in improving the quality of the foams, since the geometry of the die determines the pressure decay rate and absolute pressure drop, by which both cell density and cell morphology are dominantly affected. In some manufacturing processes a preliminary partial gazeification of the polymer beads is applied. The processing techniques used for rigid PU foams for insulation can also be used for PIR, including laminate processing, pour-in-place processing and spray processing (Ashida, 1991). The largest volume of rigid PU foams is produced in sheet form on machines known as laminates that are essentially double conveyors, between which foam rises to a controlled thickness. The coated sheet products obtained on laminators are widely used for construction applications as roofing or facing for frame construction (Backus, 1991). Studies done on LDPE extrusion with different nucleating agents and pressure profiles show the importance of high pressure gradients on the homogeneity of celled foams (Walter and Robert, 2002).

10.3.3 Injection molding This kind of processing uses screw preplasticizing injection molding equipment capable of supplying blowing agent, plasticity, kneading and maintaining retention time for gas/polymer solution. Some main parameters like injection velocity, molding temperature, retention time, plunger stroke and dwelling pressure were studied in the case of microcellular PS. It was found that cell size decreases and cell density increases with an increase in injection velocity. Cell size decreases about linearly with an increase in velocity and becomes about half size for 5 times of velocity (Hidetaka and Minoru, 2003). The molding time depends on the type of polymer as well as on the size and shape of the foam item. The two major molding techniques used in the PU industry are the hot cure and the cold cure (or high resiliency) processes. The latter uses high reactive components (polyols) so that less curing (energy) is needed (Bicerno et al., 2004).



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A new foam molding technology is able to generate single-phase polymer/ blowing agent solutions and is therefore capable of producing parts with smaller cell sizes, and more uniform cell size distributions (Xu et al., 2008). Foams can also be obtained by a casting and leaching technique which consists of dissolving the polymer in a highly volatile solvent and casting the solution in a mold containing a solid porogen (NaCl, KCl). After the solvent evaporation, the foam is washed with water to eliminate the salt. The technique has the advantage of controlling the morphology and pore size. An injection molded foam consists of compact skin layers and a cellular core; the former dominate the mechanical behavior. The thicker the skin layers the higher the mechanical values. For injection-molded foams, weight, tensile strength, flexural strength and stiffness decrease with increasing of the main process parameters such as melt temperature, mold temperature and injection velocity, whereas they increase with increasing back pressure (Chien et al., 2004). The mold temperature can affect the foam surface (skin) degradation. A recent study presents some possibilities to control the morphology of structural foams by an intelligent mold and process design. Parameters affecting the morphology of the foam like temperature, cavity pressure and the expansion ratio were varied. Structural properties of the foam like surface finish, overall density and skin layer thickness as well as mechanical properties have been examined (Spoerrer and Alstaedt, 2007). Other researchers (Guo et al., 2007) found that the cell structure (cell density and dimension), an important aspect of morphology, could be improved greatly by optimizing key parameters such as shot size, blowing agent amount, back pressure, injection speed and melt temperature.

10.4

Thermoplastic and thermosetting foams

The most used thermoplastic foams are made of polymers like PS, PVC, PE, PP, ABS, PET, cellulose acetate. Among these the construction industry uses mostly the first two. Rigid closed cell PS for the construction industry with densities between 16 and 180 kg/m3 are obtained at around 120 °C from expandable PS beads (produced by suspension polymerization). Extrusion, which takes place at up to 260 °C, leads to the production of PS foams with densities 16–80 kg/m3. The extruded foam has a more regular structure than the molded one, better strength and higher water resistance. PS foam is affected by temperatures close to its Tg, i.e. in the range of 71–77 °C. In the construction industry, PS foams are used mainly for perimeter insulation, roof deck insulation and masonry wall insulation (Feldman, 2005). PVC can be foamed through free expansion or constrained, known as the Celuka process (Patterson, 2002), with or without plasticizer for achieving flexible or rigid characteristics.


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Some of the best advantages of PVC rigid foams are: low volume cost, good mechanical properties (high tensile, compressive and shear strength), good chemical and weather stability, low thermal conductivity, low water permeability, resistance to termite and bacterial growth, good fire resistance and they do not crumble under impact or vibration (Feldman, 2005). Rigid PU foam is widely applied for thermal insulation purposes in the building and construction industry where it is available as foam board, sandwich panels, sprays, aerosol cans and shaped and molded foams. For their manufacture, PIR technology is frequently applied in addition to PU technology. Both PU and PIR construction foams are characterized by their excellent thermal conductivity with l between 20 and 30 mW/mK, low densities ranging from 30 to 100 kg/m3 at high mechanical strengths of 10–100 kPa. They can be used over a temperature range from –80 to +100 °C. The very low thermal conductivity enables application of thinner panels when space filling is an issue. Spray PU foams combine thermal insulation, filling, mechanical strength and adhesion in a very cost-effective way. Foam panels of up to 20 cm thickness are produced continuously at a double band laminator. Depending on the type of faces (films, paper, aluminum foil, wood) on the panel, we can distinguish flexible-faced or rigid-faced foam panels (Grunbauer et al., 2004).

10.4.1 Properties Density The properties of polymer foams are strongly related to their density or, in other terms, to the relative amounts of the polymer and gas phases. Many properties (E) are related to density (r) by a power function of general form

E = Krn

10.1

where constant n is normally in the range 1 to 2, and K is related to both polymer characteristics and temperature (Backus, 1991). Some density values of polymer foam are: ∑ ∑ ∑ ∑

PS extruded plank 35–53 kg/m3 (Suh, 1991) PS expanded plank 16–80 kg/m3 (Suh, 1991) PS extruded sheet 96–180 kg/m3 (Suh, 1991) PE extruded sheet 27–40 kg/m3 (Park, 1991).

Thermal conductivity Thermal conductivity l of polymer foams is the sum of the thermal conductivities of the solid phase, lp, and of the gas, lg, convective, lc, and radiative, lr components:



l = l p + l g + l c + l r

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10.2

In the construction industry, K factor term is preferred instead of thermal conductivity. One of the main requirements of insulating materials is a low thermal conductivity. Here are some examples of the values of this characteristic for polymer foams used in the construction industry. ∑ ∑ ∑ ∑ ∑ ∑

Rigid PU foam 0.014–0.016 W/mK (Backus, 2004) PIR rigid foam 0.018–0.020 W/mK (Ashida, 1991) PS extruded plank 0.030 W/mK (Suh, 1991) PS expanded plank 0.035–0.037 W/mK (Suh, 1991) PS extruded sheet 0.035 W/mK (Suh, 1991) PE extruded plank 0.056–0.089 W/mK (Park, 1991)

Some studies demonstrate the ability to calculate thermal conductivity and elastic properties on three-dimensional images of industrial polymer foams. Obtained data are well defined over a broad range of phase fractions by structure–property correlation which differ from common theoretical and recently derived empirical bounds. Results for thermal conductivity and elasticity show little scatter and provide the basis for accurate empirical correlation between phase fraction and properties. They highlight an exciting potential to predict physical properties from images of complex materials and the future development of more accurate correlations between structure and mechanical/transport properties for microstructures (Saadatfar et al., 2004). Thermal conductivity is a time-independent physical property. The rate at which heat is transferred through foam is governed by thermal diffusivity a which is a time-dependent physical property with units m2/s. Thermal diffusivity is the ratio of thermal conductivity to the product of density and heat capacity. For high density foams, thermal diffusivity is nearly independent of foam density. For low density foams, thermal diffusivity increases nearly linearly with decreasing foam density (Throne, 2004). Mechanical properties Mechanical properties of the foams are poor. Any factor affecting melt rheology can influence cell formation during processing and hence the properties of the final foam. An important problem with polymer foams used as insulating materials is the decrease of mechanical strength with the increase of the size of the cells. The solution is the nanocomposite foams that contain nanoparticles. Additives such as montmorillonite, carbon nanofibers, spherical silica particles, polyhedral oligomeric silsequioxane nanocrystals (McCabe and Nutt, 2002), etc. (Jacobs et al., 2008) have been experimented with. Foams with hectorite nanoparticles exhibit improved thermal stability and mechanical properties when compared with neat polymer foams (Velasco et al., 2007).


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PS has a higher flexural strength in comparison with other thermoplastics. This indicates that at the same density it demonstrates a better rigidity. This characteristic, coupled with a fine cell structure, is ideal for thermal insulation application. For production control the average MW of PVC can be calculated from the Mark–Houvink–Sakurada equation expressed in terms of the Fikentscher K value or viscosity number. Commercially available PVC grades are supplied in Europe with K values ranging from 50–80. Increasing the MW of PVC polymer improves the mechanical properties of the foam by upgrading the solid material of cellular walls. The effect is greatest in rigid foams and at high densities. It is possible to produce rigid PVC foam with exceptional strength through a cross-linking reaction with maleic anhydride, isocyanate and a catalyst. The ingress of water and mainly of sea water and its damaging effects on polymeric foams were investigated and explained by a mechanics model. It has been shown that exposure to sea water damages the structure of cellular foams and degrades the delamination fracture toughness at the core/facing interfaces of sandwich layups. However, those detrimental effects are confined to the outer layers of the foams and the near vicinity of crack tips (Weitsman et al., 2005, Suh, 2004). A recent published review provides the current state of polymer foams biodegradation (Gautam et al., 2007). Flammability The acceptance of polymer foams in the construction industry led to the need to produce materials of low combustibility with a low smoke evolution during fires. Their low thermal inertia permits the surface to respond rapidly to any imposed heat flux and consequently to ignition. Addition of flame retardants is the most common route to improve the non-flammable behavior of foams. In commercial use are flame retardants based on phosphorus, halogen, mixture of halogen compounds, antimony oxide, N2 and boron compounds, alkali metal salts, and hydrates of metal oxides. PS foam can produce molten drops (especially in ceiling application) and in some cases these drips burn. However, the presence of bromine-containing flame retardant is able to delay the ignition of molten PS. In large fires PS foam burns with the generation of a dense smoke. If the foam is behind plaster or concrete facing, the extent of burning is usually limited to a small area near the original site of the fire (Irvine et al., 2000). PU foams do not melt in fire but burn to produce pyrolysis gases, dense smoke and some char. The rate of the burning depends on the type and amount of fire retardant present in the foam (Feldman, 2005). From the point of view of combustion, PIR is superior to PU. PIR behaves similarly to fire retardant PU foam in the early stages of a fire, but the char



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formation significantly restricts the spread of the fire. Recognition of the efficient fire barrier characteristics of glass fiber reinforced PIR foam chars has allowed new PIR formulations, even some without flame retardant. Such composites generate less smoke than PIR foam (Feldman, 2005). Technologies for manufacturing PU foams led to the use of reactive flame retardants able to provide a permanent effect. Polyols, isocyanates containing phosphorus, halogens (usually bromine) or both are the most used (Troitzsch, 1990).

10.5

Future trends

Future foam applications may emphasize structural characteristics with a limiting of application requiring primarily thermal insulation. Consideration of their full range of properties should lead to a greater variety of use. Novel nano-composite foams based on the combination of functional nano-particles and supercritical fluid foaming technology can lead to a new class of materials that are lightweight, have high strength, good barrier properties, improved dimensional and thermal stability, whilst maintaining the same thermal conductivity as PIR. For foam products, various desirable cell morphologies (e.g., small vs. large cells, open vs. closed cells) must be attainable through successful control of nucleation and growth of bubbles.

10.6


Gibson LJ and Ashley MF (1997), Cellular solids; structure and properties, 2nd edn, Cambridge, Cambridge University Press. Randall D and Lee S, eds. (2002), The polyurethanes book, New York, John Wiley. Mils NJ (2005), Plastics; microstructure and engineering applications, 3rd edn, London, Butterworth Heinemann. Eaves D (2004), Handbook of polymer foams, Shrewsbury, Rapra Technology Ltd. Muller N and Ehrenstein GW (2004), ‘Evaluation and modeling of injectionmolded rigid polypropylene integral foam’, J. Cell Plast. 40, 45–59. Rachtanapun P, Selke SE and Matuana LM (2004), ‘Relationship between cell morphology and impact strength of microcellular foamed high-density polyethylene/polypropylene blends’, Polym. Eng. Sci. 44, 1551–1560.

10.7

References

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Backus JK (1991), ‘Rigid polyurethane foams’, in Klempner D and Frisch KC, Polymer foams, Munich, Hanser, 74–93. Backus JK (2004), ‘Rigid polyurethane foams’, in Klempner D and Sendijarevic V, Polymeric foams and foams technology, 2nd edn, Munich, Hanser, 121–140. Bicerno J, Dawssin RD, Elwell MJA, van der Wal HR, Berthevas P, Brown M, Casati F, Farrissey W, Fosnaugh J, de Genova R, Herrington R, Hicks J, Hinze K, Hock K, Hunter D, Jeng L, Laycock D, Lidy W, Mispreuve H, Moore R, Nafziger L, Norton M, Parrish D, Priester R, Skaggs K, Stahler L, Sweet F, Thomas R, Turner G, Wiltz G, Woods T, Christenson CP and Schrock AK (2004), ‘Flexible polyurethane foams’ in Lee ST and Ramesh NS, Polymeric foams, Boca Raton, FL, CRC Press, 173–253. Bogdan MC, Parker RC and Williams DJ (1999), ‘Utilization of HFC–245fa in the construction industry’, J. Cell Plast, 35, 11. Bomberg M and Kumaran M (1995), ‘Procedures to predict long term thermal performance of boardstock foam insulations’, International report No. 694NRC-CNRC, National research Council of Canada, Ottawa, Ontario, Canada. Cao X, Lee LJ, Widya T and Mocosko C (2005), ‘Polyurethane/clay nanocomposites foams; processing, structure and properties’, Polymer, 46, 775–783. Chien RD, Chien SC, Lee PH and Huand JS (2004), ‘Study on the molding characteristics and mechanical properties of injection-molded foaming polypropylene parts’, J. Reinf. Plast. Comp., 23, 429–444. Feldman D (2005), ‘Plastics and polymer composites’ in Akovali G, Polymers in construction, Shrewsbury, Rapra Technologies Ltd., 237–301. Feng JJ and Bertelo CA (2004), ‘Prediction of bubble growth and size distribution in polymer foaming based on a new heterogeneous nucleation model’, J. Rheol. 48, 439. Francis TJ, Wassner E, Rieger J, Moreira A, Schiler P and Lopez P (2006), ‘Time resolve bubble nucleation in polymer foams’, Blowing agents and foaming processes 2006, 8th International Conference Munich, May 16–17 2006 P22/1–P22/8, Shrewsbury, Rapra Technology Ltd. Gautam R, Bassi AS and Yanful EK (2007), ‘A review of biodegradation of synthetic plastic and foams’, Appl. Biochem. and Biotechnol., 141, 85–108. Gedron R (2005), ‘Rheological behavior relevant to extrusion foaming’, in Gedron R (ed.), Thermoplastic foam processing, Boca Raton, FL, CRC Press, 43–104. Gedron R, Huneault M, Tatibouet J and Vachon C (2002), ‘Foam extrusion of PS blown with HFC-134a’, 4th International Conference proceedings: Blowing agents and foaming processes, Heidelberg, Germany, May 27–28, Shrewsbury, Rapra Technology Ltd, 97–111. Goel SK and Beckman E (1994), ‘Generation of microcellular polymeric foams using supercritical carbon dioxide. I: effect of pressure and temperature on nucleation’, J. Polym. Eng. Sci., 34, 1137. Grunbauer HJM, Bicerano J, Clavel P, Daussin RD, de Vos HA, Elwell MJ, Kawabata H, Kramer H, Latham DD, Martin CA, Moore SE, Obi BC, Parenti V, Schrock AK and van den Bosh R (2004), ‘Rigid polyurethane foams’, in Lee ST and Ramesh NS, Polymeric foams, Boca Raton, FL, CRC Press, 253–311. Guo MC, Heuzey MC and Carreau PJ (2007), ‘Cell structure and dynamic properties of injection molded polypropylene foams’, J. Polym. Eng. Sci., 47, 1070–1081. Hidetaka K and Minoru S (2003), ‘Effect of key process variables on microstructure of injection molded microcellular polystyrene foam’, Cell Polym., 22, 175–190.



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10.8

Appendix: Abbreviations

ABS Acrylonitrile–butadiene–styrene terpolymer



CFC chlorofluorocarbon CO2 carbon dioxide HFC hydrofluorocarbons HCFC hydrochlorofluorocarbons K factor reflecting PVC molecular weight KCl potassium chloride MLS Montmorillonite-layer silicate MW molecular weight N 2 nitrogen NaCl sodium chloride ODP ozone depletion potential PE polyethylene PC polycarbonate PET polyethyleneterephthalate PIR polyisocyanurate PLA polylactide PS polystyrene PU polyurethane scCO2 supercritical carbon dioxide T g glass transition temperature a thermal diffusivity l thermal conductivity r density


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11

Thermal insulation material for building equipment M. Z e i t l e r, FIW München, Germany

Abstract: This chapter discusses the characteristics of insulating materials and their usefulness as insulations for building equipment. The chapter first describes the bases of the heat transfer in insulating materials. Then the difference between the nominal values and the calculation values is discussed. All relevant standards for the characteristics and classifications for the insulating materials are pointed out. Key words: thermal conductivity of insulation materials, insulation system, fire behaviour, water vapour diffusion, attestation of conformity.

11.1

Introduction

Insulation materials need to have properties suitable for their application purpose. Insulation systems for industrial installations in technical building equipment need to cater for both the task of thermal as well as cold protection, and must occasionally even sustain changing temperatures. Insulation material manufacturers specify the area of applicability for their insulants as precisely as is needed and as generally as possible. To be able to guarantee an insulation system without deficiencies for a given application purpose, the thermal conductivity, the maximum service temperature and additionally, for employment in cold protection, the water vapour diffusion coefficient are considered the most important properties. Proof of behaviour in fires is compulsory. A possible release of dangerous substances, too, needs to be given proper attention. Insulation material properties are given as declared values by the manufacturer for the product as placed on the market. The declared thermal conductivity, which is an important determinant of market value, is also the starting point for heat protection calculations. Since the thermal conductivity of an insulant is temperature dependent and since it is susceptible to change through different influences dependent on the actual employment, the insulating quality of the system is determined through the so-called operational thermal This chapter is dedicated to Dr Walter F. Cammerer on the occasion of his 90th birthday.


Thermal insulation material for building equipment

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conductivity in connection with the insulation layer thickness and depending upon the service boundary conditions. The basis for the calculation of thermal and cold protection is the design value of thermal conductivity, which must be greater than or at least equal to the operational thermal conductivity of the insulation system under the given conditions. It follows that for the determination of heat losses and the related energy saving measures, it is not the declared thermal conductivity that should be used but the values actually occurring in practice. The properties influencing the performance of an insulation system are: ∑ temperature difference ∑ open joints ∑ employed apparent density ∑ convection ∑ moisture ∑ ageing ∑ IR permeability. These should be considered for the intended application, when the insulation system is designed. EN ISO 23993 [1] provides the most important conversion factors for the calculation. Dependent on the insulation material chosen, the insulating effect of the system is influenced through possibly required insulation system related components. The fire behaviour of the insulation material is always to be heeded, since it determines the applicability in the context of the size and intended purpose of the building (residential or public building, number of floors).

11.2

Insulation materials

11.2.1 Fundamentals Insulation materials are on the market in different forms of supply. Depending on the form of supply or on the basic material, they already in themselves constitute the insulation system for given application purposes. In general, however, they constitute a basic material, which is employed together with other materials in the insulation system and the properties of which are changed through the mounting, respectively the mode of application chosen. The expert design of an insulation system requires an appropriate knowledge of the materials used. In the sections below, therefore, next to special forms of supply, available dimensions and a variety of properties in a general form, the fundamental laws of heat transfer in porous substances have been described, to be able to assess the performance profile of an insulation material as placed on the market. This is an important condition


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for the design, so that the performance of the final insulation system meets the intended requirements. Basic materials and structures For the insulation of industrial installations in technical building equipment, insulants are available on the market, consisting of basic materials on inorganic or organic bases. They consist of a solid structure of minerals, such as stone, glass, calcium silicate, etc., or of plastic materials such as polyurethane, phenolic resin, polystyrene, polyvinylchloride and vinylrubber with a porous, a closed cellular, an open cellular or a fibrous structure. Forms of supply and dimensions Insulation materials come as dimensionally stable or as flexible products. Dimensionally stable products are boards and pipe sections. Flexible products are mats and felts. Form of supply and flexibility determine their application. They are available with or without facings and support material. Preferred dimensions depend upon form of supply and basic material. Pipe sections are available in lengths from 1000 mm to 1200 mm and in thicknesses up to 200 mm, depending on the inner diameter. Mats and felts are available up to a width of 1200 mm. The lengths depend on the declared thicknesses and lie between 2500 mm and 10000 mm. Declared thicknesses range from 30 mm to 120 mm in 10 mm steps. Boards are available in widths up to 625 mm and lengths up to 1250 mm. They are available in declared thicknesses up to 200 mm, normally also in 10 mm steps. Tubes and insulation sleeves are supplied in a length of 2000 mm and with declared thicknesses from 3 mm to 60 mm. Levels of declared thicknesses must be obtained from the manufacturer. In Table 11.1 the most readily available insulation products for the insulation of industrial installations in technical building equipment have been compiled. Insulation materials may also be produced in situ. Polyurethane in situ foam is an insulation material for flexible employment, preferably for cold protection. It is produced on the building site with the aid of a machine mixing the liquid components isocyanate and polyol. In the so-called pouring method, the liquid mixture is poured through holes into the preassembled cladding. The chemical reaction of isocyanate with the polyol produces heat. The blowing agents in the polyol component evaporate and blow the polyurethane (physical driving). The blowing agent becomes totally or partially the cellular gas. Where water is part of the polyol mixture, the water reacts with part of the isocyanate and generates CO2 which functions as driving gas. However, over time, it will diffuse out of the foam, because



277

Table 11.1 Readily available insulation products – material standards, forms of supply and package Insulation material (abbreviated term)/standard/ working document

Form of supply

Mineral wool Pipe sections (MW)/EN 14303 AGI Q 132 Lamella mats Wired mats Boards Felts Loose Fill

Description

Package

Pipe sections Individual of resin bonded packages glass- or rock or wool, with packaged or without in bales grid reinforced aluminium foil facing. Resin bonded glass- or rock wool lamella mat, onesided facing of grid supported aluminium foil

Rolls, bales

Mat made of mineral wool stitched to the wire mesh with galvanized or austenitic steel yarn

Rolls, bales

Boards made of Packages, resin bonded palettes glass- or rock wool Felts made of resin bonded glassor rock wool

Rolls

Loose glass- or rock wool with or without a minimal bonding component

Sacks


278


Table 11.1 Continued Insulation material (abbreviated term)/standard/ working document

Form of supply

Flexible Tubes elastomeric foam (FEF)/EN 14304/AGI Q 143-1 and Polyethylene foam (PEF) EN 14313/ AGI Q 134-1 Boards or laps Insulation sleeves Extruded Pipe sections polystyrene (XPS) EN 14307/ AGI Q 133-2 Expanded polystyrene (EPS) EN 14309/ AGI Q 133-1 Polyurethane Boards (PUR) and polyisocyanurate foam (PIR) EN 14308/ AGI Q 133-3 Phenolic foam (PF) EN 14314

Description

Package

Tubes of reticulated elastomeric foam or of reticulated or non-reticulated polyethylene foam

Packages

Boards made of resin bonded glassor rock wool

Packages, palettes

Pipe tubes with a non-concentric insulation layer made of nonreticulated or reticulated polyethylene foam

Packages

Pipe sections of Packages extruded/ expanded polystyrene/ polyurethane/ polyisocyanurate/ phenolic foam Boards of Packages extruded/ expanded polytyrene/ polyurethane/ polyisocyanurate/ phenolic foam



279

Table 11.1 Continued Insulation material (abbreviated term)/standard/ working document Cellular glass (CG) EN 14305/ AGI Q 137

Form of supply

Boards

Description

Package

Boards of cellular glass

Packages

Table 11.2 Employability as insulant for the different components of industrial installations in technical building equipment, dependent upon the form of supply Form of supply

Vessels flat

Boilers Pipes Ducts

Elbows

Fittings

round

Pipe sections (x) x x Segments Mats x x x x x x Boards x x Felts x x x x Tubes x x x Loose fill x x Granulates x x x x x PUR-in situ foam x x x x x

x

x x x

of its good solubility in polyurethane. But mineral insulants in the form of granulates, too, can be applied with blowing technique, filling cavities behind pre-assembled casings. Applicability and operational temperature range The applicability of the insulation material is determined through its form of supply too (Table 11.2). Its maximum service temperature and/or the fire behaviour, but also its resistance against water vapour diffusion or its mechanical properties, such as compression and tensile strength, determine the limits of its employability. The possible temperature application ranges are shown in Fig. 11.1. It must be observed, however, that for certain applications, use is only possible


280

Materials for energy efficiency and thermal comfort in buildings Flexible elastomeric foam (FEF) Extruded polystyrene foam (XPS) Rigid polyurethane foam (PUR) Polyisocyanurate foam (PIR)

Maximum service

Expanded polystyrene (EPS) Polyethylene foam (PEF) Phenolic foam (PF) Vacuum Insulation Panel (VIP) Cellular glass (CG) Mineral wool (MW) Calcium-magnesium-silicate (CMS) Microporous insulants Minimum service Calcium silicate (CS)

–180 °C

–20 °C

10 °C

100 °C

600 °C

1000 °C

11.1 Temperature application ranges for insulation materials.

in special constructions, and that the manufacturer information regarding the applicability must be heeded. This applies specifically to the use of open cellular insulation materials for a cold insulation. Mineral wool products, for example, may only be employed behind a double skin covering [2] if boundary conditions guarantee that the danger of condensation formation at the object or in the insulation material can be excluded. Important insulation material properties The thermal conductivity l of insulation materials has no constant value. For a suitable design for the area of application of industrial installations, its dependence upon the temperature and the apparent density must be known. In principle the thermal conductivity is a multi-dimensional function of the temperature J, the density r and the thickness s for all insulation materials. Figures 11.2 and 11.3 show these interdependences in a qualitative way. Depending on the apparent density, thermal conductivity increases progressively towards higher temperatures. The thermal conductivity of insulation materials describes a combined heat transfer by thermal conduction via the solid structure and the air plus the radiation exchange. The different portions have different effects depending on the amount of solid structure (apparent density). For example, with materials of apparent densities below roughly 30 kg/m3, the thermal conductivity increases proportionally with increasing temperatures, starting from a relative elevated level at 50 °C. With products of apparent densities above 60 kg/m3 the l;J curves show a slightly lesser dependence upon temperature. Figure 11.3 also shows that the thermal conductivity between raw densities of 40 kg/m3 and 50 kg/m3 shows a relative minimum at a temperature of 50 °C and increases again



281

350 in mW/(m.K) Thermal conductivity

300 250 200 150 100 50

1 Ap pa

25 re nt

50

0 0

50

35

50

de

ns

ity

in

45

0

kg

5 /m 3

50 65

15

0 75

0

0 r 25 pe m e T

55

0

4 0 C 35 in ° e r atu

0 50

11.2 Thermal conductivity of insulation materials depending on apparent density and temperature [1].

with increasing apparent densities. At higher temperatures this minimum moves towards higher apparent densities. The understanding of these interdependences is very important for the choice of the optimal product for an intended application. The dependence of thermal conductivity on thickness is of lesser importance compared to the temperature dependence and needs to be reflected in declared values given by the manufacturer according to the provisions of European material standards EN 14303:2009 [3], EN 14304:2009 [4], EN 14305:2009 [5], EN 14306:2009 [6], EN 14307:2009 [7], EN 14308:2009 [8], EN 14309:2009 [9], EN 14313:2009 [10] and EN 14314:2009 [11]. Principally, the so-called thickness effect is a result of the solid structure’s permeability for infrared (IR) radiation. It is very pronounced with insulation materials with apparent densities below 10 kg/m3 and decreases for insulation materials of higher apparent densities. With commercial insulation materials



W/(m•K tivity in

)

282

0.045

0.025

m

0.08

0.04

hi

0.02

nt

0.08 0.1

rt

0.06 yq uot ie

ye

nsit

La

App 0.04 are nt d e

ne

ss

in

0.06

0.02

m

0.01

ck

c l condu Therma

0.04 0.035 0.03

0

11.3 Thermal conductivity of insulants depending on apparent density and thickness.

of apparent densities above 60 kg/m3 the influence is already negligible for most applications. For the insulation of industrial installations in technical building equipment this effect may be significant with insulation materials made of polyethylene, polystyrene and with the so-called lamella mats made of mineral wool. Lamella mats are used as a multifunctional product for pipes as well as for vessels. Their operational thermal conductivity, therefore, will be dependent upon pipe diameter and insulation layer thickness because of the varying compression. The thermal conductivity is measured according to the following test standards dependent upon the thickness, the form of supply or the temperature range: Test standards for the determination of thermal conductivity lP for boards and mats: ∑ EN 12667:2001: Thermal performance of building materials and products – Determination of thermal resistance by means of guarded hot plate and heat flow meter methods – Products of high and medium thermal resistance. ∑ EN 12939:2000: Thermal performance of building materials and products – Determination of thermal resistance by means of guarded hot plate and heat flow meter methods – Thick products of high and medium thermal resistance.



∑

283

prEN15548-1:2006: Thermal insulation products for building equipment and industrial installations – Determination of thermal resistance by means of the guarded hot plate method – Part 1: Measurements at elevated temperatures from 100 °C to 850 °C.

Test standards for the determination of thermal conductivity lR for pipe sections and tubes: ∑ EN ISO 8497:1996: Thermal insulation – Determination of steady-state thermal transmission properties of thermal insulation for circular pipes (ISO 8497:1994) [12]. Fire behaviour The fire behaviour of insulation materials is determined according to EN 13501-1 [13] on the basis of test results obtained using the test standards listed below: ∑

EN ISO 1182:2002: Reaction for fire tests for building products – Non combustibility test. ∑ EN ISO 1716:2002. Reaction to fire tests for building products – Determination of the heat of combustion (ISO 1716:2002). ∑ EN ISO 9239-1:2009: Reaction to fire tests for floorings – Part 1: Determination of the burning behaviour using a radiant heat source (ISO/DIS 9239-1:2008). ∑ EN ISO 11925-2:2002: Reaction to fire tests for building products – Part 2: Ignitability when subjected to direct impingement of flame (ISO 11925-2:2002). ∑ EN 13238:2001: Reaction to fire tests for building products – Conditioning procedures and general rules for selection of substrates. ∑ EN 13823:2002: Reaction to fire tests for building products – Building products excluding floorings exposed to thermal attack by a single burning item. ∑ EN 15715:2007: Thermal insulation products – Instructions for mounting and fixing for reaction to fire testing – Factory made products. As for the determination of thermal conductivity, because of the provisions of EN 15715, the boundary conditions of the test influence the result of the fire behaviour test. According to EN 13501 the following classifications are possible: ∑

Class A1, A1L: Class A1 and A1L products will not contribute in any stage of the fire including the fully developed fire. For that reason they are assumed to be capable of satisfying automatically all requirements of all lower classes.


284

∑

∑ ∑

∑

∑ ∑


Class A2, A2L: Satisfying the same criteria as class B and BL for the EN 13823. In addition, under conditions of a fully developed fire these products will not significantly contribute to the fire load and fire growth. Class B, BL: As class C and CL but satisfying more stringent requirements. Class C, CL: As class D and DL but satisfying more stringent requirements. Additionally under thermal attack by a single burning item they have a limited lateral spread of flame. Class D, DL: Products satisfying criteria for class E and EL and capable of resisting, for a longer period, a small flame attack without substantial flame spread. In addition, they are also capable of undergoing thermal attack by a single burning item with sufficiently delayed and limited heat release. Class E, EL: Products capable of resisting, for a short period, a small flame attack without substantial flame spread. Class F, FL: Products for which no reaction to fire performances are determined or which cannot be classified in one of the classes A1, A2, B, C, D, E, A1L, A2L, BL, CL, D L, E L.

Additional classifications are possible regarding development of smoke and burning dripping. Additional classifications for the smoke development: ∑ ∑

s3 No limitation of smoke production required. s2 The total smoke production as well as the ratio of increase in smoke production are limited. ∑ s1 More stringent criteria than s2 are satisfied. Additional classifications for the burning dripping/falling ∑ ∑

d2 d1

∑

d0

No limitation. No flaming droplets/particles persisting longer than a given time occurred. No flaming droplets/particles occurred.

Especially in the tests according to EN 13823, the result of which is decisive for the classification into the classes A2, B, C and D, the products can frequently only be tested under end-use conditions, reflecting the applications. Products with fire behaviour classes A2, B, C and D have more or less the same fire behaviour in the condition as placed on the market and under application conditions. This applies to insulation products, e.g. tubes, which in themselves already constitute the insulation system but also where



285

the conditions of application are similar to those of the test and no other building components can change the fire behaviour after the application. In particular, fastening means, such as self-adhesive tapes, but also substrates (e.g. material of a pipe which is supposed to be insulated) influence the fire behaviour in the application (see pages 300 and 301, fire behaviour of the insulation system). Since not all possible applications and execution variants can be verified in tests, the limiting conditions of the test are given in EN 15715 for all standardized products for both the product as placed on the market, and the product under end-use conditions. For the declaration of conformity as precondition for the use of the CE mark, only the fire behaviour of the product as placed on the market is relevant. Maximum service temperature The maximum service temperature is determined based on the form of supply according to: ∑ EN 14706:2005: Thermal insulation products for building equipment and industrial installations – Determination of maximum service temperature. ∑ EN 14707:2005+A1:2007: Thermal insulating products for building equipment and industrial installations – Determination of maximum service temperature for preformed pipe insulation. The maximum service temperature is the temperature at which the insulation layer thickness may only decrease by 5% of its original value. For mats this means that, in an application at this temperature, they lose part of their pre-compression. The insulation layer thickness and thereby the thermal resistance remain roughly the same. Eccentricity occurs with the pipe sections at the maximum admissible decrease in thickness, which results in a change of the thermal resistance below 1%. Only for boards does the thickness decrease have a directly proportional effect on the thermal resistance. It also decreases by 5%. It must be remembered, however, that certain applications, especially vibrations or oscillations, constitute an additional strain on the insulation system. Appropriate minimisation factors, dependent upon the application, determine the service temperature [14]. For prefabricated pipe insulation materials such as pipe sections, the maximum service temperature may also constitute the maximum application temperature in installations without vibrations. For a minimum service temperature no test method exists. According to the European product standards, the following properties have to be declared by the manufacturer as a function of the temperature:


286


∑ contraction coefficient ∑ transverse tensile strength ∑ Young’s modulus, E. This enables assessment of the behaviour of the insulation layer against the movement of the object wall. Another important criterion is brittleness. The consequence of an embrittlement of the insulating material is that the material can crumble, which can lead to a drop of lifetime. The manufacturer should therefore make in any case a statement about the temperature at which an embrittlement of the material begins. Water vapour diffusion For cold protection at industrial installations an insulation system will only maintain its effectiveness when protected against the ingress of moisture. Dependent upon the partial pressure decline between the ambient air and the boundary between insulant and object wall, a water vapour diffusion flow will develop, wherever the water vapour diffusion resistance of the insulation material is limited. Over time this leads to a moistening of the insulation layer. Moisture promotes corrosion and also increases thermal conductivity of the insulation material; the heat ingress increases. Insulation systems at cold insulations must therefore be so designed that no or only very little amounts of moisture can enter. This is either achieved by a so-called vapour barrier, made of vapour-tight layers, e.g. aluminium foil with an sd value > 1500 m, or by using insulation materials whose water vapour diffusion resistance is so high that during the life cycle of the installation no significant moisture ingress occurs. The water vapour diffusion permeability of insulation materials is determined according to the following test standards: For boards and mats ∑ EN 12086:1997: Thermal insulating products for building applications – Determination of water vapour transmission properties For prefabricated pipe insulants (tubes) ∑ EN 13469:2001: Thermal insulating products for building equipment and industrial installations – Determination of water vapour transmission properties of preformed pipe insulation As a property characteristic for low water vapour permeability the water vapour diffusion resistance factor m is used. The water vapour diffusion equivalent air layer thickness (s d value) is given by the following equation.



sd = m · s

287

11.1

where sd is water vapour diffusion equivalent-air layer thickness in m, m is water vapour diffusion resistance factor, and s is thickness of the layer in m.

11.2.2 Declared value as a property in the condition as delivered Against the background of the product liability statute (Produkthaftungsgesetz) [15] the manufacturer shall specify the application purpose. On the one hand, it is frequently kept as general as possible to improve success on the market; on the other hand, high risks are involved with too broad a definition of application purposes. Principally the manufacturer declaration is limited to the profile of properties of the product as placed on the market. The profile of properties given indicates the fitness of the insulation material according to the quoted product standard [3] – [11] for the application given. The profile of properties is declared by the manufacturer in the product data sheet. However, the properties of insulants do change in many cases as a result of the installation and the ambient conditions. To convert the properties of the product as placed on the market into the value for the intended application, a satisfactory knowledge of the material and of the pertinent influential factors is required. In particular, knowledge of which influential factors upon their respective property have already been taken into account in the measurement is of high importance. Declared values must be given by the international material manufacturer and be guaranteed for agreed periods. They are always based on measured values and take care of product specific tolerances. Dependent upon the form of supply and/or the components of the insulant, the effects of different influence factors may already be contained in the declared value, since they have been detected in the measurement or are part of the result as purely material specific values. For the declared thermal conductivity these values are: ∑

the temperature difference: high temperature differences during the measurement lead to an integral reference mean value ∑ for joints: additional losses through opening joints caused by different expansion between insulant and object are taken into account in the pipe tester ∑ compression: change of thermal conductivity through change of apparent density through compression during assembly ∑ convection: for air permeable insulants, a small convective part may be contained in the measured value in the pipe tester, e.g. for gaps between test pipe and insulant


288


∑

gas exchange: thermal conductivity changes through gas exchange as the insulant ages ∑ IR permeability: leads to values dependent upon thickness (thickness effect). For fire behaviour, the substrate and the fastening element can influence the flash behaviour and/or the flue gas development. For maximum service temperatures the test ambient conditions are very important. Thermal conductivity To assess the declared thermal conductivity and its suitability for the application, knowledge of which influential factors are already contained in the declared values is of high importance. Figure 11.4 explains the interrelations. The test method must be chosen dependent upon the form of supply. Pipe sections and pipe tubes may be tested in the pipe tester [3]; boards, however, only in the hot plate test. Multifunctional products, which can equally be used for the insulation of flat objects as of pipes, e.g. wire mats, felts or lamella mats, may be tested in the hot plate [4]–[6] as well as in the pipe tester. How permeable an insulation material is for IR radiation or in how far the cellular gas after the production is in gas exchange with the ambient air, must be established independent of the form of supply. As can be seen in the flow chart in Fig. 11.4, the effects of different influential factors are reflected when determining the thermal conductivity. It is obvious that for plain products such as boards, but also mats and felts, the pure material value is measured if the thermal conductivity is determined in the hot plate apparatus. It can be declared for the intended apparent density dependent upon the temperature. For form pieces (pipe sections or tubes), on the other hand, the measured value already reflects the service thermal conductivity under the given conditions. If the fringe conditions of the test are identical to those in the intended application, no additional conversion factors are needed when determining the design value. It follows that declared thermal conductivities may be used to compare the products of different manufacturers, however, not without knowledge of the influential factors for the comparison of different forms of supply for the intended application. This condition is only met by the service thermal conductivity, e.g. the design value (see Section 11.7.2). The determination of declared values of thermal conductivity is according to the following standards: ∑ EN 13172:2008: Thermal insulating products – Evaluation of conformity. ∑ EN ISO 13787:2003: Thermal insulation products for building equipment



289

Insulation product

IRpermeable

no

Thickness effect

no

With cell-gas

Ageing

As pipe section or as tube

no

yes no

Application range

no

yes Guarded hot plate

Pipe section

Compression Losses via gaps

no

l = f(J)

Air permeable

Convection

Test pipe

lB = f(Jm)

11.4 Flow chart to explain the pertinent influential values dependent upon the product and the test method.


290


and industrial installations – Determination of declared thermal conductivity (ISO 13787:2003). With the aid of a statistical method the production tolerances are taken into account: lP becomes lN,P and lr becomes lN,r, where P is plate, R is pipe section and N is nominal or declared. The thermal conductivity is given in the manufacturers’ literature (product data sheet) as a function of temperature for the intended temperature range, e.g. in the form of a third grade polynomial. this is done in the form of tables, graphs or with the aid of coefficients for the polynomial. In tables the declared thermal conductivity is given with two indicative digits according to the rounding rules of the european standard. For a more exact determination and for the assessment of conformity according to EN 13787 coefficients are being used. For application as insulant in technical building equipment with a limited temperature range between 0 °C and 100 °C, it is sufficient for most insulants to declare the thermal conductivity as a linear equation: lN,P (J) = a0 + a1 · J

in W/((m · K)

11.2

lN,r (Jm) = a0 + a1 · Jm

in W/(m · K)

11.3

where lN,P is declared thermal conductivity for boards and mats in W/(m · K), lN,r is declared thermal conductivity for preformed pipe insulation material in W/(m · K), a0 is: coefficient in W/(m · K), a1 is coefficient in W/(m · K2), J is temperature in °C, and Jm is arithmetic mean between warm and cold side temperature in °C. Since in the display of thermal conductivities with a linear dependence no difference exists between the two equations, it may be harmonised for the given application area in a special form for all products, written like this: lN (Jm) = a0 + a1 · Jm

in W/(m · K)

11.4

where lN is declared thermal conductivity for boards and mats in W/(m² · K), a0 is coefficient in W/(m · K), a1 is coefficient in W/(m · K2) and

Jm =

JW + JK 2

in °C

11.5

where ϑW is temperature of the warm side in °C, JK is temperature of the cold side in °C and Jm is reference mean temperature between warm and cold side temperatures in °C. For tables it is recommended to give the declared values in two indicative digits for the range of reference mean temperatures between 0 °C and 50 °C in steps of 10 °C. table 11.3 gives approximate values for insulation materials for industrial installations in technical building equipment.



291

Fire behaviour In section 5 of EN 15715 [16] rules are given for the test conditions of insulation materials for the declaration of fire behaviour in the condition of delivery. This is the precondition for the attestation of conformity and the basis for the declaration of conformity to the standard by the manufacturer. The results of these tests shall be used for the classification according to EN 13501 [13]. Classification is possible in the classes A1, A2, B, C, D and E (see Table 11.4). The class F means ‘not tested’. It can also be the consequence of a test under the provisions for class E not being passed. The fire behaviour in the condition of delivery shall be declared on the label.

Table 11.3 Approximate values for insulation materials for industrial installations in technical building equipment Insulation material Form supply (abbreviated term)

Declared thermal conductivity in W/(m.K) at reference (mean) temperature* in °C

0

10

50

Mineral wool (MW)

Pipe sections Lamella mats Wired mats Boards Felts Loose-wool

– – – – – 0.033–0.036

0.032–0.038 0.036–0.039 – 0.032–0.038 0.034–0.037 0.034–0.037

0.036–0.042 0.043–0.049 0.036–0.044 0.035–0.043 0.039–0.046 0.038–0.042

Flexible elastomeric foam (FEF), Polyethylene foam (PEF)

Tubes/insulation 0.033–0.046 0.034–0.047 sleeves Boards/laps 0.036–0.045 0.037–0.046

0.038–0.050

Extruded polystyrene (XPS), Expanded polystyrene (EPS), Polyurethane (PUR) and polyisocyanurate foam (PIR), Phenolic foam (PF)

Sections Boards

0.025–0.033 0.026–0.034 0.025–0.036 0.026–0.037

0.029–0.038 0.029–0.041

Cellular glass (CG)

Sections Boards

0.042 0.043 0.037–0.039 0.038–0.040

0.051 0.044–0.045

0.038–0.051

*Dependent on the determination method: in the guarded hot plate apparatus ‘reference temperature’, in the pipe tester (pipe sections and tubes) ‘reference mean temperature’. When testing with the guarded hot plate apparatus (boards, mats), the results constitute pure material values: the thermal conductivity is given in relation to temperature. When testing in the pipe tester, the mounting influences are already contained in the measuring result: the thermal conductivity is given in relation to the reference mean temperature.


292

Materials for energy efficiency and thermal comfort in buildings Table 11.4 Possible fire behaviour classes for insulation materials Insulation material made of

Fire behaviour class according to EN 13501

Mineral wool (MW)

A1, A2

Elastomeric foam (FEF), Polyethylene foam (PEF)

B, C, D, E

Polystyrene (XPS) or (EPS), Polyurethane (PUR), Polyisocyanurate foam (PIR),

B, C, D, E

Phenolic foam (PF)

A2, B, C, D, E

Cellular glass (CG)

A1

Table 11.5 Minimum and maximum service temperatures for different insulants Insulation material

Limit service temperatures in °C

Minimum

Maximum

Mineral wool (MW)

< 10*

> 100

Elastomeric foam (FEF), Polyethylene foam (PEF)

< 0

> 90

Polystyrene (XPS) or (EPS)

< 0

< 80

Polyurethane (PUR), Polyisocyanurate foam (PIR), Phenolic foam (PF)

< 0

< 100

Cellular glass (CG)

< 0

> 100

*As an insulant in cold protection additional measures must be taken, e.g. double skin.

Maximum service temperature The maximum service temperature is a property of the insulant as placed on the market. With standardised products it is declared with the denomination code ST(+). For the defined lower service temperature ST(–) the contraction coefficient, the transverse tensile strength and the E-modules have to be declared additionally in the manufacturer’s literature. Insulation materials for the application range of industrial installations in the technical building equipment must have a minimum temperature service range from approximately 0 °C to 90 °C (see Table 11.5).



293

Water vapour diffusion The water vapour diffusion resistance factor m is declared at a temperature of 20 °C and shall be declared for standardised products as part of the insulation material designation code (see Table 11.6).

11.2.3 Product data sheet The product data sheet is the manufacturers’ literature with a description of the employment purpose and a profile of properties of the insulation material as placed on the market. The declared values are the basis for the conformity assessment according to the provisions of the standards for both a CE mark and a voluntarily quality certificate, e.g. VDI-Keymark. It is possible to declare properties for special applications. This applies especially for the declaration of a design value of thermal conductivity, or for fire behaviour or application temperatures. Application-related properties may deviate from the material properties as placed on the market.

11.2.4 Declaration of conformity and designation Insulation material manufacturers confirm with a manufacturer’s declaration that their products have been produced in line with the requirements of the respective European material standard and that the declared properties are being met by the average of the production according to the provisions of EN 13172 [17]. For this, the insulation material properties are determined within the framework of an initial type test (ITT) and the products are subject to a singular evaluation of conformity according to EN 13787 by a so-called notified body. The notified bodies are certified test institutions according to Table 11.6 The water vapour diffusion resistance factor m is declared for a temperature of 20 °C Insulation material

Water vapour diffusion resistance factor µ

Mineral wool (MW)* Elastomeric foam (FEF) and polyethylene foam (PEF) Polystyrene (XPS) Polystyrene (EPS) Polyurethane (PUR) and polyisocyanurate foam (PIR) Phenolic foam (PF) Cellular glass (CG)

1 1000–10000 80–250 20–100 30–100 10–50 sd > 1500 m

*As an insulant in cold protection additional measures must be taken, e.g. double skin.


294


EN ISO 17025 [18] and they have been registered according to the European Directive 89/106/EEC (Construction Products Directive). An obligation for repeated tests in the framework of a third party control, for instance, only exists where the fire behaviour is influenced by additives in the production process (e.g. fire-retarding materials, or limitations of the content of organic substances). The properties of insulants are declared in the manufacturer’s literature (product data sheet). On the package, the different properties of products as placed on the market are given in a designation code and are confirmed by the CE mark. Table 11.7 lists the symbols for the properties, levels and Table 11.7 Symbols for the properties, levels and classes for the designation code Symbol

Declared property, value, level or class

AP AW BS CC(i1/i2/y) sc CL CS(10) CS(10\Y) CS(Y) CV DS(N) DS(T–) DS(T\L) DS(T+) DS(TH)

level of practical sound absorption coefficient level of weighted sound absorption coefficient level for bending strength level for compressive creep level of soluble chloride ions level for compressive stress at 10% deformation level for compressive stress or compressive strength level for compressive strength value for closed cell content class for dimensional stability under normal laboratory conditions value for dimensional stability at –20 °C level for dimensional stability under load and temperature value for dimensional stability at specified temperature value for dimensional stability under specified temperature and relative humidity conditions level of soluble fluoride ions level of water vapour diffusion resistance factor level for water vapour diffusion equivalent air-layer thickness level of soluble sodium ions declared class for flatness tolerance level of the pH value level for penetration under point load class for squareness tolerance level of soluble silicate ions level for minimum service temperature level for maximum service temperature single number descriptor of structure-borne sound class for thickness tolerances level for tensile strength parallel to faces level for tensile strength perpendicular to faces level for water absorption by diffusion level for water absorption by total immersion level for short-term water absorption level for water vapour permanence level for water vapour permeability level for water vapour transmission value for water vapour resistance

F MU MV NA P pH PL(P) S SI ST(–) ST(+) SW T TP TR WD(V) WL(T) WS WVP WVPE WVT Z



295

classes for the designation code, and Table 11.8 gives an example of the composition of a designation code for a mineral wool insulant. AGI Working documents of the Q-series for insulants for operational installations in the industry are private sector regulations of constructing divisions in globally operating companies. The working documents do not contradict European standards but show a selection of property profiles of insulation materials which are considered relevant for the application in this area. The product is described and designated using a five-position insulation material designation code. As with European standards, the properties are only declared for the product as placed on the market. These properties are certified through a voluntary conformity attestation procedure, e.g. on the basis of EN 13172, Annex A, and the VDI/Key mark scheme rules through repeated tests by certified test institutes. An evaluation of conformity is rendered by independent certification bodies, authorised according to EN ISO 17011 [19]. The certificate according to the provisions of VDI/Keymark ascertains, on the one hand, the conformity of the product with the information given in the product data sheet and the manufacturer declaration. On the other hand, and simultaneously, it is confirmed that the production process is so controlled that the product is continuously manufactured with unvarying quality. The declaration of properties for special applications is possible on the label and in the product data sheet. A clear division between the properties of the product as placed on the market, and the information for possible applications, e.g. for the insulation for heating installations according EnEV [20] on the label is taken care of (see Fig. 11.5 and Section 11.8.2).

11.2.5 Quality assurance/third party control Properties like thermal conductivity or fire behaviour cannot be checked by the user. Quality assurance systems such as the certification according to the provisions of the VDI/Keymark are confidence building measures and confirm for the user that the products are in line with the information given in the product data sheet. The VDI/Keymark confirms by a continuous quality control manufacturers’ declarations regarding the properties of their products. Unlike the manufacturer’s Table 11.8 Example for the composition of a designation code for a mineral wool insulant Product Number of Thickness abbreviation standard tolerances

Maximum Water Water service absorption soluble temperature chloride ions

MF

ST(+) … …

EN 14303

T2

WS


CL

296


Manufacturer logo

Pipe Section Mineralwool pipe section EN 14303 Reaction to fire – Class A1 T2–ST(+)250–WS1–MV1–CL6–pH9,5

0123 Manufacturer 08 Thermal conductivity see Manufacturer’s Literature Address 01234-CPD-00234 AGI Q 132 insulation designation code: 10.04.02.25.99 08120501

Manufacturer Z-23.14-XXXX

EAN code

Inner diameter mm

Insulation thickness mm

Length mm

Number of pieces per pack

22

23

1000

20

Application: Insulation of pipes within buildings according to German EnEV Design value of thermal conductivity: 0.037 W/(m • K) at 40 °C mean temperature Reaction to fire – Class A2 other Applications….

11.5 Example of a label for the designation of an insulation material for the insulation of operational installation in the technical building equipment.

declaration, which must be given by the manufacturer to obtain the right to use the CE mark, and which is solely based on an initial type test, the VDI/ Keymark notifies continual product reliability. In cases, e.g. for defined markets, the provisions of locally responsible building authorities’ certificates, against the background of national building law (e.g. in Germany with a general building authority certification), demand third party tests. The resulting and required declaration of conformity confirms for users and builders the acceptability of this building product under national building law. The properties tested according to these criteria may deviate from the properties of the product as placed on the market (see Sections 11.7 and 11.8).

11.3

Form pieces and form parts

Fittings and elbows in operational installations in technical building equipment are being insulated with insulation material form pieces, which are clad by form parts, e.g. boxes, or with loose wool, which is stuffed into prefabricated form parts. Examples are given in Figs 11.6 and 11.7. The thermal insulating effect of fittings and other installation related thermal bridges is expressed with the product of the thermal transfer coefficient U or k and the heat releasing surface A (U ¥ A or k ¥ A). The heat loss via



297

11.6 Example of a form piece made of polyurethane with a cladding for the insulation of fittings and valves [1].

11.7 Example of a pre-manufactured form piece for the insulation of fittings and valves with loose wool [1].

an insulated fitting is calculated with the following equation if k and A are known:

Q˙ WB = k · A · (JM – JL)

in W

11.6

where Q˙ WB is heat loss in W, k is thermal transfer coefficient of the box insulation in W/(m² · K), A is surface of the box in m², JM is medium temperature in °C, and JL is ambient temperature in °C.

11.4

Support and spacer ring constructions

Support constructions are components of insulation systems to transfer loads, resulting from the cladding. Spacer ring constructions are supported by the object wall or the insulant and keep the cladding of the insulation system at the correct distance.


298


Support and spacer ring constructions are insulation-related thermal bridges. They are taken into consideration when calculating the service thermal conductivity with an additive term Δl. Approximate values for this additive term are given in EN ISO 23993 [1] or in VDI 2055 Part 1 [21], dependent upon the type of spacer ring construction.

11.5

Vapour retarder materials

In order to prevent or reduce moisture penetration of the insulating layer with objects with temperatures under ambient temperature it is to examine, according to EN 14114 [22] or VDI 2055 part 1 [21] it is necessary to determine whether a suitable vapour barrier on the side of the higher temperature is necessary (see Table 11.9).

11.6

Claddings

The cladding is a mechanical protection of the insulation material. The cladding material may change the fire behaviour of the insulation system. The emissivity of the cladding influences the resulting surface temperature and the density of heat flow rate (see Table 11.10).

11.7

Insulation system

11.7.1 General Occasionally, the conformity to the standard of insulants is a fundamental prerequisite for the construction of a professionally satisfactory insulation Table 11.9 Vapour retarder materials Vapour retarder materials

sd = m . s

Polyethylene film (s = 0.1 mm to s = 1.2 mm) Polyvinyl chloride film (s = 0.3 mm to s = 0.8 mm) Aluminium foil (s ≥ 0.050 mm)

10–100 30–40 > 1500

Table 11.10 Emissivity e of various surfaces Material and conditions of surface

Emissivity e

Aluminium foil, bright Aluminium, bright-rolled Aluminium-zinc, smooth polished Steel, galvanised, bright Paint-coated sheet metal Plastic casing Elastomer foamed material

0.05 0.05 0.16 0.26 0.9 0.9 0.9



299

system. Dependent on the form of supply, the thickness of the insulation product as delivered will also be the insulation layer thickness of the insulation system. This is especially true for boards, pipe sections and tubes. With multifunctional and flexible insulation materials the installed insulation layer thickness may be influenced by the application. The insulation layer thickness determines, together with the thermal conductivity, the insulating efficiency. At least for plane walls it is directly proportional to the thermal resistance of the insulation system. For pipe insulations, the logarithm of the quotient of external and internal diameters determines the thermal resistance. Tolerances for length, width and rectangularity of insulation products influence indirectly the quality of the system and its efficiency. Differing measures or deviations from rectangularity of individual boards or pipe sections constitute unnecessary sources for mistakes in the application or require additional attention and thus an increased effort in the insulation work.

11.7.2 Properties Operational thermal conductivity of the insulation system When applying the insulation material in an insulation system, the thermal insulating properties may change. Dependent upon the insulation material chosen, probably required insulation system related components, such as support and spacer ring constructions, influence the thermal insulating effect of the system. The effective thermal and cold protection is determined by the operational thermal conductivity of the insulation system. Insulation contractors guarantee that it is smaller or in extreme cases equal to the design value of thermal conductivity. A precondition is that while applying the materials chosen, such as insulant, support and spacer ring construction, additives and cladding, the following is made sure in every case: ∑ ∑

no open joints, circumferential or longitudinal, stepped joints in multilayer insulations, no cavities no gaps between object wall and insulant through appropriate contact pressure, and for cold insulations the installation of undamaged vapour retarders to prevent the ingress of moisture or at least to lessen it.

A professional insulation contractor is also obliged to check the fitness of materials used for the intended application within the framework of his goods input control, and raise objections where appropriate. Since it is not practically feasible to check all properties on the building site, such as declared thermal conductivity, longitudinal air flow resistance, hydrophobic qualities, AS quality, water vapour diffusion resistance, maximum service temperature, emissivity of cladding sheet metal, and thermal bridges effect of


300


support and spacer ring constructions, it is recommended always to require the manufacturer to present an attestation of conformity by an accredited certification body. Only the thickness and the conformity of apparent density can be checked on the building site. In particular, the apparent density of insulation materials is considered the most important indirect quality mark (see Figs 11.2 and 11.3). The radiation permeability and thereby the dependence of temperature and thickness are being determined by it. With wired mats, lamella mats and felts, the apparent density changes in the application and thus properties, especially thermal conductivity, do change. Unlike with most applications when insulating building envelopes, apparent density plays an important role in the insulation of industrial installation also as direct quality requirement. With high insulation layer thicknesses, the insulation material constitutes loads that cannot be disregarded. This is the reason why the apparent density will always be declared by manufacturers, even if not mandatory according to European standards. Where deviations from the intended and ordered product properties are detected in the goods input control, it must be checked whether the conversion factors according to EN ISO 23993 [1] or VDI 2055 part 1 [21] for the calculation of design and values are still valid. It is calculated from the nominal values of the thermal conductivity:

l = Pf · lN + Dl in W/(m · K)

11.7

where Pf is product of all effective factors according to EN ISO 23993 or VDI 2055 Part 1, lN is declared thermal conductivity, and Dl is reference values for losses from thermal bridges or inserts. For the given application in technical plants and in technical building equipment, this amounts to Pf ª 1 and Dl = 0, so that for the given temperature range

l ª lN in W/(m · K)

11.8

can be set. Fire behaviour of the insulation system Products, especially with fire behaviour classes A2L, BL, CL and DL, may display more or less the same fire behaviour under the conditions of delivery and in the application, comparable to products, the thermal conductivity of which has been tested in the pipe tester according to EN ISO 8497 [12]. This applies for insulation products, e.g. tubes, constituting in themselves the insulation system, or where similar conditions prevail in the application as in the test and where no additional components can change the fire behaviour after the application.



301

especially with tests according to eN 13823 where results are decisive for the classifications into classes A2, B, C and D, respectively A2l, Bl, Cl and Dl, it is frequently only possible to test products under extreme conditions approaching end-use conditions. Fastening means such as adhesive tapes, but substrate materials too (e.g. the material of the pipe to be insulated), influence fire behaviour in the application. Since it is impossible to check with tests every application case and all possible execution variations, the test fringe conditions are given for all standardised products in eN 15715 [16] determining product parameters (as placed on the market) and application parameters (in end-use condition). For the declaration of conformity, as a precondition for the right to use the Ce mark, only properties of products as placed on the market are relevant. However, occasionally they also may cover certain application cases.

11.8

Heat loss of the operational installation in technical building equipment

the heat loss of an operational installation in the technical building equipment, respectively the heat ingress, is not determined solely by the service thermal conductivity of the insulation system, but naturally also by the insulation layer thickness installed, and by the surface heat transfer conditions. the choice of suitable cladding sheet metal, observing the emissivity, can likewise reduce the density of heat flow rate. Plant-related thermal bridges, too, such as supports, fittings and valves exert decisive influence on the total heat loss Q˙ of an operational installation in the technical building equipment. the overall heat loss Q˙ is decisive for the primary energy effort required. Under neglect of the internal heat transfer coefficient the total heat loss is valid for: Pipes Q i =

JM – Jl · l in W d 1 1 · ln a + 2·p ·l di a a · da · p

11.9

Boiler and vessels

J – Jl Q i = M · A in W s+ 1 l aa

11.10

where JM is medium temperature in °C, Jl is ambient temperature (air) in °C, l is designed value in W/(m · K), da is diameter of the insulation exterior in m, di is diameter (insulation internal) in m, s is layer thickness in m, l is


302


length of the pipe in m, A is surface of boiler or vessel in m², and aa is heat transfer coefficient (cf Table 11.11). Also the plant-related thermal bridges such as supports, armatures and valves exert crucial influence on the total heat loss Q˙ gen,is [24] of a technical plant for building equipment and industrial installations. The total heat loss Q˙ is contributing for the necessary primary energy expenditure.

n

m

i =1

j =1

Q gen,is = ∑ Q i + ∑ Q WB, j

in W

11.11

where Q˙ gen,is is total heat loss in W, Q˙ i is heat loss over the thermal insulation of the components in W, and Q˙ WB,j is heat loss from plant related thermal bridges (supports, fittings and valves) in W.

11.8.1 Required insulation layer thicknesses Insulation layer thicknesses required should, therefore, today be designed under the aspects of energy saving in connection with the challenge of climate protection and not only according to the criteria of contact protection or the prevention of condensate at the surface of the insulation. Heat losses via installation-related thermal bridges must be taken into account, to allow for an energy consumption related evaluation of an industrial installation. Where the heat loss cannot be directly reduced through insulation measures, it is recommended to increase the insulation layer thickness for pipes and vessels compared to insulation layer thicknesses calculated under purely operational aspects. The design can be assisted by economic considerations. Dependent upon the cost for the primary energy needed to operate the installation, a so-called economic insulation layer thickness results, which leads to marginal cost increases when further increased, but shows nevertheless very short payback periods. Insulation measures over and above the dimension required to fulfil operational demands lead to a reduction of overall heat losses. The difference Table 11.11 Reference values for the heat transfer coefficient of claddings for insulation of building equipment estimated according EN 12421 [23] and VDI 2055 Part 1 [21] Material and conditions of surface Emissivity e

heat transfer coefficient ao in W/(m² · K)

Aluminium foil, bright Aluminium, bright-rolled Aluminium-zinc, smooth polished Steel, galvanised, bright Paint-coated sheet metal Plastic casing

4 4 5 6 9 9

0.05 0.05 0.16 0.26 0.9 0.9



303

ΔQ˙ of heat losses according to these two criteria [21] and Technical Letter No. 6 [25], constitutes an important contribution to energy savings.

11.8.2 National regulations With EnEV (Energy Savings Directive) [20] Germany shows a pragmatic approach. Dependent upon whether or not heat losses contribute to the thermal economy of a building, pipes up to DN 100 must be insulated with a layer thickness representing 50% or 100% of the pipe diameter. Upwards of DN 100 the required insulation layer thicknesses are 50 mm or 100 mm. The insulation layer thicknesses apply to a design value (service thermal conductivity) ≤ 0.035 W/(m · K). For insulation materials with other design values the insulation layer thicknesses must be calculated in relation [26]. From an operational point of view, respectively according to the criteria of contact protection, these insulation layer thicknesses are totally oversized; however, according to the criteria of energy saving, they make sense and they are recommended for the energy efficiency of the installation. The design value for insulation materials is regulated together with the classification of fire behaviour in a general Building Authority Acceptance (ABZ) of the DIBt, Berlin. Where insulation materials are being produced according to European standards [3]–[11], they must be designated according to the provisions of those standards. Special properties, e.g. a design value or a fire behaviour class for special applications, shall be declared clearly distinct from the CE marking (see Fig. 11.5 and Section 11.2.4). With the Ü-mark, confirmation is given to the user of the insulation material, that ‘conformity’ prevails between the product and the provisions of the General Building Authority Acceptance for the described purpose of application ‘insulation of heating installations’ and is especially authorised. A proper execution of this directive by insulation contractors, using accepted insulation materials and layer thicknesses demanded by regulations, constitute a small contribution to climate protection.

11.9

References

1. EN ISO 23993:2008: Thermal insulation products for building equipment and industrial installations – Determination of design thermal conductivity. 2. DIN 4140: Dämmarbeiten an betriebstechnischen Anlagen in der Industrie und in der technischen Gebäudeausrüstung – Ausführung von Wärme- und Kältedämmungen; Ausgabe: 2008-03 [Insulation work on industrial installations and building equipment – Execution of thermal and cold insulations; Version: 2008–03]. 3. EN 14303:2009: Thermal insulation products for building equipment and industrial installations – Factory made mineral wool (MW) products – Specification. 4. EN 14304:2009: Thermal insulation products for building equipment and industrial installations – Factory made flexible elastomeric foam (FEF) products – Specification. © Woodhead Publishing Limited, 2010

304


5. EN 14305:2009: Thermal insulation products for building equipment and industrial installations – Factory made cellular glass (CG) products – Specification. 6. EN 14306:2009: Thermal insulation products for building equipment and industrial installations – Factory made calcium silicate (CS) products – Specification. 7. EN 14307:2009: Thermal insulation products for building equipment and industrial installations – Factory made extruded polystyrene foam (XPS) products – Specification. 8. EN 14308:2009: Thermal insulation products for building equipment and industrial installations – Factory made rigid polyurethane foam (PUR) and polyisocyanurate foam (PIR) products – Specification. 9. EN 14309:2009: Thermal insulation products for building equipment and industrial installations – Factory made products of expanded polystyrene (EPS) – Specification. 10. EN 14313:2009: Thermal insulation products for building equipment and industrial installations – Factory made polyethylene foam (PEF) products – Specification. 11. EN 14314:2009: Thermal insulation products for building equipment and industrial installations – Factory made phenolic foam (PF) products – Specification. 12. EN ISO 8497:1996: Thermal insulation – Determination of steady-state thermal transmission properties of thermal insulation for circular pipes (ISO 8497:1994). 13. EN 13501-1:2007: Fire classification of construction products and building elements – Part 1: Classification using data from reaction to fire tests. 14. AGI Q 101: Insulation work on power plant components – Execution; Version: 2000-07. 15. ProdHaftG Gesetz, 1989-12-15. Gesetz über die Haftung für fehlerhafte Produkte (Produkthaftungsgesetz) geändert durch ProdHaftRVÄndG, Gesetz 2000-11-02. 16. prEN 15715:2007: Thermal insulation products – Instructions for mounting and fixing for reaction to fire testing – Factory made products. 17. EN 13172:2008: Thermal insulating products – Evaluation of conformity. 18. EN ISO/IEC 17025:2005-08: General requirements for the competence of testing and calibration laboratories (ISO/IEC 17025:2005). 19. EN ISO/IEC 17011:2004: Conformity assessment – General requirements for accreditation bodies accrediting conformity assessment bodies (ISO/IEC 17011:2004). 20. EnEV: 2007-07-24: Verordnung über energiesparenden Wärmeschutz und energiesparende Anlagentechnik bei Gebäuden (Energieeinsparverordnung – EnEV). 21. VDI 2055 Part 1: Thermal insulation of heated and refrigerated operational installations in the industry and the building services – Calculation rules: Technical rule, 2008–09. 22. EN 14114:2002: Hygrothermal performance of building equipment and industrial installations – Calculation of water vapour diffusion – Cold pipe insulation systems. 23. EN 12421:1998: Magnesium and magnesium alloys – Unalloyed magnesium. 24. EN 15603:2008: Energy performance of buildings – Overall energy use and definition of energy ratings. 25. Technischer Brief Nr. 6: 2008-10: Hohe Rentabilität bei umweltgerechten Isolierschichtdicken – High profitability through ecologically based insulation thicknesses. 26. DIN V 4108-4:2007-06: Thermal insulation and energy economy in buildings – Part 4: Hygrothermal design values. © Woodhead Publishing Limited, 2010

12

Reflective materials and radiant barriers for insulation in buildings

D. W. Y a r b r o u g h, R&D Services Inc., USA

Abstract: Reflective insulations and radiant barriers are products used to reduce radiative transport across air spaces. The physical basis for these products, methods of testing, and the standards for reflective-type products are discussed in this chapter. Selected references to the existing literature are included. Key words: reflective insulation, radiant barriers, heat flux, convection, radiation, reflective air space, emittance.

12.1

Background and definitions

Thermal insulations used in buildings use the low thermal conductivity of gases to reduce heat flow. Mineral fiber, cellulosic, and open-cell plastic insulations use air as the low thermal conductivity component. The purpose of the fibrous or cellular material is to prevent free convection and reduce radiation. Cellular plastic insulation containing low thermal conductivity gases can provide thermal resistance greater than the air products as long as the gas remains in the cells. High thermal performance insulations based on evacuated powders or fibers or insulations with small diameter fibers or particles provide very high thermal resistance by reducing gas-phase conduction. Reflective insulations and radiant barriers are a class of materials that also rely on the low thermal conductivity of air for their performance. The thermal resistance of a reflective system is always less than the thermal resistance of an air space without convection or radiation since reduced pressure or nano-scale particles are not involved. Reflective insulations and radiant barriers are materials used to reduce the transport of energy across air spaces in a building envelope. Both types of materials include surfaces with low emittance and high reflectance in the thermal spectrum (2–50 mm), i.e. near infra red (NIR) to mid infra red (MIR). These surfaces significantly reduce radiation across adjacent air spaces. Reflective insulation systems are characterized by having enclosed air spaces adjacent to low-emittance surface(s) while radiant barrier systems are associated with large ventilated or unventilated air spaces. Reflective insulation systems generally include air spaces up to about 0.25 m in the 305 © Woodhead Publishing Limited, 2010

306


direction of heat flow. Radiant barriers systems generally have air spaces larger than 0.5 m. Reflective insulation materials consist of one or more aluminum foils or metallized aluminum films bound to substrates (core material) primarily for mechanical support. The supporting materials include paper-type materials, plastics, or wood. In some cases the supporting material has a thermal resistance because of its thickness. Plastic bubble products or fiberglass batt material, for example, are used as core material. The thickness of the core material is generally in the range 3–25 mm. The thermal resistance of the core material is usually a small part of the overall resistance of the reflective air space. The thermal resistance is the result of reduced radiative transport across the enclosed air space and limited free convection. Reflective insulation systems have thermal resistances or R-values that can be measured and calculated. The thermal resistance of a reflective air space depends on orientation since there is a convective component of heat flow across the air space. Reflective insulations are commonly labeled with R-values for heat flow up, horizontal, and down. Radiant barrier materials are most commonly single sheet materials consisting of aluminum foil or film bonded to reinforced plastic or paper or in some cases wood sheathing. A well-established North American product consists of foil bonded to oriented strand board for use as roof sheathing. Radiant barrier systems that consist of both a low-emittance surface facing a large air space with one or more enclosed reflective air spaces are also in use. These systems provide reduced radiative transport across the air space (radiant barrier) and thermal resistance associated with the reflective air spaces. Radiant barrier systems are evaluated in terms of reduced heat flow across the bounding air space rather than R-value. The performance of a radiant barrier system depends on the heat flow direction. This means that winter and summer performance for attic or roof mounted radiant barriers will be different. The performance of attic radiant barriers can be estimated for a given geographical location using computer modeling such as that described in ASTM C 1340. The modeling provides annual load reductions based on local weather data. Both reflective insulation systems and radiant barrier systems can be achieved with liquid coatings having emittances in the range 0.20 to 0.30. At the present time, however, liquid coatings with emittances as low as aluminum foil or film are not available. The energy savings potential of liquid radiant barrier systems, therefore, is lower that that of the foil or metallized film products. Aluminum has been the material of choice for reflective products because of its low emittance and relatively low cost. The emittance of aluminum foil products is in the range 0.03 to 0.05 on a scale of 0 to 1. Aluminum film products have slightly higher emittances because the deposited metallic


Reflective materials and radiant barriers for insulation

307

surface must be protected from the surrounding atmosphere. The results of thin coatings to protect the metallized surface are emittances in the range 0.04 to 0.08. Long-term performance of a reflective system requires maintenance of low emittance. Contamination by dust or other debris or water damage increases the emittance and reduces savings. Cook et al. [1] have measured the effect of dust on emittance by distributing measured amounts of fine dust on a foil surface and measuring the resulting emittance using ASTM C 1371. The emittance increases from 0.03 to about 0.85 as the weight per unit area of dust was increased from 0 to 8 mg/cm2. Many of the common building materials have emittances in the range 0.8 to 0.9. The heat flux, Q/A (W/m2), from a warm surface to a cool surface is described by a heat transfer coefficient for conduction and convection, hc (W/m2·K), and a radiation term, Ehr, as shown by Eq. 12.1,

Q/A = (hc + Ehr) ∙ DT

12.1

where DT is the difference in temperature between the surfaces. The term hc includes conduction and convection while hr accounts for the radiative transfer. Equations and data for hc and hr will be discussed in following sections. The E in Eq. 12.1 is the effective emittance for the air space. E depends on the emittances of the surfaces bounding the enclosed air space. Reflective insulations and radiant barriers are produced with surfaces that result in E near zero. Equation 12.1 can be used to describe the heat flux to or from a surface to adjacent air if E is taken to be the emittance of the surface. Reflective insulation systems have R-values that depend on orientation, bounding temperatures, and the physical design of the system. Radiant barrier system performance is commonly stated in terms of heat flow or heat flux reduction. Heat flow is the product of heat flux and area. The commonly used units for these quantities are given in Table 12.1.

12.2

Applications and assemblies

The thermal performance of a reflective insulation system depends on heat flow direction, temperature difference across the air space, the average temperature, and the thermal emittances of the surfaces bounding the air space. Table 12.1 Units for reflective systems Description

Scientific Inch-Pound international

Conversion from SI to IP

Thermal Resistance (RSI) Heat flux Heat flow

m2∙K/W

ft2·h·°F/Btu

Multiply by 5.678

W/m2 W

Btu/ft2∙h Btu/h

Multiply by 0.319 Multiply by 0.293


308


The dependence of RSI on the distance across the air space is illustrated in Fig. 12.1 for three directions of heat flow. The thermal resistance for heat flow upward or horizontal maximizes at about 25 mm while the thermal resistance for downward heat flow increases with the distance across the air space. The downward heat flow is affected by stratification of the air with low-density air being at the top of the space. R-values for commercial products are usually less than R-values calculated for large parallel planes partly because of radiation absorbed by framing or supports (multi-dimensional effects). A reflective insulation installed in a space of fixed dimensions can consist of one air space or multiple air spaces. Subdivision of a given space into two or more smaller air spaces will increase the overall thermal resistance by reducing both the radiative and the convective components of heat transfer. The convective component has a strong dependence on the temperature difference across the air space. Dividing a space into two regions, for example, will reduce the temperature drop across each space by a factor of about two with a corresponding reduction in convection. Table 12.2 contains calculated values for the thermal resistance of reflective air spaces at mean temperature Table 12.2 Calculated thermal resistances for selected reflective air spacesa Thermal Resistance (m2·K/W) Air space

Temperature difference

Heat flow direction

(mm)

(°C)

(Up)

(Horizontal)

(Down)

12 24 36 12

10 10 10 15

0.33 0.36 0.38 0.30

0.41 0.60 0.57 0.40

0.42 0.76 1.03 0.42

Tmean = 25 °C, emittances 0.9 and 0.03 1.4 1.2 Thermal resistance

a

1.0 Down

0.8

Horizontal

0.6

Up

0.4 0.2 0.0

0

10

20

30 40 Air space (mm)

50

60

12.1 Thermal resistance (RSI in m2·K/W) at T = 25 °C and DT = 10 °C.



309

25 °C bounded by large parallel planes that illustrate the effect of air space dimensions and temperature difference on the thermal resistance. The installation of a single layer reflective insulation material in the middle of a wall cavity or between floor joists to produce two enclosed air spaces is a very common application. Table 12.3 contains calculated thermal resistances for a fixed space divided into small regions. The results are based on large, parallel thin reflective membranes installed in a 50 mm wide space. The heat flow direction is horizontal and membrane thickness is neglected for the thermal resistances in Table 12.3. The cost associated with producing two low emittance surfaces for each air space often rules out this type of design since the added thermal performance shown in Table 12.3 is modest. Commercial products have existed with up to nine enclosed air spaces in series when a high R-value system is required. Figure 12.2 shows examples of assemblies that have been used for woodframe or metal-frame structures. The core material for the reflective material can be either a single-sheet material, foam, or plastic bubble structure.

12.3

Basis for thermal performance

The performance of reflective insulations and radiant barriers is based on a reduction in heat transfer by radiation between hot and cold surfaces. The radiative flux Q/A (W/m2) from a surface at an absolute temperature, T, with emittance e, is readily calculated from the Stefan–Boltzmann equation where s, the Stefan–Boltzmann constant, equals 5.67 ¥ 10–8 W/m2·K4. The radiative flux is directly proportional to the emittance, a property that varies from 0 to 1. Reflective insulations and radiant barriers utilize materials with low emittance surfaces. Aluminum foils with emittances in the range 0.03 to 0.04 have been a popular choice. Metallized aluminum films with protective coatings have come into use in recent years.

Q/A = e∙s∙T 4

12.2

Table 12.3 Effect of dividing a fixed space to produce multiple air spacesa Number of air spaces

R-values (m2·W/K)

One side of each air space with low emittanceb

Both sides of each air space with low emittancec

1 2 3

0.44 1.20 1.63

0.46 1.27 1.68

a

Total space 50 mm, bonding temperatures 15 °C and 25 °C Low emittance side 0.03 with high emittance side 0.9 c Both sides of air space with emittance 0.03 b


310


Single foil products

Two foils (one or two air gaps)

Three foils (three or four air gaps)

Multifoil configurations

Support structure

Reflective surface

Non-reflective surface

12.2 Examples of reflective insulation assemblies.

The net radiative transfer between parallel surfaces depends on the temperatures of the surfaces, the emittances of the surfaces, and the orientation of the surfaces. If surface 1 is hot and surface 2 is cold, then the net radiative transfer (Q12) is given by Eq. 12.3 where the term F12 is the overall interchange factor.

Q12 = s ∙A1∙ F12·(T14 – T24)

12.3

The evaluation of F12 is discussed in detail in the heat transfer literature. In the case of reflective insulation systems the discussions are generally limited to radiative transfer between large parallel surfaces with emittances e1 and e2. In this case, the interchange factor is expressed by Eq. 12.4. The interchange factor in this case is commonly called the ‘effective emittance (E)’.

F12 = E = 1/(1/e1 + 1/e2 – 1)


12.4


311

In building application, the temperature differences (T1 – T2) are relatively small with the result that the radiative heat flux can be approximated as follows. Q12/A1 = 4·E·s·Tm3 ·DT = E·hr Tm = (T1 + T2)/2 DT = T1 – T2

12.5

The heat flux due to convection-convection is represented by hc with the resulting total heat flux being the sum of the two terms. The units for hc and hr are W/m2·K. Qtotal/A = (E·hr + hc)·DT

12.6

A combination of Eq. 12.6 with the definition of R-value gives an expression for the thermal resistance of the enclosed air space. R-value =

DT = 1 Qtotal (E ·hr + hc ) total /A

12.7

The term hc depends on the heat flow direction since it includes buoyancy driven free convection. The standard ISO 6946 contains equations for estimating hc for two important cases. Case 1 is an unvented space with length and width both more than 10 times the thickness, d. Figures 12.3, 12.4, and 12.5 show hc based on ISO 6946 as a function of the air gap, d, for the three major heat flow directions for the case DT across the air gap less than 5 K. Case 2 involves regions that don’t conform to the Case 1 dimensional requirements. There are many cases where a small temperature difference across the air gap is valid. If d is small, convection is absent and hc gives conductive transport at 10 °C. There are two curves in each of the three figures. The curves shown with diamonds represent a conduction dominated region. The Downward

14 12 hc (10 °C)

10 8 6 4 2 0

0

5

10

15

20

25

30

35 40 45 50 Air gap (mm)

55

60

65

70

75

80

12.3 Downward heat flow values for hc (10 °C) based on ISO 6946 for DT £ 5 K.


312

Materials for energy efficiency and thermal comfort in buildings Upward

14 12 hc (10 °C)

10 8 6 4 2 0

0

5

10

15

20

25 30 Air gap (mm)

35

40

45

50

55

12.4 Upward heat flow values for hc (10 °C) based on ISO 6946 for DT £ 5 K. Horizontal

30 25 hc (10 °C)

20 15 10 5 0

0

5

10

15

20 25 30 35 Gap thickness (mm)

40

45

50

55

12.5 Horizontal heat flow values for hc (10 °C) based on ISO 6946 for DT £ 5 K.

curves shown as squares are for convection. R-values are calculated using the greater of the two values at a given gap thickness. The figures illustrate the onset of convection in a reflective air space. The curves for downward heat flow intersect at 60.7 mm. The curves for upward heat flow intersect at 12.8 mm while the curves for horizontal heat flow intersect at 20 mm. Convection is absent for air gap thicknesses less than the preceding values. Table 12.4 contains hc values for d greater than the intersection points. ISO 6946 provides a method for calculating the radiative term when the air space does not satisfy the dimensional requirement stated earlier or the temperature difference across the air gap exceeds 5 K. Both of these conditions will reduce the thermal resistance of the air gap. An increase in the temperature difference results in increased convective transport while narrowing the width of the air space increases the radiative transport. Annex B in ISO 6946 contains the equations for making R-value calculations. An extensive set of hot box measurements for reflective air spaces were made at the US National Bureau of Standards by Robinson and Powell in 1954 [2]. A second major set of hot box measurements were published by



313

Table 12.4 Values for hc for air gaps that exhibit convection

hc (W/m2·K)

Air gap (mm)

Down

Horizontal

12.8 20.0 1.25 60.7 0.41 1.25 65.0 0.40 1.25 70.0 0.39 1.25 75.0 0.38 1.25

Up 1.95 1.95 1.95 1.95 1.95 1.95

Desjarlais and Yarbrough in 1991 [3]. These data have been used to establish correlations for hc that include dependence of air gap temperature, air gap temperature difference, effective emittance, and heat flow direction. The graphical results in Fig. 12.1 were calculated using the correlations from the hot box tests. Equations for calculating hc that are based on the NBS hot box data are in the 1991 paper mentioned above.

12.4

Measurement of thermal performance

The thermal performance of reflective air spaces is generally determined using a hot box facility. Relatively large test specimens (2.5 ¥ 3 m, for example) can be evaluated using a hot box facility. A flat test specimen is bounded on one side by a region containing a heater. The second side of the specimen is bounded by a temperature-controlled space. At steady state, the energy input to maintain a constant temperature on the hot side establishes the heat flux through the test specimen. The temperature difference between the hot and cold regions is the DT. Equation 12.7 can then be used to calculate the R-value which is the thermal resistance from the air space on the hot side of the specimen to the air space on the cold side of the specimen. Addition instrumentation provides the thermal resistance between the exterior or interior surfaces of the test specimen. Parallel-path heat flow calculations are used to isolate the thermal resistance of an enclosed cavity containing a reflective air space. Figures 12.6 and 12.7 contain photographs of a rotating hot box facility in operation at the Oak Ridge National Laboratory in Oak Ridge, TN, USA. The hot box is positioned for a horizontal heat flow measurement. Figure 12.7 shows the two parts of the hot box separated for insertion of a building element for testing. An apparatus of this type was used to generate the collection of thermal resistance values mentioned in previous sections. Relatively small test specimens that include a reflective air space can be evaluated using a heat-flow meter apparatus. The apparatus pictured in Fig. 12.8 can be used to test a 600 ¥ 600 mm specimen up to about 150 mm thick with heat-flow up or down. This type of measurement, however, is usually


314


12.6 A rotatable hot box positioned for horizontal heat flow (photo courtesy of the BTRIC, Oak Ridge National Laboratory, USA).

12.7 Hot box apparatus with sides parted to install test specimen (photo courtesy of the BTRIC, Oak Ridge National Laboratory, USA).



315

12.8 Heat-flow meter apparatus with door closed for test.

limited to air gaps less than 50 mm. Smaller versions of the heat-flow meter apparatus have been mounted in a frame and rotated for horizontal heat flow measurements. Kanade and Yarbrough describe this type of measurement which is limited to small specimens that do not include framing or other types of obstructions [4]. Laboratory equipment of the type shown in Figures 12.6,12.7, and 12.8 is not used to evaluate radiant barriers. A large-scale facility that can be used to evaluate attic radiant barriers is shown in Fig. 12.9. The test specimen in Fig. 12.9 is a flat rectangular element. A small enclosed attic with pitched roof can be mounted in the frame and tested under a variety of conditions. The apparatus pictured in Fig. 12.9 operates as a hot box facility. The usefulness of this apparatus is enhanced by the fact that specimens with nonrectangular shapes can be tested (a specimen with triangular cross-section, for example). The type of data that can be obtained from the apparatus in Fig. 12.9 is like that obtained with the conventional hot box. R-values for specific layers within the test element and overall heat transfer coefficients are determined using Eq. 12.7 with DT being the measured temperature difference across the element to be evaluated. The heat flux across the specimen is the same at all locations when steady-state conditions are achieved.

12.5

Codes and standards

Reflective insulation and radiant barrier products are subject to a variety of codes and standards around the world. Primary consideration is given


316


12.9 Test element installed in the large scale climate simulator (photo courtesy of the BTRIC, Oak Ridge National Laboratory, USA).

to thermal performance evaluations including the measurement of the key physical property – thermal emittance. In addition, properties such as water vapor transmission, resistance to bleeding and delamination, resistance to fungal growth, combustion properties, and tear strength are examples of additional product requirements. Table 12.5 lists standards and specifications that are used in various parts of the world. The standards in Table 12.5 will guide the reader to test methods and computational methods for evaluating reflective insulations and radiant barriers.

12.6


There is a vast literature associated with reflective insulations and radiant barriers. The literature includes handbook information and published research papers. A representative collection of available material is listed below. Literature reviews ASHRAE Transactions 95 (Part 2) 1989 by W. P. Goss and R. G. Miller ‘Literature Review of Measurement and Predictions of Reflective Insulation System Performance, 1900–1989’. Oak Ridge National Laboratory ORNL/TM-8891 by D. W. Yarbrough ‘Assessment of Reflective Insulations for Residential and Commercial Applications’. © Woodhead Publishing Limited, 2010


317

Table 12.5 Standards and specifications related to reflective products Identification

Title

ISO 6946

Building components and building elements – Thermal resistance and thermal transmittance – Calculation method (Versions of this document exist for individual countries) Common understanding of assessment procedure: products with radiant heat reflective component for use in thermal insulation systems of building envelopes Standard practice for installation and use of reflective insulation in building constructions Standard specification for reflective insulation for building applications Standard practice for installation and use of radiant barrier systems in building construction Standard specification for sheet radiant barriers for building construction applications Standard practice for evaluation of heat gain or loss through ceilings under attics containing radiant barriers by use of a computer program Methods of determining the total thermal resistance of parts of buildings Materials for the thermal insulation of buildings: General criteria and technical provisions Acceptance criteria for reflective foil insulation

CUAP

ASTM C 727 ASTM C 1224 ASTM C 1158 ASTM C 1313 ASTM C 1340

NZS 4214:2006 AS/NZS 4859 ICC-ES AC02 ISO ASTM NZS AS/NZS ICC-ES

International standards organization American society for testing and materials New Zealand standard Joint Australian/New Zealand standard International code council-evaluation services

Reference books Heat Insulation, John Wiley and Sons, New York, Chapter 6 ‘Reflective Insulation’ by G. B. Wilkes (1950). Her Majesty’s Stationery Office 1955 by G. D. Nash, J. Comrie, and H. F. Broughton The Thermal Insulation of Buildings – Design Data and How to Use. ASHRAE Handbook of Fundamentals (2005) Chapter 25 (American Society of Heating, Refrigerating and Air-Conditioning Engineers). Papers with collections of data, derivations or correlations H. E. Robinson and F. J. Powell, ‘The Thermal Insulation Value of Airspaces’, Housing Research Paper 32, National Bureau of Standards Project ME12, USA (1956). Andre O. Desjarlais and David W. Yarbrough, ‘Prediction of the Thermal


318


Performance of Single and Multi-Airspace Reflective Insulation Materials’, ASTM STP 1116 (1991) pp. 24–49. F. B. Rowley and A. B. Algren, ‘Thermal Resistance of Air Spaces’, Transactions of the American Society of Heating and Ventilating Engineers, 35, 165–191 (1929). M. Hollingsworth, ‘Experimental Testing of Reflective Insulations’, ASTM STP 922 (1987) pp. 506–517. K. G. T. Hollands, T. E. Unny, G. D. Raithby, and L. Konicek, ‘Free Convection Heat Transfer across Inclined Air Layers’, J. of Heat Transfer (May 1976) pp. 189–193. Leon R. Glicksman, ‘Two-Dimensional Heat Transfer Effects on Vacuum and Reflective Insulations’, J. of Thermal Insulation, 14, 281–294 (1991). Joe C. Cook Jr., D. W. Yarbrough, and K. E. Wilkes, ‘Contamination of Reflective Foils in Horizontal Applications and the Effect on Thermal Performance’, ASHRAE Transactions, 95(2), 677–681 (1989).

12.7

References

1. D. C. Cook Jr., D. W. Yarbrough and K. E. Wilkes, ‘Contamination of Reflective Foils in Horizontal Applications and the Effect on Thermal Performance’, ASHRAE Transactions 1989, volume 95, Part 2, 677–681. 2. H. E. Robinson and F. J. Powell, ‘The Thermal Insulation Value of Airspaces’, Housing Research Paper 32, National Bureau of Standards Project ME-12, USA (1956). 3. Andre O. Desjarlais and David W. Yarbrough, ‘Prediction of the Thermal Performance of Single and Multi-Airspace Reflective Insulation Materials’, ASTM STP 1116 (1991) pp. 24–49. 4. P. Kanade and David W. Yarbrough, ‘Use of a Heat-Flow Meter Apparatus to Evaluate Reflective Insulation Systems’, Proc. of the International Conference on Thermal Insulation, 16, 89–109 (2002).


13


C. - H. Y u, Q. J. F u and S.C.E. T s a n g, University of Oxford, UK

Abstract: Aerogel materials have recently received much attention since they give many exciting applications in a wide range of areas. This chapter highlights the processing of these materials, the resulting physicochemical properties and their applications. Thus, fundamental understandings in the techniques for processing of aerogel materials including conventional drying, supercritical drying, freeze-drying, ambient-pressure drying with regards to material density and void size distribution, thermal conductivity, optical and acoustic properties are provided. In addition, a number of chemical posttreatments for surface engineering of aerogel materials are included. Finally, potentially new applications of using these materials as thermal insulation for building, optical sensor, space dust collector and catalysis are discussed. Key words: silica, aerogel, pore structure, surface, sol-gel process, application.

13.1

Introduction

13.1.1 General properties Aerogels have received significant attention over the last few decades because they possess many unique and characteristic physicochemical properties. For example, solid aerogel can be fabricated at a lower density than any other solid materials, with a density ranging from 1.5 to 50 mg/cm3. It also gives exceptionally low thermal insulation, in the order of 0.005 W/mK, high dielectric constant, at 3–40 GHz with k = 1.008–2.27, low acoustic properties (sound velocities as low as 100 m/s) and good optical transmittance, over 90% even at 632.8 nm (Fricke and Emmerling, 1992; Kim and Hyun, 2001; Schultz et al., 2005, Dorcheh and Abbasi, 2008). Its extraordinarily high specific surface area, corresponding to a high surface-to-volume ratio, is particularly significant. Recent commercial applications of aerogels include thermal window insulation (Duer and Svendsen, 1998; Wagh and Ingale, 2002), supercapacitors (Gouerec et al., 1999; Li et al., 2006), acoustic barriers (Vicente et al., 2005; Gronauer and Fricke, 1986), and catalysis (Pajonk, 1991; Cao et al., 2007; Schneider and Baiker, 1995). A number of possible new applications, such as modified materials for wall decoration, nuclear waste storage (Reynes et 319 © Woodhead Publishing Limited, 2010

320


al., 2001), adsorbents (Carraher, 2005; Han et al., 2000) and dust capture (Tsou, 1995; Burchell et al., 2006) have been described. Modifying functional groups on the gel surface to carry foreign materials (Yu et al., 2009; Tsang et al., 2006) and total encapsulation of a range of different chemical species in the gel (Maury et al., 2004; Buisson et al., 2001; Orcaire et al., 2006; Yu et al., 2009) could offer new uses in many areas. Thus the range of technologies using aerogels is expanding rapidly to meet new and changing industrial demand.

13.1.2 Background We start by defining terms related to the sol-gel process that will be used throughout this chapter. In a sol, (seed) colloidal particles with diameters in the range of 1–1000 nm are dispersed in a liquid. A gel consists of a sponge-like, three-dimensional solid network whose pores are filled with liquid. When gels are prepared by hydrolysis and condensation of metal or semi-metal alkoxides or other hydrolyzable metal compounds (through the sol stage), the liquid in the pores consists mainly of water and/or alcohols. The resulting wet gels are therefore called aquagels, hydrogels, or alcogels. When the pore liquid is replaced by air without decisively altering the network structure or the volume of the gel body, aerogels are obtained (or cryogels, when the pore liquid is removed by freeze-drying). A xerogel is formed upon conventional drying of wet gels, that is, by increase in temperature or decrease in pressure, and is accompanied by large shrinkage (and mostly destruction) of the initially uniform gel body. Thus the general term ‘aerogel’ describes a high surface area solid material with an internal structure of pores and networks that were originally filled with liquid during synthesis but this has since been replaced with air. Aerogel was first discovered by the scientist Samuel S. Kistler in the late 1920s. Its characteristic properties have been revealed gradually since the initial synthesis (Kistler, 1931, 1932, 1935). Kistler’s work led to many other new and fascinating materials of different composition being developed along the same lines. The first recorded commercial silica aerogel was produced by the Monsanto Chemical Co. at Boston, MA, in the 1940s (Ayers, 2000). Synthesis of aerogel involves three steps: sol and gel formation, aging of the gel and solvent drying. This creates the internal porous structure, which gives the material its characteristic properties. The pores consist of cavities, voids, channels, holes or interstices which can be arranged regularly, similar to zeolite, or mesoporous and molecular sieves. However, aerogels more commonly have a rather irregular porous structure, as the different parameters of the chemical processes in seeding, aging and drying damage the internal ordering structure of the pores. Since the physical and chemical properties are greatly dependent on the conformation of pores, including



321

pore size, shape, porosity, and density, etc., controlling the size and shape of the pores and their network is a major challenge. The drying process is undoubtedly the most important step affecting the internal porous structure of the final aerogel material. The gel typically shrinks during drying. Many researchers have devoted their efforts to investigating mild drying techniques in ambient pressure in order to maintain the porous network (Schwertfeger et al., 1998; Prakash et al., 1995; Rao et al., 2007; Shi et al., 2006). In recent studies, modifications of surface functional groups, solvent exchange and use of different precursors (Schwertfeger et al., 1998) have been explored as methods to maintain or improve the porosity of aerogels rather than using expensive and cumbersome supercritical techniques. In this chapter, we first review the general synthesis of sol-gel materials and then discuss aerogel synthesis in order to elucidate the key parameters in controlling the internal porous structure of aerogels. For interested readers, more detailed information is available from the literature (e.g. Dorcheh and Abbasi, 2008; Husing and Schubert, 1998; Pierre and Pajonk, 2002, Pajonk and Venkateswara Rao, 2001).

13.2

Processing material and properties

13.2.1 Sol-gel processing The sol-gel process is a wet-chemical technique which is widely adopted for making high surface area oxide materials. The sol-gel process is composed of three stages: hydrolysis, condensation and drying (see Fig. 13.1). A liquid sol particle (the smallest seed particles in the solvent) is first created by hydrolysis of a chemical precursor, followed by condensation to form a gel. The first precursors used in sol-gel processing were metallic salts MXn,

Sol

Gelation/wet gel

t Ex

Mixing of the precursors

r

ti ac

on

o

o fs

lve

nt

Solid

Evaporation

Am

Start of hydrolysis Polycondensation and condensation of the sol particles/ aging

bie

nt-

pr

es

su

Air

re

dr

yin

g

13.1 A typical scheme for preparing aerogels by sol-gel process and drying processes.


322


in which a metal M is linked to some number n of anions X. In aqueous solution, the chemical precursors are presented as ionic species in which the metal atoms exist as solvated cations M[H2O]nz+. The reactions to form sol particles and gels comprise hydrolysis reactions, in which H2O groups are replaced with OH– groups (alkaline catalysed reactions, see Eq. 13.1) with the loss of protons, and condensation reactions, which lead to the construction of M–O–M ‘oxo’ bridges with the elimination of water molecules (nucleophilic attack of the metal cation, see Eq. 13.2). Thus, the sol particles gradually condense to form a three-dimensional continuous network (base-catalysed sols). In this stage, the porous gel structure is filled with solvent molecules. A more interconnected, wet gel (or hydrogel) forms during the aging period. During the final step, drying, solvent or liquid is removed from the pores of the wet-gel material. Conventional drying is typically accompanied by large shrinkage in volume. M O

H

+ OH

M O

d-

H

d+

H H O H d–

d+

13.1 M O H + H2O

M O

H H O H

d = 0, Leaving group

nucleophile

H d+

d–

M OH + M OH

M O

M OH 13.2

d+

M O

M

+ H2O

H M O

d–

OH M

To understand particle agglomeration, it is important to appreciate the surface characteristics of the particle and the type of aqueous species surrounding it. In general, zero-charged particles give higher aggregation rates to form larger particles, which up to a critical size may settle as precipitate under gravity. Figure 13.2 shows zero-charge nanoparticles in the liquid phase, which collide with each other and form larger clusters (this is thermodynamically favourable because it reduces the overall surface energy of the system). If each particle



323

13.2 Uncharged nanoparticles undergo a more rapid coalescence than those of charged particles.

13.3 Electrical double layers can protect colloid particles from aggregation.

carries the same electrical charge, mutual electrostatic repulsion will keep them apart in solution, as shown in Fig. 13.3. Typically, the small silica sol (nanoparticles) synthesized by the sol-gel method at pH 7.4 are in the form of a stable and homogeneous colloid which does not undergo aggregation. This is because of the lower isoelectronic point (pH ~ 2.0) of silica compared to the solvent medium, which renders the particles negatively charged. Particles possessing high charges can induce double layers in an aqueous environment and will remain discrete and well dispersed from each other.


324


13.2.2 Synthesis and structure of metal alkoxides In core-shell nanoparticle morphology, a core metal particle is protected from oxidation and corrosion by a silica coating (Santra et al., 2001). This structure is becoming more popular, and is created using microemulsion techniques. Hydrolysis and condensation of sol-gel precursors such as silicon alkoxide displace the surfactant molecules in micelle assemblies and produce an ultra-thin porous coating on the core particle at the water/organic interface. This process is based on hydrolysis of precursors such as tetraethoxysilane (TEOS) in the presence of water and catalysts, followed by condensation with surface metal hydroxyls. An M–O–Si chemical linkage is established between surface metal atoms and TEOS, followed by the formation of a three-dimensional network of siloxane bonds (Si–O–Si) with increasing TEOS concentration. Once the precursor has been condensed into a gel, the solvent is removed by drying. It is worth noting that the surfaces of silica gels can have different chemical functionalities, i.e. alkoxides, silanol and siloxane, depending on the preparation procedure, reagents used and posttreatments (see Fig. 13.4).

13.2.3 Possibility for advancement using other alkoxides for aerogels Although silica alkoxide is most commonly used, other metal alkoxides (M-OR) can be used for the sol-gel process. For example, titanium, zirconium, tin, carbon and aluminium can be prepared in a similar way to give porous microstructures (Sanchez and Ribot, 1994). The rates of hydrolysis and condensation for these metal alkoxides are much faster than those for silicon as they are much stronger electrophiles. Carbon aerogels can be prepared from pyrolysis of an organic matrix. The surface area of synthesized carbon aerogels ranges from 400 to 800 m2g–1. The pore size distribution is found to depend on the density of the carbon aerogel (Zhang et al., 1999). Carbon aerogel made using the pyrolysis process has been shown to have the largest specific area at 600 °C. After further pyrolysis at 2100 °C, the internal structure revealed by small-angle X-ray scattering (SAXS) contained a large amount of micropores (Reichenauer et al., 1998). Carbon aerogels also show

Si

OR

Alkoxide

Si

OH

Silanol

Si

O

Siloxane

13.4 Silicon–oxygen groups.


Si


325

density-dependent electrical conductivity, rendering them suitable for use as supercapacitors. They also find use as solar energy collectors because of their lower reflectance of radiation. Alumina aerogel is synthesized using either Al(OsBu) 3 (ASB) or Al(OtBu)3(ATB) as a precursor (Husing and Schubert, 1998). These aerogels can be doped with metallic promoter(s) to introduce new physiochemical properties. For example, Kwak et al. (2005) used a platinum–cobalt–alumina aerogel for the selective oxidation of carbon monoxide to carbon dioxide in hydrogen gas. Controlling the homogeneity of zirconia silica aerogel was demonstrated by Miller et al. (1994). Aerogels based on titania–silica and MTiO2 (M=Mg, Mn, Fe, Co, Zn) were tested as catalysts for photocatalysis (Cao et al., 2007; Ahmed and Attia, 1995; Kapoor et al., 2005). Zirconia aerogels were prepared using combined alcohothermal and supercritical fluid drying techniques, resulting in materials that were characterized by mesopores with a narrow pore size distribution and high specific area (Cao et al., 2002).

13.3

Aerogel formation

13.3.1 Drying techniques Making aerogels requires an efficient drying stage to replace trapped solvent molecules in the wet sol-gel materials with air without damaging the internal porous structure. The drying process is very complex, involving interplays between surface tension, capillarity and diffusion of the solvent with different pore size, shape and wall materials. When liquid is evaporated from occupied pores, it diffuses from the interior of the pore to the outer surface. A cohesion force holds the liquid molecules together (reflected by the phenomenon of surface tension), and adhesion forces stick the liquid molecules to the wall material. If different sizes of pores are present, the larger pores retain solvent molecules less easily than the smaller ones due to the smaller capillary potential under the same pressure conditions. The change in capillary force and the stress created on the wall of the pore are also different when solvent molecules leave the pore structure. A smaller pore network can become brittle, leading to cracking (see Fig. 13.5), since the capillary potential is inversely related to pore radius. Dynamic drying processes also affect the degree of shrinkage. A more rapid evaporation of solvent molecules from the interior pore surface will cause a greater pressure gradient change, which can easily lead to cracks in the pore structure. Thus, the large shrinkage of a gel body upon evaporation of the pore liquid is caused mainly by capillary forces acting on the pore walls as the liquid retreats into the gel body. This results in the collapse of the filigrane, the highly porous inorganic networks of aquagels and alcogels.


326

Materials for energy efficiency and thermal comfort in buildings Pores

Network

Pore liquid

Differential crack

13.5 Evaporation of solvent molecules from smaller pores induces more structural damage (crack) than larger ones.

During the conventional drying process, there are also chemical changes that affect the degree of shrinkage of the aerogel materials. For example, the inner wall surface of a pore is likely to be covered with –OH groups after wet chemical synthesis. In the case of silica gel, the surface Si–OH groups in close proximity can interact with each other to form siloxane bridges during drying. Thus, the porous network will become stiffer, with a smaller pore radius. Elevated temperatures will speed up these chemical changes. When the stress due to solvent evaporation is no longer capable of further deforming the network, the structure may collapse, creating substantial shrinkage in the gel material.

13.3.2 Supercritical drying Supercritical drying is a very efficient method for producing aerogel. Two supercritical drying techniques have been demonstrated. One is described as high temperature supercritical drying (HTSCD) and the other low temperature supercritical drying (LTSCD). HTSCD was the first technique employed in the synthesis of silica aerogel by Kistler in the 1930s and is still widely used to prepare aerogel materials today (see Fig. 13.6). The drying principle is illustrated in Fig. 13.6, which shows a critical point in the phase diagram of a fluid at which the density



327

Supercritical point Pc

A B P

C

T

Tc

13.6 Schematic to illustrate the principle of supercritical drying.

of its gas and liquid forms is identical. A wet gel containing a large amount of solvent (e.g. methanol, ethanol, acetone, 2-propanol, or water) is first placed in an autoclave, and the temperature and pressure of the container are slowly increased to above the critical point of the solvent (Step A). The drying conditions are kept constant and this allows homogeneous extraction of the solvent because, at the supercritical drying stage, there is no interface between gas and liquid. Thus, the pressure/stress change imposed on the pore structure due to removal of solvent is not significant. The next step, B, is to slowly vent the contents of the autoclave at constant temperature until it reaches ambient pressure. The final step, C, is to decrease the temperature to room temperature, yielding a dried aerogel. However, the supercritical states of most alcohols and acetones are characterized by critical temperatures about 250 °C and critical pressures between 5 and 8 MPa. It is important to note that the combination of high temperature and high pressure could cause undesirable chemical reactions or rearrangements in the surface of the gel network. Kocon et al. (1998) therefore studied the effect of pH on silica aerogel in an attempt to minimize material shrinkage during supercritical drying of ethanol from the structure. Under these conditions, a low density aerogel (3 kg/m3) can be obtained. Gross et al. (1998) used a rapid supercritical drying process to produce silica aerogel


328


from tetramethoxysilane (TMOS) with ammonia mixed in alcohol. This process involved loading the sol in a stainless steel autoclave and immediately heating the mixture to supercritical conditions. The aging process took place and the reaction rate was very fast due to the high temperature. Once the supercritical point of the alcohol was reached, the supercritical fluid was vented rapidly. The whole process lasted less than one hour (Gross et al., 1998). An advantage of this technique is that the process is carried out in one single step, and a short period of time means an increase in production volume. The disadvantage is that this technique may still induce chemical change in the material due to the high temperature and pressure, but the fast supercritical drying stage may reduce undesirable chemical change to some extent (Poco et al., 1996; Scherer et al., 2002; Gauthier et al., 2004). Another supercritical drying technique is the employment of carbon dioxide (LTSCD) for solvent extraction (Tang and Wang, 2005; Sui et al., 2004). The advantage of this method is that it takes place near to room temperature (< 32 °C) and under moderate pressure (< 80 bar). Figure 13.7 shows the phase diagram of carbon dioxide. A wet gel is placed in an autoclave and carbon dioxide is pumped in. The temperature is then raised to 40 °C and the pressure is kept at about 100 bars to reach the supercritical state of carbon dioxide. The time required for exchanging the trapped solvent with carbon dioxide depends on the dimension of the pores in the gel material. After extraction, the pressure of the applied carbon dioxide is slowly reduced and the temperature is dropped to room temperature. The fluid is then vented from

B

Supercritical carbon dioxide

Pc

A

C

P D

T

Tc

13.7 Schematic illustrating principle of supercritical drying at the lower temperature using carbon dioxide.



329

the autoclave. Repeated extractions in the same manner are carried out. Silica aerogel can be obtained using this technique (Dorcheh and Abbasi, 2008). Since water and carbon dioxide are rather immiscible, modifier(s) such as alcohols are required to produce highly porous silica aerogels (Husing and Schubert, 1998).

13.3.3 Freeze-drying process Freeze-drying is another mild drying method used to produce aerogels. The liquid solvent in the pores is rapidly frozen and then sublimed under reduced pressure. Using this drying method for the preparation of carbon or TiO2 aerogel materials is well documented (Mukai et al., 2004; Babic et al., 2004; Xu and Yang, 2003; Yamamoto et al., 2002). There are some prerequisite conditions that have to be achieved before the drying process can begin. First, the porous network of the wet material has to be extensively strengthened during the aging period. Secondly, the trapped solvent must have a low sublimation temperature, otherwise it will have to be exchanged with another solvent with lower sublimation temperature to facilitate diffusion and sublimation during drying. Salt or modifier may be used to lower the solvent freezing temperature. Maintaining the integrity of the porous network of a gel is challenging during crystallization of solvent molecules in the pores. The aerogel produced using this method is also known as ‘cryogel’.

13.3.4 Ambient-pressure drying process (APD) Another common drying method in aerogel production is known as ‘ambientpressure drying’ (APD). The drying is carried out at ambient pressure preferably using reduced or elevated pressures. As a potentially cheap and safe drying process for large-scale production of aerogels, this method has received considerable attention from industry. However, reducing shrinkage of the material during the drying process is a significant challenge, and the porous network has to be strengthened considerably, probably through surface modification, prior to drying. In addition, the contact angle between the trapped solvent and the pore wall must be measured to decide whether chemical modification of the inner surface is needed in order to minimize the change in capillary forces following solvent removal (Dorcheh and Abbasi, 2008). The general method for chemical modification of the surface is to use a mixture of precursors before gelation, although modifying the surface after gelation by using silylating agents is also effective. As the silylation reaction commonly takes place in an organic solvent, multi-step processes for solvent exchange are sometimes necessary. For example, mixed water and alcohol is used as a bulk solvent to exchange the trapped water in the gel pores before the silanol group on the surface is silylated by adding a


330


1 –R

O–Si–R2 –R 3

1

–R

OH

–R3

OH

–R

R2 i– –S –R3 O

–R1

O–Si–R2 –R 1

O –S –R1 i– R2

H

O

H

O

CTMS or HMDZ

O– R1 Si – –R 2

O–Si–R2 –R 1

R2 i– –S –R3 O

H

H

Silylation

–R3

O–Si–R2 –R 3

O

–R1

O

3

OH

–R

OH

modifier (e.g. CTMS, HMDS, HMDZ or others) (see Fig. 13.8). After the surface reaction, the material becomes hydrophobic due to the presence of surface capping groups (Si–O–R). Multi-stepped exchange processes also require long pre-treatment time and large amounts of solvents, which causes economic and environmental concerns. Ambient-pressure drying still results in a degree of material shrinkage, because severe stress is created in the gel material due to the unavoidable pore narrowing with the formation of siloxane. The pore structure can be returned to its original form upon re-wetting under defined conditions, the so-called spring-back effect (Lee et al., 2002) (see Fig. 13.9). Schwertfeger et al. (1998) presented a competitive method using inexpensive waterglass, a simple exchange solvent technique, and drying in ambient pressure with modification of the internal surface. They employed hexamethyldisiloxane (HMDSO)/trimethylchlorosilane (TMCS) solution to modify the inner surface of the wet gel. IPA/TMCS/n-hexane solution was used to prepare a silica aerogel by Kim and Hyun (2003) and Lee et al. (2002). Rao et al. (2005b)

3

13.8 A typical surface silylating process on aerogel by chlorotrimethylsilane (CTMS) or hexamethyldisilazane (HMDZ).

Springback

Springback

Dried gel

Aerogel

13.9 Demonstration of a ‘spring-back effective’ gel. Reproduced with the permission of Elsevier BV.



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developed a two-step drying process at ambient pressure applicable to various solvents, including hexane, cyclohexane, heptane, benzene, toluene and xylene. They prepared aerogel from TEOS and modified the surface with hexamethyldisilazane (HMDZ) in different solvents. They reported that heptane gave highly transparent, porous but low density, low thermal conductive and hydrophobic aerogels. FTIR spectra of their pre-treated aerogels clearly indicated the change in surface functionalities (the IR bands at 1600 and 3500 cm–1 corresponding to Si–OH and H–OH were substantially attenuated but new bands at 840 and 1250 cm–1, corresponding to Si–C and C–H were formed (Rao et al., 2005a)).

13.3.5 Controlling density and void size distribution When a wet gel is dried by removing trapped solvent molecules, an inherent porous structure can be reclaimed. In the case of inorganic gel, the skeletal structure becomes more rigid during solvent evaporation. Liquid in the pores is removed to form an extensive porous network. However, heat treatment will concomitantly cause the porous structure to collapse, making the network more dense. The chemical reactions that take place during heat treatment occasionally isolate the pore structure, resulting in closed pores. It is important to avoid cracking and pore blockage in order to maintain an ordered porous structure in an aerogel. A sharp void size distribution can be obtained by adding reagents such as formamide, oxalic acid or glycerol to the gel precursors. The size of macroporous silica aerogel can be tailored by tuning the concentration of polymer added. For example, Nakanishi et al. (1998) successfully prepared a narrow void distributed aerogel by adding polyethylene glycol (PEG) to their sol precursor. A high concentration of PEG improved the strength of the solid matrix, whereas low concentrations of PEG reduced its strength. Harreld et al. (2002) modified silica gels by exchanging a mixture of solvents in order to reduce structural shrinkage during drying at ambient pressure. The porosity, pore size distribution and surface area of their gels were much improved by using appropriate ratios of acetone/alkane. Tabata et al. (2005) dried an alcogel (less shrinkage than aerogel) at ambient temperature to produce an extremely low density gel material of 9 kg/m 3, which corresponded to a reflect index of 1.002. The world’s lowest density silica aerogel was prepared by Larry Hrubesh at Lawrence Livermore National Laboratory (LLNL) in the late 1980s. The claimed density of the aerogel was 3 kg/m3, which is approximately only three times that of air (Arlon and Michael, 2004). Widenmeyer and Anwander (2002) used surfactants ([CH3(CH2)nNMe2(CH2)mNMe2(CH2)nCH3]2+2Br–) of different chain lengths under hydrothermal methods at pH > 9 to prepare tuneable pore sizes and highly ordered cubic mesoporous silicas (MCM-48). Kim and Ryoo (1999)


332


prepared tuneable pore sizes of cubic mesoporous silica SBA-1 under acidic conditions using tetraethyl orthosilicate and hexadecyltriethylammonium bromide. The pore diameter of the SBA-1 materials was controlled over 1.4–2.7 nm by tuning the surfactant concentration. Rao et al. (2003) used methyltrimethoxysilane (MTMS) precursor, methanol (MeOH) solvent and ammonium hydroxide (NH4OH) catalyst to prepare aerogels. The optimum reagent ratio was obtained to yield a low density but highest contact angle (~173°) aerogel. BET is the most common method for characterizing porosity, pore size distribution and surface area of a synthesized aerogel.

13.4

Physical properties of aerogels

13.4.1 Thermal conductivity Kistler was the first researcher to discuss the low thermal conductivity of silica aerogel (Kistler, 1932, 1935). This led to a fundamental analysis of the factors affecting the overall thermal transportation of aerogels consisting of gas phase, solid phase and radiation components. Figure 13.10 shows a model of the thermal transportation properties of an aerogel as a function of density (Husing and Schubert, 1998). There is a sharp increase in conductivity when the density of the gel material is raised. However, gas and radiation 0.02

Total

Solid

0.01 l/w m–1 K–1

Gas Radiation 0 50

150 r/kg m

250

350

–3

13.10 Thermal conductivity of aerogels: total, solid, gas and radiation transport l, are dependent on the density r of aerogel materials (Husing and Schubert, 1998; Fricke, 1988).



333

components show the opposite trend for thermal conductivity with increasing density. Thus, the lowest overall thermal conductivity peaks should be minimized at a density of about 150 kg/m3. It was anticipated that smaller pore size played an important role in lowering gaseous thermal conductivity due to the Knudsen effect (Pierre and Pajonk, 2002), and that gas thermal conductivity could be further minimized under reduced pressure. This led to the production of a new silica aerogel which showed thermal conductivity as low as 0.020 W/mK at ambient pressure, with a further decrease to 0.010 W/mK under vacuum at 300 K (Husing and Schubert, 1998; Fricke, 1988). Thermal conductivity also relates to the nature of the porous structure: whether the pores are closed or open will give different thermal transport properties. Heat transport through infrared radiation is an important factor affecting the thermal conductivity of silica aerogels. This is affected by density, temperature and optical thickness. Reducing thermal conductivity by doping aerogels with infrared opacifiers such as carbon or TiO2 is well documented (Husing and Schubert, 1998). Silica aerogels show the lowest thermal conductivity of all non-flammable solid materials (see Fig. 13.11), thus they can be used as a coating to modify construction materials. Aerogels can be used to coat the walls of buildings. The poor infrared radiation prevents heat from a heated room penetrating outside, but visible and UV radiations from external daylight can pass through the material. Aerogels are therefore used for window insulation, daylight windows (for example car windshields, or bathroom, staircase or 45 40 35

l/mW m–1 K–1

30 25 20 15 10 5 0 Aerogel PUR/CFC Air

EPS XPS CFC

Mineral- wool

PUR/CO2

13.11 Comparing thermal conductivity of aerogel with other commercially available materials. PUR: polyurethane foam, CFC: chlorofluorohydrocarbons; EPS, XPS: expanded and extruded polystyrene (Husing & Schubert, 1998).


334


ceiling windows) and cooling and heating storage applications (Husing and Schubert, 1998; Pierre and Pajonk, 2002). Solar energy can be captured by panelling house walls (see Fig. 13.12), or by solar collectors that use aerogel materials (Fricke et al., 1987; Svendsen, 1992). It is well known that transmitted light suffers from the scatter phenomenon, causing poor optical quality in conventional materials, but significant improvements in the optical quality of aerogels have been reported recently (Duer and Svendsen, 1998). Schultz and co-workers reported a super insulating glazing using a monolithic SiO2 aerogel sheet of approximately 55 × 55 cm2. Anti-reflective treatment of aerogel-coated glass can also improve their visual quality. Solar energy transmittance can reach 76% (Schultz et al., 2005; Jensen et al., 2004).

13.4.2 Optical properties Silica aerogels are between transparent and translucent, depending on their internal structure and moisture content. The porous networks of ordered Shade Outside

Inside

Glass panes

40 °C Wall

Aerogel

20° Insulation

Thermal transport

–15°C

Light-absorbing layer Heat loss

Heat gain

13.12 Principle of energy saving by using aerogel to capture solar energy (Husing and Schubert, 1998).



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silica aerogels have smaller structural units than the wavelength of visible light, hence they should appear to be transparent. But if some amorphous structures are introduced in the ordered units, these lead to scattering in the visible light region, known as Rayleigh scattering (De Leo et al., 2001; Tajiri and Igarashi, 1998). Such non-homogeneity in silica aerogel is responsible for reduced transparency, and most silica aerogels appear yellowish under light and bluish under a dark background (Husing and Schubert, 1998). Their optical properties can be tuned by controlling the content of amorphous structures in the aerogel during synthesis. Optical transparency is important for some applications. For example, a new type of solid-state Cerenkov particle detector requires a high quality, transparent aerogel (Adachi et al., 1995). Controlling the processing parameters during synthesis gives arerogels with different degrees of optical transparency. The drying process, degree of water adsorption on Si–OH groups, adsorbed organic components on Si–OCH3 or Si–CH3, heat treatments (Buzykaev et al., 1999), type of precursors, sol-gel process, silation agent and solvent used were all found to affect the optical properties of aerogels (Rao et al., 2001). It was also claimed that two-step synthesis methods give more transparent aerogels than one-step methods, which could be related to the lower degree of amorphous silica present in the aerogel (Rao et al., 2005b; Danilyuk et al., 1999). Supercritical drying of the gel materials with CO2 commonly resulted in materials with better transparency than those dried using other organic fluids. Lower reaction temperature and lower amounts of water appeared to be effective in making aerogels that were more transparent (Husing and Schubert, 1998). Waterglass, a cheap aerogel precursor, gave a low transparency gel, probably because of the presence of large silica particles and sodium ions in the sample.

13.4.3 Acoustic properties Overall, aerogels are excellent acoustic insulators. The acoustic propagation in aerogels depends on the interstitial gas type and pressure, density and, more generally, texture. The sound energy transferred from the gas to the solid phase is partly lost, thus reducing the amplitude and velocity of the acoustic wave. The longitudinal acoustic velocity is typically in the order of 100 m/s. This property makes silica aerogels suitable for applications in acoustic impedance ì/4 matching layers in ultrasonic transducers, range finders, speakers, etc. Their application in house floor-covering and for football pitch sound insulation has been mentioned (Husing and Schubert, 1998). Other applications concern acoustic insulation in transportation and machinery (Gross and Fricke, 1992; Gross et al., 1992). Silica aerogels have also been investigated for applications in anechoic chambers (Pierre and Pajonk, 2002; Forest et al., 1998; Zimmermann et al., 1995).


336

13.5


Applications of aerogels

13.5.1 Solid state insulation As aerogels have an extraordinarily low thermal conductivity and high acoustic insulation, a striking number of applications have been developed in window glazing (Tewari et al., 1986; Duer and Svendsen, 1998). Reim et al. (2002, 2005) undertook a large programme to develop a highly insulating but translucent glazing material. A sandwich structure construction of gel filled with granules was set up. Krypton gas was used as the filler to optimize thermal insulation. The resulting heat transfer coefficient was less than 0.4 W/mK, with total solar energy transmittance of 35%.

13.5.2 Space applications As low density and lightweight materials, aerogels can successfully capture cosmic dust particles in space. NASA’s stardust (spacecraft) mission programme employed silica aerogels for such capture. The speed of a cosmic dust particle is at least six times that of a bullet. Thus, when a dust particle hits the mechanically rigid silica aerogel, a bullet shaped track can be located in the gel. The extraordinarily lightweight silica aerogel also protects space mirrors (Hrubesh, 1998). The European Recoverable Carrier (EURECA) spacecraft is also using silica aerogel for catching falling cosmic dust (Tsou, 1995). This technology could be expanded to capture gases and dust in buildings and vehicles.

13.5.3 Optical applications Transparent aerogels may find applications in solar windows (Tewari et al., 1986; Rubin and Lampert, 1983; Fricke, 1986), whilst translucent aerogels are proposed for use as solar covers and collectors (Svendsen, 1992; Jensen, 1992). Ultralow density aerogels are currently being considered as lightweight mirror backings (Hotaling, 1993; Hrubesh, 1998). Silica aerogels doped with lanthanides are used as laser glasses (Tillotson et al., 1994). If they are doped with radioactive tritium and a phosphor, radioluminescent light/power sources can be obtained (Ashley et al., 1992). If treated with microwave-energized reducing gases, silica aerogels could become permanent visible photoluminescent sources with emission wavelengths ranging from 460 to 500 nm, due to oxygen-defective centres (Ayers and Hunt, 1997).

13.6

Future trends

Silica aerogels have high surface areas, are highly porous and lightweight but mechanically rigid and will no doubt find a wide range of applications © Woodhead Publishing Limited, 2010


337

from pharmaceutical to semiconductor industries. Silica aerogels can also be chemically modified to suit specific applications. A popular area of research is seeking applications for silica aerogel in biotechnology. Functionalizing the surface of silica aerogels to carry biomolecules can be divided broadly into two methods. The first method is based on non-covalent binding of bio-species by simple electrostatic interaction. At a pH above the isoelectric point (IEP), the surface of a silica aerogel becomes negatively charged, whereas at a pH below the IEP, the surface is positively charged. If the biomolecule of interest carries the opposite charge to the aerogel in solution, it attaches to the silica surface by electrostatic attraction. The advantage of this method is that it does not require complex processes for chemical binding of the biomolecule, which therefore maintains its original conformation and activity (see Fig. 13.13). The alternative technique is covalent linkage of the biomolecule onto the surface, which provides a much stronger binding affinity and site specificity for the biomolecule attachment. General methods for covalent conjugation of biomolecules with surface functional end groups such as –NH2, –SH, and –COOH are available (see Fig. 13.14). Silica aerogel can therefore carry a wide variety of bio-species following surface modification, including small drug, antigen and protein molecules. After the silica surface is tagged to the bio-species, interesting applications such as in vitro bio-separation and in vivo targeting of cancer cells can be envisaged, and chemical modification of aerogels has been studied intensively over the last decade. El Rassy et al. (2003) have recently tailored the surfaces of aerogels with different degrees of hydrophobicity and hydrophilicity through different coverage of Si–OH, Si–OCH3 or Si–CH3 end groups. Bass and Katz (2006) have prepared silica surfaces with bi-functional groups consisting of thiol and primary amine groups, which were characterized using solid-state UV/Visble, Si-19 CP/MAS NMR and C-13 CP/MAS NMR spectroscopy. Zhang et al. (2008) prepared helical meso-structured silica as a drug carrier. Their report revealed that drug release rate could be controlled by the helicity and morphology of the meso-structured silica. Chandra and Bhaumik (2009)

Silica aerogel

Bio-species

13.13 Schematic to illustrate the formation of non-covalent biospecies conjugates through electrostatic attraction.


338

Materials for energy efficiency and thermal comfort in buildings O

O + NH2 – Biomolecule

NP

C

NP

SH + HO –(CH2)6–S–S– Biomolecule

OH

NP

C

NP

NH

S–S

Biomolecule

Biomolecule

O O NP

NH2 +

N–O–C– Biomolecule

O NP

NH

C

Biomolecule

O

13.14 Schematic representation of covalent bio-conjugation protocols for the attachment of biomolecules on silica nanoparticles (Wang et al., 2006). Reproduced with the permission of American Chemical Society (ACS).

prepared a new functionalized mesoporous polymer with silica for the removal of pollutant anions. For example, mesoporous poly-triallylamine (MPTA-1) showed high anion exchange efficiency for the removal of pollutant anions such as MnO4–, CrO42–, AsO33–, NO3– and PO43–. Attia et al. (1994) prepared alkaline earth oxide modified aerogels for the capture of greenhouse gases such as CO2, SO2 and NOX. The high surface area of silica areogel combined with the basicity of CaO or MgO has proved to be efficient in capture and/ or adsorption of these acidic pollutants. Other smart materials containing silica aerogel, such as silica-coated magnetic nanoparticles (Yu et al., 2007, 2009; Tsang et al., 2006) and luminescent silica aerogels (Fricke, 1992; Ashley et al., 1992; Leventis et al., 1999) have been described. Heavy metals have been removed from industrial effluents using novel magnetic nanocomposites synthesized by embedding metal nanoparticles in a silica aerogel (Xu and Dong, 2008). Applications of these materials in capturing hazardous substances from water have been demonstrated (Bryant et al., 2003; Wingenfelder et al., 2005; Perez-Quintanilla et al., 2006). The possible applications of these smart and related materials to solving current environmental problems have been widely discussed (Zhu et al., 2007; Mureseanu et al., 2008; Puanngam and Unob, 2008; Shevchenko et al., 2008; Yantasee et al., 2008). As a result, study on grafting reactive species, including bio-molecules, inorganic catalyst species or composites, onto material surfaces has intensified. New applications, such as developing enzymatic or photo-catalytic paints for wall decorations to remove pollutants (e.g. incorporating photoactive TiO2 and ZnO into aerogels) may become possible in the near future.



13.7

339

Conclusions

This chapter only gives a limited amount of information on the chemistry and techniques involved in synthesizing aerogels. Many inorganic polymeric oxides are remarkable by themselves, with a very rich chemistry that can be tailored to a wide range of applications in fundamental science and modern technology. Recent developments in the preparation of ‘aerogel-like’ materials at ambient pressure, which eliminates the need to use an autoclave, will open new fields of applications at the industrial level and require further chemical characterization. Organic, inorganic and composite aerogels clearly deserve to be studied as a new class of chemically designed architectures, and much remains to be done for materials other than silica.

13.8

References

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14

Hygrothermal materials for heat and moisture control in buildings

M. R. H a l l, University of Nottingham, UK

Abstract: This chapter seeks to explain what makes a ‘hygrothermal’ material, with reference to their specific ability to regulate indoor environmental conditions of air temperature and relative humidity. The volume averaged physical properties of these materials is addressed along with the experimental and numerical techniques employed to characterize and study their behaviour. The main material classes of candidate hygrothermal materials are explained in detail. Finally, the application of these materials in relation to buildings is explored along with predicted future research trends in this field. Details of the main active research groups and numerical modelling techniques available are provided for those wishing to engage in the subject. Key words: hygrothermal behaviour, porous materials, humidity buffering, thermal buffering.

14.1 Introduction The balance (or buffering) of indoor air temperature and relative humidity (RH) in buildings is very important from the point of view of respiratory health and comfort of the occupants (Toftum et al., 1998a), healthy skin by avoiding dryness (Toftum et al., 1998b), perceived indoor air quality (Fang et al., 1998a), to avoid deterioration of the building materials (Lucas et al., 2002), and to avoid mould growth and the problems associated with condensation (Sedlbauer, 2002). The use of mechanical systems for heating, cooling, ventilation and air conditioning for air temperature and RH control is not always the best solution due to the levels of energy consumption required, the issue of noise for occupants, the need for regular maintenance, and the potential threats associated with security of energy supply needed to run the equipment.

14.1.1 What makes a material hygrothermal? Hygrothermal behaviour, with respect to materials and buildings, considers the simultaneous (and inter-dependent) occurrence of heat absorption, storage and release, and moisture (liquid/vapour) absorption, storage and release. It is by definition a fully transient approach normally tackled through a combination 345 © Woodhead Publishing Limited, 2010

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of experimental testing, physical modelling and numerical modelling solutions, as detailed in Chapter 2. The approach is typically applied to materials such as timber, plasterboard, insulation materials, masonry, concrete, brick, lime plaster, etc., and more recently earth materials. All materials and adjacent control volumes (e.g. a room or zone in a building) can be assessed hygrothermically, but not all exhibit desirable properties for the given application. The ones that do are referred to as ‘hygrothermal materials’ and have good capacity for enthalpy buffering, i.e. simultaneous buffering of heat energy (sensible and latent energy in air) and mass concentration (vapour and associated partial pressure). In order to predicatively model hygrothermal behaviour, knowledge of several material functional properties is required: ∑ dry density ∑ bulk porosity and apparent porosity ∑ dry state and moisture-dependent heat capacity at constant pressure ∑ dry state and moisture-dependent thermal conductivity ∑ sorption isotherms including absorption/desorption moisture storage functions at relevant temperatures ∑ sorptivity (or water absorption coefficient) ∑ vapour permeability ∑ hydraulic conductivity. Much experimental data has been recorded and published for these properties for many conventional and existing materials, though there is much work still to do. The data is included in the databases for various computer software packages as listed in Section 14.6. The key advantages to using hygrothermal materials in buildings include: ∑ ∑ ∑ ∑ ∑ ∑

Increased thermal comfort Increased perceived indoor air quality Reduced ventilation and cooling in summer time Reduced mould formation Reduced dust mite population (asthma trigger) Reduced structural degradation of materials due to moisture.

14.1.2 Buffering capacities Conventionally, we can study thermal (heat) energy buffering and moisture vapour buffering. Both involve absorption, desorption and volumetric storage capacity changing as a function of time in response to driving potentials in the adjacent control volume. For details of the fundamentals behind these processes, refer to Chapter 2. In the case of buildings, the material constitutes


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a portion of the indoor environment (e.g. walls) and the control volume is an occupied space filled with air containing moisture vapour and other gases. We know that for thermal energy, the driving potentials include the heat energy differential between air and material with heat therefore travelling from the hotter body to the cooler body, e.g. warm air heating a cooler surface. The increase in surface temperature of the material generates a temperature gradient within that material resulting in transfer of heat, principally by conduction. The rate of energy transfer is determined by the thermal conductivity of the material and also the steepness of the temperature gradient. If the material density is high, normally accompanied by a high specific heat capacity, the energy requirement for temperature gradient formation is higher. Hence, if the rate of heat flow is constant from the control volume (assumed steady state) the response time of the material to distribute heat throughout its mass can be given by the thermal diffusivity, a (m2/s), which is simply calculated from l/rcp. Thermal effusivity, b, is the ability of the material to exchange heat with its surroundings (i.e. heat entering or leaving the material) and is given by l r c p . In the same way that ‘thermal mass’ can be used to stabilize the temperature of occupied spaces, hygroscopic materials have a hygric capacity that can be used to buffer relative humidity fluctuations. Excessive humidity can result from activities such as bathing, washing and drying clothes and occupant perspiration, while low humidity is associated with heated spaces in the winter months. Humidity buffering is therefore a desirable property of a material to absorb water vapour from the air, when relative humidity is high, and release this water as vapour when relative humidity falls. This property can be quantified using the experimentally-determined moisture buffer value (MBV) as defined by Rode (2003) and the NORDTEST Project. For this test a sample of material is subjected to a high humidity (23 °C 75% RH) environment for a period of 8 h followed by a low humidity environment (23 °C 33% RH) for 16 h. This cycle is repeated for at least three days and until the change of mass between cycles is less than 5%. Specimens are sealed on all but one exposed surface over which the air velocity should be 0.10 m/s ± 0.05. An example of experimental test data for the MBV test is shown in Fig. 14.1. The MBV (g/m2 %RH) is calculated from the change in mass per square metre per % relative humidity (Rode, 2003): MBV =

Dm AD RH

14.1

Using this approach, materials may be classified under one of five categories based on their ability to contribute to the control of humidity in a room as shown in Table 14.1. A numerical method for estimating the MBV using material physical


Materials for energy efficiency and thermal comfort in buildings Mass change per unit surface area (g/m2)

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8h

16 h Elapsed time

14.1 Typical graph of experimental data for the moisture buffer value (MBV). Table 14.1 Moisture buffer value classification scheme (adapted from Nordic Innovation Centre, 2005) MBV class

Minimum MBV Maximum MBV g/(m2 %RH) @ 8/16 h

Negligible Limited Moderate Good Excellent

0 0.2 0.5 1.0 2.0

0.2 0.5 1.0 2.0 –

properties was also proposed by the NORDTEST Project (Nordic Innovation Centre, 2005). It was based on a mass transport solution and assumes no boundary layer resistance. For these reasons it is distinguished by being called the ideal moisture buffer value or MBVideal and represents a theoretical maximum value where: MBV Videal = 0.00568 psat bm t p

14.2

Note that the value, bm, is the moisture (or mass) effusivity, which is analogous to the thermal effusivity described above, and is calculated by:

d p rd bm =

∂wm ∂j

psat

14.3

Enthalpy buffering is the minimization of ∆H, where H = U + PV. In the case



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of psychrometrics, for the air in a building, internal energy, air pressure and air volume can change as a consequence of air temperature, and also partial pressure due to changes in mass concentration (moisture content of the air). This air of course interacts with the material fabric inside the building. The rates at which energy can be transported (in the form of heat) from the air to the material and back again, and the rate at which mass can be transported (in the form of water vapour) in and out of the material, can effectively create a balance effect in indoor air enthalpy. Hygrothermal materials derive their name from the fact that their hygrothermal functional properties are particularly useful for enthalpy buffering, as previously mentioned. The first most important set of functional properties relates to the control of absorption and desorption of water vapour from the air in order to maintain small changes in the humidity ratio. The second most important parameter relates to the control of absorption and desorption of energy (as heat) from the air in order to maintain small changes in dry bulb temperature. This can be visualized using a psychrometric chart (as discussed in Chapter 4) in which the influence of both humidity ratio and dry bulb temperature on air enthalpy at constant relative humidity can clearly be seen.

14.2

Characterization of hygrothermal functional properties

The following section explains how hygrothermal functional properties can be characterized along with various appropriate test methods and procedures for providing data.

14.2.1 Volume-averaged properties The main functional properties of a hygrothermal material are listed in Section 14.1, and the fundamentals behind these properties are described in detail in Chapter 2. A good hygrothermal material has, amongst other things, a high moisture buffer value (MBV) and the equivalent of a high thermal buffer value (TBV) which manifests itself as a delayed thermal response and fabric energy storage. These things are controlled by material physical properties at the microstructural level which are effectively quantified as volumeaveraged properties at the macrostructural level. The important concept behind hygrothermal behaviour, however, is that the absorption, storage and release of both heat energy and mass (moisture) are interdependent and occur simultaneously, i.e. thermal performance becomes more complicated and more important when moisture is present, and vice versa. Empirical relationships between l and constituent volume fractions, bulk density and moisture content have been obtained from experimental testing of a wide range of porous building materials by various researchers. Auto


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coherent homogenization has been used to try simple predictions of l for two phase (air and solid) or three phase (air and mixture of solid A and solid B) porous materials by assuming that the heat transfer is analogous to that occurring across a multilayered sphere of concentric homogenized phases (Bederina et al., 2007). The cubic lattice and capillary bridge model (Ewing & Horton, 2007) proposes a mathematical description of l for dry porous materials (when the void ratio is varied), and for moist porous materials when the degree of saturation is varied. The Random Mixed Cube Model (Zhang et al., 2006) uses the three phase continuum model as a base to represent relative quantities of each phase as small discrete cubes randomly arranged within a larger finite volume cube (i.e. analogous to a Rubik’s cube). The heat transfer between adjacent cubes is modelled to provide a cumulative effect. This allows good consideration of the boundary effects but cannot accurately measure the effects of void geometry which is crucial in determining the independent functions of the matrix structure. There is much research work still to do in order to fully understand how and why the volume-averaged hygrothermal properties that we measure for a given material originate by understanding their microstructure.

14.2.2 Measurement techniques Many standard modelling techniques for thermal and hygric properties of porous materials are detailed, along with the fundamentals behind them, in Chapter 1. The important properties for a hygrothermal material are listed in Section 14.1 and the techniques for measuring them are described here. Thermal conductivity can generally be assessed using a heat flow meter (HFM), dual probe heat pulse meter (DPHP), or a guarded hot box. Thermal conductivity testing using computer-controlled heat flow meter apparatus, for example, must comply with ISO 8301 Thermal Insulation – Determination of Steady State Thermal Resistance and Related Properties (ISO 8301, 1991). For moisture-dependent thermal conductivity testing, the same HFM apparatus is used but the methodology is adapted to comply with ISO 10051 Determination of Thermal Transmissivity of a Moist Material (ISO 10051, 1996). An alternative to direct measurement is ISO 10456 Procedures for Determining Declared and Design Thermal Values (ISO 10456, 1999) which provides methodologies for the theoretical calculation of the change in thermal conductivity when influenced by, for example, variations in specimen moisture content, temperature and age. The rate of water vapour flow through a porous specimen can be determined using the wet cup/dry cup method in a temperature and humidity controlled environmental chamber (BS EN ISO 12572, 2001). By using a saturated salt solution for the ‘test’ vessel, a vapour pressure gradient can be imposed across the sample thickness thus inducing diffusive mass transfer and allowing



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the water vapour permeability to be calculated. Sorption and desorption isotherms can be determined using small representative samples that have been oven dried to constant mass. Using an array of sealed desiccators containing different saturated salt solutions, to provide a wide range of stable relative humidity environments, the dry specimens can be progressively placed in each of the desiccators and permitted to absorb moisture vapour until equilibrium moisture content is achieved. The relationship between the moisture content of a porous material and the relative humidity of the surrounding environment is quoted using the moisture storage function (MSF). Sorptivity and rate of liquid water flow in unsaturated specimens can be measured using the partial immersion test described in Chapter 1. The hydraulic conductivity, as a function of moisture content, can be calculated based on the MSF value. Much more precise physical modelling of liquid water movement can be performed using gamma ray scanning devices and nuclear magnetic resonance (NMR) spectrometry (Hall & Hoff, 2002).

14.3

Material classes

Several engineering materials can be described as ‘hygrothermal’ exhibiting strong behaviour. In the case of building physics applications, these chiefly fall amongst the categories described in the following sub-sections although there is still much need for further research and many institutions are now actively pursuing this field.

14.3.1 Clay materials The term ‘clay’ can mean many things. It can, for example, refer to a clay soil which can be a formal geological classification, or it can refer to the behaviour of the material (clay-like) in terms of cohesion and plasticity. In terms of chemical composition, clay is a mineral belonging to the group know as phyllosilicates on account of their layered sheet-like structure. In civil engineering terms, clay is a soil material originating from a parent rock mineral and has a mean particle diameter of 2 microns or less. Clays are naturally-occurring phyllosilicates formed from various parent rock minerals, usually as the result of weathering. The many variations of naturally-occurring clay minerals are formed from layers of silicate and aluminate minerals that produce either 1:1 or 2:1 lamellar structures. They form discrete particles known as micelles, as shown by the examples given in Fig. 14.2. For further details on clay chemistry and clay mineral classification, refer to Velde (1992). Since water is a polar molecule, it is attracted to the net electrostatic charge on the surfaces of clay platelets. H + in H2O is attracted to O– in the platelet shell. The higher the specific surface area of the clay, the more water molecules will be attracted both in terms


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Oxygen or hydroxyl

Various cations

Packed according to charge and geometry

Repeated to form a sheet Tetrahedral

Octahedral

Stacked in ionic and covalent bonding to form layers 1:1 Semi-basic unit

2:1 Semi-basic unit

Stacked in various ways

Stacked in various ways

Water ions Water + ions Water + Mg Potassium

Water layer Kaolinite

Halloysite Pyrophyllite

Smectite Vermiculite

Illite

Potassium

Chlorite

Mixed layer

14.2 Composition of typical clay mineral structures (Mitchell, 1993).

of quantity and distance of the effect. The accumulation of water (often containing hydrated metal cations) forms what is known as a double diffuse layer around the clay platelets. The thickness of this layer (or distance from the platelet surface) is determined by the surface potential of the clay, i.e. the magnitude of the net electrical charge. Some clay minerals have very low surface potentials, such as kaolinite, and hence their double diffuse layer is relatively thin in relation to the platelet size. This means that the quantity of water absorbed, when equilibrium is achieved between charged surface and water molecules, is relatively small. Other clays, such as montmorillonite, have very large specific surface areas (up to 800 m2/g) and consequently their surface potential and double diffuse layer thickness is extremely high, see Fig. 14.3. These minerals can absorb very large quantities of water but obviously they expand considerably in volume whereas less reactive clays do not. Clays generally have a small range of achievable l, and a slightly bigger range for dry density/porosity. This means that thermal resistance, R, cannot change much for a given thickness (in the dry state) between materials. However, when water is introduced, even in small amounts it, can vary considerably depending upon pore structures and their properties. This means that thermal storage and also thermal diffusivity can be altered, and hence thermal buffering can be manipulated. Crucially, clay minerals are very hygroscopic due to their chemical composition as described above. Unfired clay bricks are often referred to as ‘green bricks’ as they use less energy during the production process and are simply air-dried rather than



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40 nm Adsorbed water 100 nm

20 nm

Kaolinite crystal

20 nm

Montmorillonite crystal (1 nm thick)

40 nm

14.3 Relative sizes of adsorbed water layers surrounding a montmorillonite clay particle (left) and a kaolinite particle (right) (adapted from Holtz & Kovacs, 1981).

being fired in a kiln. They can be formed by hand or mechanically extruded and are dried until the equilibrium moisture content is reduced to around 2%. Many shapes and sizes can be created through the use of a variety of different moulds. They are designed to be used for non-structural, internal applications because they degrade when in long-term contact with water, i.e. exposure to weather. The bricks create little waste during production, require less energy to produce them, and can be either recycled or cleanly disposed of when no longer needed (Morton, 2006). Bricklaying can be done following conventional trade practices, although the bricks must either be left exposed or coated with a highly breathable paint, surface treatment or plaster. Unfired clay bricks are a relatively new idea in Great Britain; however, in other parts of Europe, particularly Germany, it has developed a good reputation and been perceived as a viable building solution. A number of UK suppliers have started to produce earth bricks to meet the ever-increasing interest in these methods. Ibstock, for example, have released a line of earth bricks known as ‘Ecoterre bricks’ and even equip buyers with a list of retailers who supply materials such as mortars and paints to be used alongside their products. The Errol Brick Company in Scotland also produces a range of green clay bricks and, along with Arc Architects, Fife, have successfully been used in a small-scale trial building to regulate relative humidity over the period of a year. The test house was designed to passively regulate internal relative humidity between 40% and 60%, and tests confirmed that in the bathroom humidity regulation was the same after a shower whether or not mechanical ventilation was in use (Morton, 2005). Water vapour sorption of 1.5–2.3% wt at 80% RH and a vapour permeability of between 8 and 16 ¥ 10–12 kg/(Pa s m) is reported by Hansen & Hansen (2002). In addition, they found that an active depth (i.e. the depth from the surface involved in sorption/desorption) of between 4.1 and 5.1 mm was available on green clay bricks for humidity buffering.


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Clays can be used in combination with fibre reinforcement (to prevent expansion/contraction cracking) as interior plaster for buildings and occupied spaces. A good example of this is the range of clay plasters produced by CLAYTEC, Germany, although other manufacturers now make similar products. One of the main advantages of clay plasters are that they have the ability to absorb and diffuse water vapour for buffering of interior relative humidity fluctuations. The plasters are also perceived to be more environmentally friendly because they are not calcined (heated at high temperatures to dissociate CO2) and do not contain any energy-intensive binders such as Portland cement or lime.

14.3.2 Stabilized earth materials It is claimed that rammed earth (RE) or stabilized rammed earth (SRE) walls can be used as a building-integrated source of ‘passive air conditioning’, which is a combination of indoor moisture buffering and air temperature buffering (Allinson & Hall, 2007). They are ‘building integrated’ because the walls themselves act as a load-bearing external envelope. SRE structures are increasingly accepted in other developed countries such as the USA, Canada and Australia where in some regions they account for up to 20% of all new build (Easton, 1996); see the example in Fig. 14.4. In the UK, the Rammed Earth Design & Construction Guidelines, published by BRE Bookshop (Walker et al., 2005) were developed as part of a DTi Partners in Innovation Project under the title ‘Developing Rammed Earth for UK Housing’. These materials have a strong ability to passively cool a building by naturally absorbing heat gains, and re-radiating the stored heat energy

Temperature buffering

Moisture buffering

14.4 Interior of an SRE house with exposed surfaces to allow hygrothermal buffering (© 2000–2009 Trevor Mein).



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when the ambient temperature falls, i.e. the thermal flywheel effect. The porous nature of the material coupled with the presence of hygroscopic clay minerals also enables the walls to passively control the humidity of a building’s internal environment, i.e. a humidity-based version of the thermal wheel. Current research by the author is investigating how to control SRE material properties such that the passive cooling and dehumidification could be specified, thus enabling these to be tailored to the design of a building. The potential future outcome could be buildings that regulate themselves so that only a relatively small intervention from mechanical building services is required. An electronically monitored SRE house in Sydney, for example, which was left unoccupied and with no curtains for over a year had an average annual indoor temperature range of 18–27 °C, when the outdoor temperature range was 7–42 °C (Mortensen, 2000). A two-storey SRE office in New South Wales also displayed good indoor environmental performance over a hot summer due to passive cooling and dehumidification (Taylor & Luther, 2004).

14.3.3 Timber Timber is a naturally-occurring cellular fibre composite that chiefly comprises cellulose (fibre) and lignin (matrix). Timber is a light, strong material, used in building structures for millennia because it is renewable, simple to manufacture, easy to use, stable and safe. It has an inherently low thermal conductivity although both its structural behaviour and durability may be adversely affected by moisture. The physical properties of timber vary greatly between species. Hardwoods are normally from broad-leaved deciduous trees (e.g. oak, ash, beech, elm, mahogany) that are slow growing and therefore have a dense, close-grained cellular structure but that can be prone to fungal attack. Softwoods are from fast-growing coniferous trees (e.g. normally evergreens such as spruce, pine, douglas fir) that have lower density, a much more open grained structure, and are mostly softer than their hardwood counterparts. The coefficient of variability for timber materials can be greatly reduced when knots and other defects are removed and/or lamination techniques are used. In hardwoods purpose-grown large vessels occur whose function is to transport fluids through the plant, whereas in softwoods this function is achieved within the main cellular structure. For further details on timber microstructure, refer to Dinwoodie (2000). Within the cellular structure of timber there are individual openings known as a ‘torus’ which can open and close to allow in response to moisture movement. The electron microscope image in Fig. 14.5 clearly shows the vertically aligned cellular tissue of a piece of European spruce where at least one torus is visible inside each broken vessel. Note that the specimen was prepared by tensile failure which


Materials for energy efficiency and thermal comfort in buildings European spruce – tensile fracture surface

800x

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Back scattered electron detector imaging using Philips XL-40 ASEM

5000x

400x

14.5 Electron microscope images of the cellular microstructure of European spruce timber.

clearly exposes the fibre composite nature of the cellular tissue. One can appreciate that the equilibrium moisture content, at a known relative humidity (which of course can be used to plot a sorption isotherm; see Chapter 1), is normally very high for timber in comparison with other building materials such as brick, concrete and plaster. Gaur and Bansal (2002) used a simplistic model of a spruce-lined room to demonstrate that it was necessary to consider moisture transfer if room air temperature was to be modelled. They found that neglecting the hygroscopic fabric resulted in temperature errors of 2–3 °C for climatic conditions in Delhi, India. Osanyintola and Simonson (2006) suggested that spruce plywood can reduce both the heating and cooling demand in a building. Heating demand could be reduced while the building was occupied (as moisture accumulated in the fabric releases heat) but increased while the building was unoccupied as energy was needed to dry out the walls. Good system control produced an overall saving. The cooling load could be reduced by a greater amount due to lowering of the enthalpy of the air through the reduced humidity. Further savings were expected from improved indoor air quality allowing for increasing indoor temperatures during the summer, reducing them in winter and lowering ventilation rates.

14.3.4 Natural fibre materials and composites Many natural fibres have the ability to absorb, store and release water vapour from the air and therefore make good candidates for hygrothermal materials in buildings. These include straw, hemp, sisal, flax, jute and many others. The



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issue of bio-degradable materials such as these is protecting the fibre from decay when moisture is present by use of stabilizing materials. ‘Hemcrete®’ is a lime–hemp composite material made using hemp shiv and a hydrated lime binder (see the cross section shown in Fig. 14.6). They have a high thermal insulation and high water vapour permeability that can reduce the development of condensation and trapped moisture within the building. As with all good humidity buffering materials, it can also improve the indoor air quality by controlling the relative humidity as well and reducing the potential for mould growth. A hygrothermal analysis of a lime–hemp composite was conducted by the Fraunhofer Institute for Building Physics in Holzkirchen, Germany using WUFI software and three theoretical case studies. Evrard (2006) reports an experimentally determined MBV of 1.39 g/m2∙%RH for lime–hemp materials and a dry thermal conductivity of 0.115 W/m K. Some manufacturers are now marketing a commercial lime–hemp product as Hemcrete®, an example application of which is shown in Fig. 14.7.

14.6 Cross section through a lime–hemp composite wall (© 2000– 2009 Lime Technology Limited).


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14.7 The Adnams Brewery facility in Suffolk, England built with Hemcrete® walls for hygrothermal buffering (© 2000–2009 Lime Technology Limited).

14.3.5 Smart materials The work conducted during the NORDTEST Project involved determining the relative humidity buffering capacity of conventional building materials such as concrete, wood, gypsum plaster and brick. Following on from this, some researchers have attempted to develop ‘smart materials’ with very high buffering capacity that can be used to better regulate the indoor environmental conditions without the use of air conditioning systems. Candidate absorbent materials included silica desiccants, zeolites and molecular sieves. Others include sodium polyacrylate, with a reported MBV of 8.97 g/m2 %RH, and cellulose with an MBV of 3.07 g/m2 %RH (Cerolini et al., 2009). These materials were evaluated for an exposure to cyclic fluctuation between 75% RH and 33% RH over test periods of 8 h and 16 h. Some hysteresis was involved, as expected from looking at a sorption isotherm; sodium polyacrylate suffers from high hysteresis whereas the cellulose-based has a lower hysteresis.

14.4

Applications in buildings and occupied spaces

Many researchers, architects and engineers are making increasing use of hygrothermal materials for the passive regulation of indoor relative humidity, to reduce/eliminate problems associated with condensation and/or degradation of materials and furniture, and to increase the quality of indoor air. This applies not only to domestic buildings but also, for example, to office buildings with relatively high daytime air moisture loads, where the ‘hygrothermal’



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material can be both cooled and dried at night time/off-peak times through the use of natural ventilation and night cooling strategies.

14.4.1 Thermal buffering, comfort and energy efficiency in buildings Thermal buffering is the combined effect of materials absorbing, storing and releasing heat energy to their surroundings in response to changes in environmental conditions. The use of exposed thermal mass in buildings to reduce peak summertime temperatures and cooling loads in buildings is well established. Peak indoor temperatures are reduced when the heat gains (especially solar and internal heat gains) are absorbed by the building fabric which is in turn cooled overnight in readiness for the next day. During the heating season, the same thermal mass tends to absorb heat intended for the interior space and thereby increases energy demand. Good, passive solar design (BRESCU, 1998) and thermally efficient buildings with continuous low level heating (TCC, 2006) can be used to overcome this problem. Ultimately thermal mass can reduce overall energy demand, and improve thermal comfort, in properly designed buildings. In the UK, climate change is predicted to increase the summertime cooling load and reduce winter heating, reinforcing the case for thermally heavyweight buildings (Arup R+D, 2005). What appears to be less understood is that effective control of the air enthalpy, by controlling air moisture content, can significantly reduce the peak cooling loads and hence allow the thermal mass to become more effective. In addition, the material properties, such as thermal conductivity, density and heat capacity, are moisture content-dependent and can vary significantly and in a non-uniform manner. Hence, more accurate consideration and predictive modelling of thermal mass behaviour, and indeed the future design of thermal mass material technologies, will require a more sophisticated understanding using the hygrothermal approach.

14.4.2 Humidity buffering and perceived comfort in buildings Similarly, the use of exposed hygroscopic building materials has the potential to control the relative humidity of the interior of a building and its structure. Peak indoor humidity is reduced as internal and external moisture gains are absorbed by the building fabric to be returned during periods of low humidity. This passive humidity control has the potential to reduce the energy needed for mechanical systems through improving the perceived indoor air quality and thermal comfort for building occupants as well as directly reducing the sensible and latent heating and cooling loads. Toftum et al. (1998a) found that the humidity of the air that we inhale has a significant effect on the


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perceived acceptability of that air. Discomfort was thought to be due to reduced evaporative and convective cooling of the mucous membranes in the respiratory tract when breathing high humidity air (Toftum et al., 1998b). By reducing the humidity, higher indoor temperatures will be acceptable, reducing the cooling load of a building. Fang et al. (1998a) also found that the temperature and humidity of inhaled air had a significant effect on the perception of indoor air quality though little influence on the odour intensity of pollutants from building materials. Acceptability was found to be linearly correlated with the enthalpy of the air for a constant pollution level. Similar, though less distinct, results were found for whole body exposure (Fang et al., 1998b). Therefore, by reducing air temperature and humidity, acceptable air quality may be achieved at reduced ventilation rates. Reducing ventilation would lead to a lowering of the heating demand in winter and air conditioning in summer.

14.4.3 Dampness and mould control in buildings In an experimental study of Swedish apartment buildings, Hameury and Lundström (2004) found that exposed massive wood contributed to the thermal mass effect and buffered indoor temperature variations. Modelling of the heat and mass transfer processes suggested that the contribution of the wood to moisture buffering was significant, especially at low air ventilation rates (Hameury, 2005). Further potential benefits of hygroscopic building materials include a reduction in dust mite populations and mould growth (beneficial for asthma and allergies) (Cunningham, 1996) and a reduction in structural degradation caused by moisture ingress (Lucas et al., 2002). This form of passive relative humidity control may be particularly appropriate to museums, art galleries and libraries, and also to heritage structures (e.g. historic buildings that allow visitor access) where the displayed items are sensitive to moisture. The use of mechanical systems (e.g. air conditioning) in these environments can often be inappropriate, especially in important historic buildings, and the need for a reliable, non-powered humidity control system is vital for the preservation of museum artefacts or sensitive media such as cloth and paper.

14.5

Future trends

There is a clear need to carefully match hygrothermal materials and their behaviour patterns with mechanical systems engineering design. Firstly, this will maximize the ability of hygrothermal materials to passively regulate indoor environmental conditions and reduce the energy use from mechanical systems, and secondly it will avoid any conflict between the operation of the mechanical system and the material response which could (in the worst case) result in over-sizing the dehumidification and cooling loads. Barbosa



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and Mendes (2008) demonstrate, using a whole building hygrothermal model, that it is important to consider the hygrothermal behaviour of the building material when sizing the heating, ventilating and air conditioning (HVAC) system. Their work, based on a case study in Brazil, shows that disregarding moisture may oversize the system by 13% and underestimate the energy required for cooling by 4%. Künzel et al. (2003) use validated computer modelling to show that the reduction in cooling load may not be as simple as many first think because the moisture absorbed during air conditioning shut off periods is re-emitted when the system is turned back on – which increases the latent heat capacity of the air.

14.6


In addition to research being conducted in this field by the author, numerous very well-established research teams and organizations are involved directly or indirectly with hygrothermal materials. A selection of them is presented in Table 14.2, although this list is by no means exhaustive. A number of computer models have been developed for use with hygrothermal material properties and in tandem with building/occupied space assessment and analysis. A selection of the most widely used and validated models is given in Table 14.3.

Table 14.2 A list of institutions and research teams involved with hygrothermal materials research Institute

Location

Fraunhofer Institute for Building Physics Oak Ridge National Laboratory (DOE) National Research Council Canada – Institute for Research in Construction (NRCC-IRC) University of Saskatchewan VTT Technical Research Centre Technical University of Denmark Royal Institute of Technology Helsinki University of Technology Tampere University of Technology Tallinn Technical University Lund University Dresden University University of Helsinki Pontifical Catholic University of Parana University of Nottingham

Germany USA


Canada Canada Finland Denmark Sweden Finland Finland Estonia Sweden Germany Finland Brazil UK

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Table 14.3 A selection of the most widely used computer models for use with hygrothermal material properties. Model

Institution

Country

1D-HAM DELPHIN 4, DIM 3.1 EMPTIED GLASTA hygIRC Hygran24 LATENTITE MATCH MOIST 3 MOISTURE-EXPERT TCCC2D TRATMO UMIDUS WALLDRY WUFI Pro/Plus WUFI ORNL/IBP

Chalmers University of Technology Technical University of Dresden Canada Mortgage and Housing Corporation Physibel Software Institute for Research in Construction Catholic University of Leuven National Research Centre Technical University of Denmark National Institute of Standards and Technology Oak Ridge National Laboratory VTT Technical Research Centre VTT Technical Research Centre Pontifical Catholic University of Parana Canada Mortgage and Housing Corporation Fraunhofer Institute for Building Physics Oak Ridge National Laboratory

Sweden Germany Canada Belgium Canada Belgium Canada Denmark USA USA Finland Finland Brazil Canada Germany USA

14.7

Acknowledgements

The author wishes to acknowledge the assistance and helpful discussion of Dr David Allinson during the preparation of this chapter.

14.8

References

Allinson D and Hall M, 2007, ‘Investigating the optimisation of stabilised rammed earth materials for passive air conditioning in buildings’, in Proceedings for the International Symposium of Earthen Structures, Bangalore 22–24 August. Arup R+D, 2005, ‘UK housing and climate change: heavyweight vs. lightweight construction’, A technical report for Ove Arup and Partners Ltd, London Barbosa RM and Mendes N, 2008, ‘Combined simulation of central HVAC systems with a whole-building hygrothermal model’, Energy and Buildings, 40(3), 276–288. Bederina M, Marmoret L, Mezreb K, Khenfer MM, Bali A and Quéneudec M, 2007, ‘Effect of the addition of wood shavings on thermal conductivity of sand concretes: experimental study and modelling’, Construction and Building Materials, 21(3) 662–668. BRESCU, 1998, Planning for Passive Solar Design, BRECSU, BRE, Watford. BS EN ISO 12572, 2001, Hygrothermal performance of building materials and products – Determination of water vapour transmission properties, British Standards Institute, London. Cerolini S, D’Orazio M, Di Perna C and Stazi A, 2009, ‘Moisture buffering capacity of highly absorbing materials’, Energy and Buildings, 41(2), 164–168. Cunningham MJ, 1996, ‘Controlling dust mites psychrometrically – a review for building scientists and engineers’, Indoor Air, 6(4), 249–258. Dinwoodie JM, 2000, Timber: its nature and behaviour, 2nd edn, Taylor and Francis, London.



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Easton D, 1996, The Rammed Earth House, The Chelsea Green Publishing Company, Vermont. Evrard A, 2006, ‘Sorption behaviour of lime–hemp concrete and its relation to indoor comfort and energy demand’, PLEA2006 – 23rd Conference on Passive and Low Energy Architecture, Geneva, Switzerland, 6–8 September. Ewing RP and Horton R, 2007, ‘Thermal conductivity of a cubic lattice of spheres with capillary bridges’, Journal of Physics D: Applied Physics, 40(16), 4959–4965. Fang LG, Clausen G and Fanger PO, 1998a, ‘Impact of temperature and humidity on the perception of indoor air quality’, Indoor Air, 8(2), 80–90. Fang LG, Clausen G and Fanger PO, 1998b, ‘Impact of temperature and humidity on perception of indoor air quality during immediate and longer whole-body exposures’, Indoor Air, 8(4), 276–284. Gaur RC and Bansal NK, 2002, ‘Effect of moisture transfer across building components on room temperature’, Building and Environment, 37, 11–17. Hall C and Hoff WD, 2002, Water Transport in Brick, Stone and Concrete, Taylor & Francis, London. Hameury S, 2005, ‘Moisture buffering capacity of heavy timber structures directly exposed to an indoor climate: a numerical study’, Building and Environment, 40(10), 1400–1412. Hameury S and Lundström T, 2004, ‘Contribution of indoor exposed massive wood to a good indoor climate: in situ measurement campaign’, Energy and Buildings, 36(3), 281–292. Hansen EJdP and Hansen KK, 2002, ‘Unfired clay bricks – moisture properties and compressive strength’, in Building Physics 2002 – 6th Nordic Symposium, 17–19 June, Trondheim, Norway. Holtz RD and Kovacs WD, 1981, An Introduction to Geotechnical Engineering, PrenticeHall, Englewood Cliffs, NJ. ISO 8301, 1991, Thermal insulation – Determination of steady-state thermal resistance and related properties – Heat flow meter apparatus, International Organization for Standardization, Geneva, Switzerland. ISO 10051, 1996, Thermal Insulation – Moisture effects on heat transfer – Determination of thermal transmissivity of a moist material, International Organization for Standardization, Geneva, Switzerland. ISO 10456,1999, Building materials and products: Procedures for determining declared and design thermal values, International Organization for Standardization, Geneva, Switzerland. Künzel HM, Zirkelbach D and Sedlbauer K, 2003, ‘Predicting indoor temperature and humidity conditions including hygrothermal interactions with the building envelope’, in Proceedings of 1st International Conference on Sustainable Energy and Green Architecture, BSRC, Bangkok. Lucas F, Adelard L, Garde F and Boyer H, 2002, ‘Study of moisture in buildings for hot humid climates’, Energy and Buildings, 34, 345–355. Mitchell JK, 1993, Fundamentals of Soil Behaviour, John Wiley & Sons, London. Mortensen N, 2000, ‘The naturally air conditioned house’, Earth Building Research Forum, University of Technology Sydney, Australia, available via: http://www.dab. uts.edu.au/ebrf/ Morton T, 2005, ‘Unfired earth brick building’, Building for the Future, 24–27. Morton T, 2006, ‘Feat of clay’, Materials World, 14(1), 23–24. Nordic Innovation Centre (NIC), 2005, ‘Moisture buffering of building materials’, Technical report BYG-DTU R-126, Technical University of Denmark. © Woodhead Publishing Limited, 2010

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Osanyintola OF and Simonson CJ, 2006, ‘Moisture buffering capacity of hygroscopic building materials: experimental facilities and energy impact’, Energy and Buildings, 38(10), 1270–1282. Rode C, 2003, Workshop on Moisture Buffer Capacity – Summary Report R-067, Department of Civil Engineering, Technical University of Denmark. Sedlbauer K, 2002, ‘Prediction of mould growth by hygrothermal calculation’, Journal of Thermal Envelope and Building Science, 25, 321–335. Taylor P and Luther MB, 2004, ‘Evaluating rammed earth walls: a case study’, Solar Energy, 76, 79–84. TCC, 2006, Thermal Mass for Housing, The Concrete Centre, Surrey. Toftum J, Jorgensen AS and Fanger PO, 1998a, ‘Upper limits of air humidity for preventing warm respiratory discomfort’, Energy and Buildings, 28(1), 15–23. Toftum J, Jorgensen AS and Fanger PO, 1998b, ‘Upper limits for indoor air humidity to avoid uncomfortably humid skin’, Energy and Buildings, 28(1), 1–13. Velde B, 1992, Introduction to Clay Minerals, Springer, Berlin. Walker P, Keable R, Marton J and Maniatidis V, 2005, Rammed Earth Design and Construction Guidelines, BRE Bookshop, Watford. Zhang H, Ge X and Ye H, 2006, ‘Randomly mixed model for predicting the effective thermal conductivity of moist porous media’, Journal of Physics D: Applied Physics, 39, 220–226.

14.9 A b m c p H m P psat R RH t p U V w m a b dp r rd l ϕ

Appendix: Nomenclature area moisture effusivity constant pressure specific heat capacity enthalpy mass fluid pressure saturation vapour pressure thermal resistance relative humidity period internal energy volume moisture content thermal diffusivity thermal effusivity water vapour permeability density dry density dry state thermal conductivity relative humidity (decimal)

m2 kg/(m2 Pa s1/2) J/kg K J kg Pa Pa m2 K/W % s J m3 kg/kg m2/s W s1/2/m2 K kg/m s Pa kg/m3 kg/m3 W/m K –


15


B. Warwicker, UK

Abstract: Desiccant is a material for modifying humidity; the chapter explains the absorption and adsorption desiccant processes. The chapter reviews the commercial, solid and liquid desiccant processes, together with selected practical applications. Health, comfort and air quality, as well as energy coefficient of performance (COP), energy efficiency ratio (EER) and seasonal energy efficiency ratio (SEER) are discussed. Finally the chapter looks to the future and reviews the more natural approach to modifying building humidity with normal building materials and reviews practical applications. Key words: desiccant, sorbent, absorbent, adsorbent, health, comfort, air quality, silica, alluminium, lithium chloride, cellular concrete, gypsum lime, wood, indoor climate, energy efficiency raho (EER), seasonal energy efficiency raho (SEER).

15.1

Introduction

Sorption is the binding of one substance to another. Sorbents are materials that have the ability to attract and hold other gases or liquids. Desiccants are a subset of sorbents. All desiccants behave in a similar way; they attract moisture until they reach equilibrium with their surroundings. Moisture may be removed from a desiccant by heating. Once the desiccant is dry it must be cooled before it can attract moisture once again. Sorption generates sensible heat equal to the latent heat of the water taken up by a desiccant, isosteric enthalpy. In addition, the heat of sorption varies between 5 and 25% of the latent heat of water vapour which is dependent on the permeability/hygroscopicity of the desiccant. These are two distinct functions. The process of attracting and holding moisture is described as either absorption or adsorption, depending on whether the desiccant undergoes change as it takes in moisture. Absorption changes the desiccant. An example of an absorbent is table salt which changes from a solid to a liquid as it absorbs moisture. Adsorption does not change the desiccant, except by the addition of the weight of water adsorbed. An example of adsorption is a sponge. Virtually all materials are desiccants; that is they attract and hold water 365 © Woodhead Publishing Limited, 2010

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vapour. Wood, natural fibres, clays, and many synthetic materials attract and release moisture as commercial desiccants do, but they lack holding capacity. Commercial desiccants continue to attract moisture even when the surrounding air is quite dry, a characteristic that the other materials do not share. Desiccants, whether liquid or solid, hold moisture through absorption or adsorption. Most absorbents are liquids and most adsorbents are solids.

15.2

Desiccant cycle

All desiccants function by moisture transfer caused by a difference between water vapour pressures at their surface and that of the surrounding air (Fig. 15.1). Liquid absorption is a dehumidification process. The vapour pressure of liquid absorption is directly proportional to temperature and inversely proportional to its concentration. The vapour pressure of a given concentration of absorbent solution approximates to the vapour pressure value of a fixed relative humidity line on a psychrometric chart. Higher concentrations provide a lower equilibrium and relative humidity, which allow the absorbent to dry air to lower levels (Fig. 15.2). Adsorbents are solid materials with a tremendous internal surface area per unit of mass that attract moisture because of the electrical field at the material’s surface. This field is not uniform in either force or charge. When the complete surface is covered, the adsorbent can still hold more moisture because vapour continues to condense into the first layer and fill the capillaries throughout the material. As with absorbents, the ability of an adsorbent to attract moisture depends on the vapour pressure between its surface and the air. 120 °C

Reg

ene

Cooling

Desiccant surface Vp

3

rati

on 2

So

on rpti

1

Desiccant moisture content

15.1 Desiccant cycle.


10 °C

Desiccant materials for moisture control in buildings Water

20

60

Glycol solutions 55

Lici solutions 50%

50 10

te Wa

r

15% 25% 45%

10

20

30 40 Solution temp (°C)

40

90%

30

98%

50

Dewpoint °C

Surface vapour pressure (kPa)

367

60

15.2 Comparison of liquid desiccants and water.

Adsorption behaviour depends on total surface area, the total volume of the capillaries and the range of capillary diameters. A large surface area will give the adsorbent a larger capacity at low relative humidity. Large capillaries provide a high capacity at high relative humidity. A narrow range of capillary diameters makes an adsorbent more selective in the vapour molecules it can hold. Water molecules migrate slowly through solid materials by diffusion, propelled by the difference between water vapour pressures on each side of the material. The molecules move slightly faster when the material is more permeable/hygroscopic and the greater the water vapour pressure difference. The unit that defines water vapour movement through a material is called ‘perm’. One perm is the number of nanograms per second that pass through one square metre with a pressure difference of one Pascal.

15.3

Desiccant applications

Desiccants can dry liquids or gases, including moisture from air and are used in many air conditioning applications. It is the air conditioning application that is of interest. Desiccant systems have already been implemented in many buildings. The technology promises to advance building technology in the 21st century and help the individual achieve ‘the right to healthy indoor air’ with minimal environmental impact.

15.4

Health and comfort

If humidity in the air has nowhere to go and levels rise, this can create an ideal environment for mites and mould, which start to thrive above 50% © Woodhead Publishing Limited, 2010

368


relative humidity (rainy day is 100% RH and middle of the desert is 0% RH). This is represented by a maximum absolute moisture content of 8 g/ kg at 21 °C in the space; below 40% RH the mites die. Humans are most comfortable between 40 and 60% RH, i.e. 6–9 g/kg at 21 °C. The influence of moisture on people is one of the least understood of all the comfort factors. ∑ Comfort is affected by skin wetness; skin also requires water content to remain supple. ∑ Dry throats and noses lower the resistance to infection. ∑ Dry itchy eyes in very dry conditions lead to a breakdown in tear film. ∑ Clothing is influenced by moisture, both insulation and permeability. The electrical properties are also affected and can create electrostatic charges as well as in surrounding materials. ∑ Health and allergies are affected; very dry conditions lower the resistance to infection and damp conditions enable moulds and mites to flourish. ∑ Odour, irritant and dust effects are perceived worse at low humidities; the release of formaldehyde gas is also affected by humidity. Figure 15.3 shows the modes of heat loss at various temperatures of a clothed sedentary person. It is interesting to note that most comfort surveys treat moisture as an independent factor unrelated to comfort. Studies show that a 30% increase in relative humidity is equal to a 1 °C rise in temperature. Thus, if humidity levels are reduced then ‘comfort’ temperatures could be allowed to rise (Fig. 15.4).

Heat loss (W)

150

100 Evaporation

Co 50

nv

ec

tio

n

Radiation 10

20 30 Temperature (°C)

40

15.3 Heat loss by clothed sedentary person.



369

28 27

Comfort (°C)

26 25

Women

24 23

Men

22 21 0

20

40

% RH

60

80

100

15.4 Influence of humidity on comfort.

Moisture is an important factor in our comfort and health. Tests and studies confirm sensitivity to both dry and humid conditions. The time taken for these sensitivities to impact vary from 15 minutes to an hour for eye dryness to a few hours for body dryness and weeks for an overall feeling of unpleasant dryness. In humid conditions the sensitivities impact more quickly, skin becomes damp, the atmosphere becomes sultry and we overheat. Research evidence indicates that at low temperatures of 20 °C and high relative humidities of 65% people are the most comfortable; however, the modern trend is to provide higher temperatures, especially in passive buildings, which makes it difficult to achieve healthy and comfortable moisture conditions (Fig. 15.5).

15.5

Air quality

Indoor air pollution poses many challenges to health. Studies have shown that people spend more than 90% of their time indoors. Volatile organic compounds (VOCs) are a class of pollutants that pose a risk to human health since they can be toxic at very low concentrations (ppb). The adverse health effects caused by these compounds can range from minor complaints, such as irritation of the nasal and ocular mucosa, to chronic complications such as the exacerbation of asthma. Removal of VOCs by conventional means such as photo catalytic oxidation, ionization, condensation and ozone oxidation have proved insufficient due to the nature of VOCs and their very low concentrations in the air. The recent focus has been on sorption filtration using desiccant materials for the simultaneous removal of both water vapour and other pollutants from


370


Comfort (°C)

30

Electric shocks at lower RH

25

20

Damp clothing

Dry eyes

Clammy forehead

Comfort

Chapped skin at lower RH

Dry throat at lower RH

Sultry at higher RH

15 0

20

40 60 Comfort (% RH)

80

100

15.5 Influence of moisture on people.

air (co-sorption) as it is an energy efficient process eliminating the need for cooling air to dew point temperatures to remove water vapour. Experimental and predictive methods have been used to determine the suitability of adsorbents for pollutant removal. The results indicate an improved indoor environmental quality (IEQ) where desiccants were used to clean the air. Adsorption involves competition for binding sites on the surface of the adsorbent by the adsorbing molecules. Research has established that although competition exists between water and the pollutant molecules on desiccants, their capacity for adsorption remains relatively unaffected.

15.6

Natural and commercial desiccants: typical materials

It is the air surrounding the material that controls its water content. The exact water content of a material depends on the temperature and moisture content of the surrounding air. The material permeability/hygroscopicity will determine its moisture content relative to that of the surrounding air. Each material has its own sorption isotherm which is a graph of equilibrium moisture content (EMC) versus relative humidity at a constant temperature (Fig. 15.6). Many materials produce similar results, e.g. cotton, lime, linen, silica gel, silk, wood, wool and concrete. The amount of water in the air is a negligible portion of the water content in the material. Thus when the temperature is increased, the relative humidity of the air is only marginally increased. But the absolute moisture content of the air increases as a direct result of the change of temperature, whilst the moisture content of the material stays relatively constant. Thus



371

30

EMC (%)

20

0 °C

10

30 °C

0 0

20

40

% RH

60

80

100

15.6 Typical variation of material moisture content.

the equilibrium curve moves horizontally across the graph. Removing the material from the air whilst reducing the temperature of the air will create an increase in relative humidity. If the material had remained whilst the air temperature was reduced, then the relative humidity would only have fallen slightly. In summary, water absorbent materials tend to keep relative humidity constant. A change in material temperature will cause a change in the surrounding air. Moisture mass is the equivalent of thermal mass in ‘passive design’. Natural moisture open, hygroscopic, materials such as clay, hemp, lime plaster, wood and mineral paints can provide ceilings, floors and walls in buildings that are reactive to moisture. Mechanical ventilation is a crude tool for removing moisture and is less efficient than hygroscopic materials. The use of hygroscopic materials in buildings must be an essential part of ‘passive design’. Commercial desiccants, liquids or solids hold moisture through absorption or adsorption. Most absorbents are liquids and most adsorbents are solids. Liquid absorption dehumidification is best illustrated by comparison with an air washer. When air passes through an air washer, its dew point approaches that of the water temperature. In a similar manner, in a liquid absorption dehumidifier, as the vapour pressure of the solution is lower than the vapour pressure of the moisture in the air (at the same temperature), it is dehumidified. The concentration of an absorbent solution approximates to a fixed humidity line on a psychrometric chart. In practice, the behaviour of a liquid desiccant is controlled by its temperature, concentration or both. Commercially available liquid desiccants have an especially high water holding capacity. For example each molecule of lithium chloride (LiCl) can hold two water molecules, in a dry state. Above


372


two it becomes a liquid and continues to absorb water; in equilibrium with air at 90% RH 26 molecules of water is held. This is equivalent to 1000% of water absorption by weight. Absorbents are solid materials with a tremendous internal surface area per unit of mass. They resemble a sponge; a single gram can have more than 5000 m2 of surface area. Adsorbents attract moisture due to the electrical field at the surface of the desiccant. The field is not uniform so specific sites attract water molecules that have a net opposite charge. As with liquid absorbents the ability of an adsorbent to attract moisture depends on the vapour pressure difference between its surface and the air. The capacity of solid adsorbents is generally less than liquid absorbents. A typical molecular sieve can hold 17% of its dry weight in water with air at 20 °C and 20% RH compared with LiCl which can hold 130% of its mass at the same temperature and relative humidity. Solid absorbents have many other favourable traits, e.g. molecular sieves continue to absorb moisture even when hot. Several solid absorbents can be manufactured to precise tolerances enabling pore diameters to be closely controlled to absorb molecules of specific diameter. This selective characteristic is useful as several desiccants with differing pore sizes can be combined to remove first water then other specific contaminants, such as VOCs. Silica gels and most other absorbents can be manufactured for a specific application, balancing capacity against strength, weight and other characteristics. Typical solid absorbents comprise activated alumina, carbons, silica gels, synthetic polymers, synthetic zeolites (molecular sieves) and zeolites. Activated aluminas are oxides and hydrides of aluminium manufactured from a thermal process. Carbons are most frequently used for the absorption of gases as they have a greater affinity for the molecules typical of organic solvents. They also have a high capacity to absorb water vapour at high humidities. Silica gels are formed by condensing soluble silicates. They are low cost and easy to customize. Synthetic polymers have a capacity that exceeds many other solid desiccants due to long molecules, such as polystyrenesulfonic acid sodium salt (PSSASS) that are twisted together like strands of string. Each of the many sodium ions in the long PSSASS molecules has the potential to bind several water molecules. Synthetic zeolites or molecular sieves are crystalline aluminosilicates manufactured in a thermal process. Control of the manufacturing process permits close control of the structure and surface characteristics. This is a more costly product than naturally occurring zeolites. Zeolites and aluminosilicates occur in nature and are mined rather than processed. Zeolites have a very open crystalline lattice that allows water vapour molecules to be held in the lattice openings.



15.7

373

Practical applications of commercial desiccants

Examining ventilation rates of occupants in non-domestic buildings leads to the conclusion that the rates recommended were not determined by the respiratory needs of people (i.e. 0.015–0.10 l/s/m²), as various bodies recommend a minimum ventilation rate around 1.20 l/s/m². The moisture content of ventilation air can be significant. Anyone who has experienced a hot, humid day will know the discomfort of perspiring without any cooling effect. This is due to a saturated atmosphere being unable to absorb any more moisture, and thus any excess we produce simply stays on us, unable to evaporate. Dalton’s Law of Partial Pressures clearly confirms this and for those of us who have forgotten, it is as follows: ∑ Each constituent in a mixture of perfect gases exerts the same pressure as if it alone were present in the space occupied by the mixture, at the temperature of the mixture. ∑ Total pressure of the gases is the sum of their partial pressures, and the volume of the mixture of gases is the same as the volume occupied by each gas at its partial pressure. ∑ Totally enthalpy of the mixture is the sum of the enthalpy of each constituent at its partial pressure. ∑ Barometric pressure represents the total pressure of atmospheric air. This total pressure is composed of all the partial pressures of all the constituents, chiefly nitrogen, oxygen and water vapour.

P = Pa + P g .

A typical person, who works in an office, produces latent heat of around 60 W at 24 ºC, which equates to 1.68 g/kg of moisture entering the atmosphere. Table 15.1 shows the effect of occupant density on dew point and humidity level. The Comfort Zone Chart, published in 1924 by American Society of Heating and Ventilating Engineers (ASHVE) after studying physiological reactions of temperature humidity and air movement, clearly set out the relationship between humidity and temperature in relation to comfort. Basically, as temperature increases, humidity had to reduce to maintain Table 15.1 The effect of occupant density on dew point and humidity level Density of people 1 1 1 1

per per per per

Latent gain (W/m²) Moisture inc. infiltration pick-up (g/kg)

10 m² 9.6 7.5 m² 11.3 5.0 m² 15.1 2.5 m² 26.1

1.8 2.12 2.64 4.9

Space air dew point (°C)

Space relative humidity (%)

9.0 9.6 10.0 14.1

45 48 50 65


374


the same relative comfort. Comfort lines similar to those in Fig. 15.7 were established in 1922. Humidity and the influence it has with regard to comfort and energy saving when cooling is required is not always considered seriously. Air distribution rules may be relaxed where lower moisture content ventilation air is introduced at/or near to room temperature. Asymmetric temperature gradients may be ignored with vapour pressure differential doing the work, taking us back to Dalton’s Law of Partial Pressures. Research and notable authors have concluded that a moisture content in air of not more than 7 g/kg is required to provide a healthy environment. This equates to a relative humidity of 40%, with a dry-bulb temperature of 23 ºC. Too dry an environment, generally below 30% RH, is not recommended. It is therefore of critical importance that the moisture within the air is properly considered and suitably controlled. The desiccant system accommodates just this problem. The psychrometric principals of a commercial desiccant system are illustrated in Fig. 15.8. Figure 15.9, shows an adsorbent (solid) desiccant air-handling system in summer, and Fig. 15.10 shows the desiccant air-handling system in winter. Both can be used in conjunction with the psychrometric description to demonstrate how a typical commercial desiccant system works. Fresh air enters through the filter and then passes over the desiccant wheel, which absorbs some of the moisture in the air, which also adds heat (approx. SHF 0.6). The drier but warmer air then passes to the thermal wheel which cools it down; it may be further cooled by an evaporative cooler, and depending upon the required final conditions, may be further cooled by means of a sensible cooling coil. The cooler, now dehumidified air is introduced to the space via an air supply fan, where it provides all the latent cooling Relative humidity (% RH) 100 80 60

40

25

Wet air temperature (°C)

20

15 0

10

0

10

15 20 30 Dry air temperature (°C)

15.7 Lines of comfort.


20

40


375

Relative humidity (% RH) 100 80 60

40

25

20

Wet air temperature (°C)

A

15 0

E 0

20

10

10

D

C

15 20 30 Dry air temperature (°C)

B 40

15.8 Typical desiccant psychrometrics.

and some of the sensible cooling requirements depending upon which type of system is in use, either an all air, or air/water system. The warm, moist return air leaving the space by way of the filter is passed through the evaporative cooler. This cool now humid air enters the thermal wheel enabling the sensible cooling to be transferred to the incoming air, which in turn is being heated by the incoming air. A regenerative heater further increases the air temperature before entering the desiccant wheel where it regenerates the desiccant. Some of the air could bypass the regeneration heater and desiccant wheel to save energy. The system is environmentally friendly, using inert materials, i.e. salt, silica gel or titanium silicate, and can use direct or indirect gas, waste or solar heat to regenerate the desiccant. Desiccant wheels also have laminar flow characteristics, thus higher air volumes are permissible without an energy penalty. During the winter months when the desiccant wheel may be inactive there needs to be an additional consideration. As supply air flows through the desiccant wheel without reactivation, moisture will build up in the desiccant wheel. Excess moisture supports fungal growth within the core of the material. Thus when dehumidification is required, the warm, humid wheel can give off odours reminiscent of wet wool until the regeneration heat kills off the microbial growth accumulated when the wheel was inactive. A regular exercise and purging regime is recommended for the wheel to prevent the build-up of microbial growth. Commercial absorbent desiccant systems present a liquid to the air stream instead of a solid. This type of equipment is less common in a commercial application; however, it does provide some unique advantages. In a liquid system a solution of LiCl coats an extended surface similar to that of


376

Regeneration heater

Evaporator cooling

Filter


Discharge air

Hot humid air

Preheat

Filter

Desiccant wheel

Thermal wheel Summer

Evaporator cooling

Supply air fan

Temperature Moisture content

15.9 Desiccant air handling system: summer psychrometrics.


Exhaust air fan

Regeneration heater

Evaporator cooling

Filter


Discharge air

Cold dry air

Preheat

Filter

Desiccant wheel

Thermal wheel Winter

Evaporator cooling

Supply air fan

Temperature Moisture content

15.10 Desiccant air humidity system: winter psychrometrics.


Exhaust air fan

377

378


cooling tower media. The air flows through the media and the air gives up its moisture to the liquid desiccant. The diluted liquid desiccant is then sent to a regenerator to have its excess moisture, equal to the moisture taken out of the air, removed. The dilute liquid is first heated then put in contact with a small amount of outdoor air; the hot liquid desiccant releases its moisture to this regeneration air. The re-concentrated liquid desiccant flows back to the media where the process is repeated. Control of the temperature and concentration of the solution enable the air to leave the liquid desiccant system at the precise temperature and moisture content needed without the after-cooling required by the dry adsorbent system of a desiccant wheel. Around the world, over the past decade, desiccant cooling systems have advanced through development in polymer technology and novel developments in heat exchanger technology. These novel heat exchanger applications provide products that can heat and cool, both air and water, as well as recycle sensible and latent energy in a very efficient manner (Fig. 15.11). These novel desiccant cycles have more than doubled the coefficient of performance (energy efficiency ratio) of desiccant cooling systems. They are up to three times more energy efficient than conventional (Pennington) systems when operating on low temperature heat such as that from solar heat collectors, fuel cells or combined heat and power (CHP) systems.

15.8

Practical applications of natural desiccants for modifying building humidity

An alternative approach is to use the water absorption properties of building materials. The advantage of using water absorbing materials as a buffer during brief periods of increased water vapour is that during these periods the absorbent material may be used as a substitute for mechanical ventilation. The heat of evaporation being recovered by the heat of absorption released by the material provides for increased comfort. The use of porous materials in walls, floors and ceilings may be used as a buffer to maintain relative humidity. Hygroscopic walls, floors and ceilings can aid stability of relative humidity and thus comfort, especially in rooms which are ventilated to less than 1 ac/h. A few centimetres of absorbent material are sufficient to buffer a daily RH cycle and about 40 cm of absorbent wall might buffer an annual cycle in a room with about 0.1 ac/h. The defining factors are the water capacity and permeability of the absorbent materials used. The common buffer building materials are wood, cut across the grain, and unfired clay brick. A specially designed lightweight clay made from bentonite mixed with perlite also gives an excellent performance (Padfield, 1998).



379

6

1

4 5

3 Combine 3 processes Heat exchange Indirect evaporate cooling Dehumidification

15.11 Novel desiccant system.

The best performer of the commonly used building materials is cellular concrete. Most other building materials have a negligible effect at moderate relative humidity. Cellular concrete is made by mixing silica, calcium oxide, aluminium powder and water. The end result is a mass of interlocking needle crystals of calcium aluminium silicate with relatively large air voids and a typical density of 750 kg/m3. These air voids provide plenty of sites for water absorption, making cellular concrete a moderately good buffer material. Figure 15.12 shows a typical absorption and desorption cycle. The combination of porous materials laid over porous absorbent materials is interesting. A combination of gypsum plaster over cellular concrete provides a sort of a diode as it allows moisture to penetrate but hinders its return. The gypsum plaster conducts liquid water, because it is wettable and has a


380

Materials for energy efficiency and thermal comfort in buildings 100

80 End grain wood

RH

60

Cellular concrete

40

20

0

12

24 Hours

36

48

15.12 Absorption and desorption cycle.

system of large pores. The cellular concrete has a similar structure on a finer scale and retains the water. This combination can only be advantageous if the cellular concrete can evaporate the moisture to the outside of the building. The need to ventilate away moisture generated by human activities could be reduced by utilizing porous and absorbent materials to buffer the moisture and thus use a lower ventilation rate and less energy. In some instances this buffering may be so effective that mechanical systems are not necessary. However, in other circumstances the average RH provided by the passive buffers will be too variable and a relatively small and simple air conditioning system may assist the material buffers during the time of peak moisture gain. There are no readily available design tools to explore the use of porous or absorbent materials to moderate indoor climate. However, when the room has absorbent buffers, the RH can be defined by the interaction of the materials with the various sources of water vapour within the room, as the RH is a consequence of the behaviour of the materials, not a controlling parameter. The sorption curve of a material defines how much water will be released or absorbed when the material moves from equilibrium with one RH to another. The inverse argument is that this same curve also defines RH change in the air that surrounds the material resulting in the material losing a certain weight of water into the space. Where there is a reasonably even and moderate RH, only that portion of the absorption curve is relevant, i.e. between 40 and 65% RH. This part is fairly straight for most materials, so the absorption curve can be replaced by a constant: the water capacity. The water capacity is defined as the weight of water that would be released from one cubic metre of material if the RH changed from 100 to 0%, assuming



381

a sorption curve gradient constant at the value for 50% RH. Water vapour permeability is also a factor. The higher permeability makes for a better performance as a buffer; however, there are other considerations when the RH is changing: as the water vapour diffuses into the material it is attracted to the pore walls by van der Waals forces, according to its absorption coefficient. A material with both high permeability and high absorption will actually show a low permeability to water vapour under dynamic circumstances, because the water is absorbed before it is diffused. Calculation of this complex process is aided by defining a unit called diffusivity, which is the diffusion coefficient divided by the absorption coefficient. This is used to calculate an active depth for moisture exchange.

d=

Dt tt

where ‘d’ is the active depth of water exchange, ‘t’ is the period of the water flux cycle and ‘D’ is the diffusivity. The active depth marks the distance into the wall at which the variation in RH in the pores has fallen to about one third of the variation in the room (Claesson and Hagentoft, 1994). Unfortunately, the analogy between heat and moisture flow is not exact. Therefore the effective depth is just an indication of the order of the useful thickness of the material for buffering relative humidity. The amount of water available is now obtained by multiplying the water capacity by this active depth. The best materials are not those generally used in building. Wood cut Water

High

Capacity

Diffusion coeff.

Clay perllite

Wool

Wood

Active depth

Available water

Lime mortar

Wood across grain

Gypsum plaster Wood across grain

Brick

Clay perllite

Gypsum board

Low

Cellular concrete

Gypsum board

Gypsum board

Cellular concrete

Gypsum plaster

Clay perllite

Cellular concrete

Wool

Wood across grain

Cellular concrete

Clay perllite

Gypsum plaster

Gypsum board

Wool

Lime mortar

Brick

Gypsum plaster

Brick

Lime mortar

Brick

Lime mortar

Wood

Wood

Wood

15.13 Performance comparison of typical building materials.


382


across the grain is by far the best buffer but may not be ideal because it outgases a variety of corrosive vapours. Clay is the second best, but some give off hydrogen sulphide. Clay walls smell much less than wood, as the hydrogen sulphide normally oxidizes to hydrogen sulphate. The active depth of clay for a daily cycle is 5 mm. Figure 15.13 provides a synopsis of the above. Absorbent walls, floors and ceilings can significantly improve climatic stability of indoor spaces that have low ventilation rates. Thus, moisture mass must have equal consideration to thermal mass in passive buildings.

15.9

Bibliography

ASHRAE Publication (1992) Desiccant Cooling and Dehumidification. ASHRAE Guide (2007) Fundamentals – Chapter 22. Beggs, Warwicker. ‘Towards zero carbon dioxide emission: the environmental impact of fabric thermal storage in UK office buildings’. www.warwicker.com Beggs, Warwicker, Howarth (1995) ‘Elimination of air conditioning in existing buildings through fabric thermal storage: A Theoretical study’. Building Serv. Eng. Res. Technol. 16(4) 215–220 www.warwicker.com Brundrett G. W. (1990) Criteria for Moisture Control, Butterworth’s, CIBSE Desiccants the Future – Robert van Zyl Professor of Engineering, Professor Brian Warwicker. www.warwicker.com CIBSE Guide Volume A, Section A7 (2006). CIBSE Guide Volume B, Section B2 (2005). Claesson, Johan and Hagentoft, Carl-Eric, (1994) ‘Basic building physics – mathematiucal modelling. University of Lund, Department of Building Science. Evans B. (1995) ‘New cooling for old buildings.’ Architect Journal May. www.warwicker. com Harriman, Brundrett, Kittler, ASHRAE Publication (2001) Humidity Control Design Guide. Padfield. The role of absorbent building materials in moderating changes of relative humidity. October 1998. Redman B. (1997) ‘Dehumidifying using Desiccants’; CIBSE Journal, June. Warwicker B. P. (1989), ‘Low Temperature Air and Ice Storage’; CIBSE Journal, February. www.warwicker.com Warwicker B. P. (1995) ‘Low Humidity Air & Air Conditioning’; CIBSE Journal, November. www.warwicker.com World Health Organisation Report, Bilthoven, Nederlands. ‘The Right to Healthy Indoor Air’ 15–27 May 2000. Wood Science and technology Journal (1995) Volume 29, Number 5/August.

15.10 Appendix: Energy efficiency ratio (EER) and coefficient of performance (COP) Energy efficiency ratio (EER) is used in the USA, and is defined as the system output in Btu/h per watt of electrical energy. Coefficient of performance



383

(COP) is the equivalent measure using SI units, which is widely used in the UK. A COP of 1.0 equates to an EER of 3.4. When comparing technologies that use electrical energy with those that use heat energy, it is useful to consider the ‘equivalent COP’ that achieves the same carbon emission levels. In the UK the generation of electricity emits 2.6 times as much carbon per watt when compared with burning natural gas. In terms of carbon emissions, therefore, a heat driven device with a COP of 2.5 would be equivalent to an electrically driven device with a COP of 2.5 ¥ 2.6 = 6.5. The seasonal energy efficiency ratio (SEER) used in the USA and equivalent ‘seasonal COP’ is a measure of the annual energy efficiency taking into account the varying weather conditions. It is calculated using weather ‘bin’ values, with a given frequency of occurrence in each temperature ‘bin’. Very high SEER (and seasonal COP) values can be expected with new desiccant cycles, as for a large part of the year no dehumidification is required and no regeneration heat is used. The SEER system is flawed, however, in that it does not consider the external humidity and therefore the requirement for dehumidification is not quantified. Until a standard is set, an objective view cannot be taken on what SEER to expect for desiccant cycles, but the SEER for novel desiccant systems could well be equivalent to an electrically driven device with a SEER in excess of 30, which is up to three times better than that of conventional air-conditioning systems.


16

Phase change materials for energy storage and thermal comfort in buildings

M. M. F a r i d and A. S h e r r i f, University of Auckland, New Zealand

Abstract: This chapter reports on recent research conducted on the use of phase change materials (PCM) for thermal comfort and heating and cooling peak load shifting in buildings. The objective here is to show experimentally and through a computer simulation that PCM impregnated in building materials can provide thermal energy storage benefits. Paraffin (RT20) has been used as the PCM because of its desirable thermal and physical attributes including its melting temperature of 20–22 °C, which is close to human comfort temperature. The PCM was impregnated into gypsum wallboards to produce an efficient thermal storage medium (PCMGW), which consists of 26%-wt PCM impregnated in gypsum boards. This PCMgypsum wallboard structure was tested in-situ in an office size building. In parallel, a thermal building simulation code (SUNREL) was used to simulate the performance of the office size rooms. Measured and simulated results in summer showed that the use of PCMGW effectively reduced diurnal daily fluctuations of indoor air temperatures and maintained the indoor temperature at the desired comfort level for a longer period of time. A major benefit of thermal energy storage in winter is to reduce electricity demand charges by limiting the need to run electrically operated heating and airconditioning devices during peak load periods. This study reveals that this application of PCM storage in buildings can lead to healthier interior spaces, and more efficient energy use in terms of demand charge reduction and use of favourable off-peak rates. Key words: PCM, peak load, comfort in buildings, paraffin wax, gypsum wallboard.

16.1

Introduction

The likelihood of a shortfall in future availability of non-renewable energy, which powers much of modern society, is increasing. It is advisable to consider the use of building materials that minimize the need for space conditioning while maintaining comfortable and healthy internal environments [1]. The fluctuation of indoor air temperature, which is a consequence of the low thermal mass of lightweight buildings, plays a special role among other factors influencing thermal comfort. It can be considered as the single largest cause of complaints from occupants in modern buildings constructed using lightweight building materials [2]. 384 © Woodhead Publishing Limited, 2010

Phase change materials for energy storage and thermal

385

Thermal energy storage in buildings, which has received an increasing attention, seems to provide better thermal comfort for occupants and ensure efficient energy use. The need for air-conditioning could be minimized through an effective incorporation of phase change materials (PCM) into building materials, which also enables strategies for electricity peak-load shifting. The use of PCM will also allow the passive capture of solar energy when it is available, which can then be stored and used later when there is a demand for it. The work presented in this chapter shows how well PCM building materials could perform in real buildings in New Zealand. Full-scale office size buildings constructed using normal and PCM-impregnated gypsum wallboard are used for that purpose.

16.2

Background

Research on PCMs and methods of thermal energy storage (TES) in buildings was initiated and encouraged by the US Department of Energy in 1982 to demonstrate the benefits of utilizing latent heat of PCMs in term of reducing variation in indoor temperature (thermal inertia) in lightweight constructions [3]. Later, many researchers confirmed the effectiveness of PCMs in improving energy efficiency and indoor thermal comfort of buildings based on numerical simulations and experimental studies [4]. In particular, Athienitis et al. [5] and Schossig and Hennin [6] carried out extensive studies in full-scale office size rooms, which were constructed using PCM building materials. It was shown that these materials could function efficiently as a thermal storage medium. Recently, Cabeza et al. [7] used microencapsulated PCM in a real size concrete cubicle and compared its performance with a control concrete cubicle containing no PCM. They showed an improved thermal inertia and efficient energy use in the PCM cubicle. Comprehensive reviews on the PCM thermal energy storage in buildings have been adequately discussed in the literature [8]. Another application of TES is peak-load shifting to take advantage of the off-peak electricity tariffs. There is a growing interest in the use of daily TES for electrical load management in both new and existing buildings. The TES technology has matured and is now accepted internationally as a proven energy conservation technology [9]. A simplified sketch in Fig. 16.1 is given by Dincer [10] to show different strategies for charging and discharging TES to meet cooling demands during peak demand hours. The full-storage strategy, which is sketched in Fig. 16.1(a), shifts the entire peak load to off-peak hours. This strategy is most attractive and likely to be economically advantageous only when spikes in the peak load curve are of short duration [11]. In the partial-storage strategy (Fig. 16.1(b)), equipment operates to meet part of the peak-load and the rest is met by drawing energy from storage. On mild


386


Tons

Reduced on-peak demand

C

C

C

A B

A B

C

B

24 hours daily cycle A: storage meets load B: chiller meets load directly C: chiller charging storage (a)

24 hours daily cycle Dotted line: chiller on Solid line: load (b)

16.1 Operating strategies: (a) full storage; (b) partial-storage [10].

spring or autumn days the partial-storage system designed for space heating at winter design temperatures may function as a full-storage system with a full peak demand shift. Promising results of peak-load shifting by using the concepts of TES have encouraged investigators to put more effort in energy efficiency and conservation.

16.3

Selection of phase change materials (pcm) and fabrication of pcm-gypsum wallboards (pcmgw)

A comprehensive list of possible material that may be used for latent heat storage are reported by Abhat [12]. Readers who are interested in such information are referred to the papers by Lorsh et al. [13], Lane et al. [14], Humphries and Griggs [15] and more recently by Khudhair and Farid [8] who have reported a large number of possible candidates for latent heat storage covering a wide range of temperatures. Paraffin compounds undergo negligible supercooling, are chemically inert and stable with no phase segregation. Pure paraffin waxes are very expensive; therefore only technical-grade paraffin should be used. Paraffin waxes are essentially available as linear alkyl hydrocarbons at almost any temperature. Furthermore, the paraffin waxes are non-polar materials; they are not subject to hydrogen bonding with other polar components of the building materials. Fatty acids and their esters can also be used as more environmentally friendly PCMs; however, they have lower latent heat than paraffin. Feldman and Shapiro [16] have analysed the thermal properties of fatty acids (capric, lauric, palmitic and stearic acid) and their binary mixtures. The results have shown that they are attractive candidates for latent heat thermal storage in space heating applications. The melting range of the fatty acids was found to



387

vary from 30 °C to 65 °C, while their latent heat of transition was observed to vary from 153 to 182 kJ/kg. Hydrated salts are attractive materials for use in thermal storage due to their high volumetric storage density (~350 MJ/m3), relatively high thermal conductivity (~0.5 W/m °C) and moderate costs compared to paraffin waxes, with few exceptions [8]. However, the high storage density of these materials is difficult to maintain and usually decreases with cycling. This is because most hydrated salts melt congruently with the formation of lower hydrated salt, making the process irreversible, leading to the continuous decline in their storage efficiency. Subcooling is another serious problem associated with all hydrated salts. A number of companies have successfully succeeded in solving these problems and commercialized CaCl2·6H2O encapsulated in different forms of containments to be used as a PCM. To allow safe and straightforward application of PCMs in different environments (e.g. within plasterboard of residential housing) they can be microencapsulated as micron-sized particles with a polymer shell as a coating [17]. This eliminates any possibility of leakage and diffusion of the liquid PCM or evaporation of volatile components over time. The microencapsulation also reduces the flammability of the PCM. The microcapsules can be easily incorporated into many systems to reduce energy consumption and maintain temperatures at a comfortable level. Prime examples include heat transfer media in heating and cooling systems or heat storage media in insulating and building materials. The commercial paraffin, RT20, was selected as a suitable PCM in this study mainly due to its melting temperature, which is close to human comfort temperature. Paraffin RT20 is stable, chemically inert and possesses a good level of latent heat. Thermal energy storage characteristics of PCMs are most accurately evaluated by using differential scanning calorimetry (DSC) analysis. DSC graphs in Fig. 16.2 show that paraffin RT20 has a melting range of 18.1–21.5 °C and a reasonably high latent heat of fusion of ~162.5 kJ/kg. RT20 melts and solidifies within a narrow temperature range so that the stored heat may be released over a limited temperature swing that must be maintained inside buildings. Repeated 100 melting/freezing cycles were conducted on samples of RT20 to study the changes in the thermal properties. The results showed that this commercial paraffin has good thermal properties and undergoes congruent melting and freezing with no supercooling. A most flexible process whereby the PCM can be incorporated into gypsum wallboards is the immersion process, which does not interfere with the manufacturing processes of the gypsum wallboards. It involved dipping the stocks of the gypsum wallboards in molten PCM. The process was found easy to control and susceptible to a wide variation of process conditions such as PCM temperature and immersion time. Impregnation of


388


24 21

10th Cycle: Peak 18 °C, Heat 162.5 kJ/kg 50th Cycle: Peak 17.75 °C, Heat 161 kJ/kg 100th Cycle: Peak 18.15 °C, Heat 162 kJ/kg

10th Cycle

18

50th Cycle 100th Cycle

15 12 Heat flow (mW)

9 6 3 0 –3

0

5

10

15

20

25

30

35

40

45

50

55

–6 –9

–12 –15

10th Cycle: Peak 21.8 °C, Heat 162.4 kJ/kg 50th Cycle: Peak 21.9 °C, Heat 162.7 kJ/kg 100th Cycle: Peak 21.8 °C, Heat 162.5 kJ/kg

–18 –21 –24

Temperature (°C)

16.2 DSC graph of RT20 after a number of thermal cycles. 28 26

Percentages of PCM-uptake

24 22 20 18 16 14 12 10 40 50 60 70 80

8 6 4 2 0

0

1

2

3

4

5 6 7 Time (minute)

°C °C °C °C °C

8

PCM PCM PCM PCM PCM

molten molten molten molten molten

9

10

bath bath bath bath bath

11

12

16.3 Effects of immersion time and PCM temperatures on the PCM absorptivity into 13 mm thick gypsum wallboards.

the gypsum wallboard with 24–26% by weight of RT20 was achieved by immersing ordinary gypsum wallboards (60 ¥ 60 ¥ 1.3 cm) for 10 minutes in a bath filled with molten RT20 at 70–80 °C. Figure 16.3 shows that the rate of paraffin uptake into the gypsum wallboards increases with PCM temperature. The rate of paraffin uptake was very high during the first three



389

minutes but diminished gradually after 10 minutes of immersion time. The alternative to this method is to microencapsulate the paraffin with a polymer. The microencapsulated product can then be mixed with the gypsum during manufacturing. This will prevent any possible migration of the paraffin from the board but will add cost that has prevented its commercial use up to now.

16.4

Full-scale testing facilities

A full-scale facility consisting of two identical office size constructions were built at the Tamaki Campus of the University of Auckland, New Zealand with the view to conduct long-term thermal performance involving monitoring and modelling work. A schematic plan view of the test room is shown in Fig. 16.4. The interior walls and ceiling of the first office (ORD) were finished with ordinary gypsum wallboards while the interior walls and ceiling of the second office (PCM) were finished using PCMGW. Each office in the test facility is a single-storey design of a typical lightweight construction. They measure 2.6 ¥ 2.6 ¥ 2.6 m giving a floor area of 5.76 m2 each. Wooden frames made of 9.8 ¥ 6.3 cm dressed pine timber were used in the construction. The interior coverings were sets of either gypsum wallboards or PCMGW panels (60 ¥ 60 ¥ 1.3 cm) mounted

1200

1100

2500

2600

1200

EQ

2400

94 mm Insulation

EQ

Electricity board

800

Door 760

*All constructions comply with NZS 3604 North

**All measurments in mm

16.4 Schematic plan view of the test room.


390


on the wooden frame. The exterior walls were 1.25 cm thick sheets of plywood. The wall cavities were filled with fibreglass thermal insulation. The insulation is installed with no gaps and no folds so as to achieve high thermal resistance to heat flow. The thickness of the insulation is 9.4 cm for both the walls and ceilings. The test facility faces north to maximize sun exposure, as this is a key aspect of energy-efficient building design. Each office was supplied with one window facing north. The two constructions were situated in a large open area with a 4.2 m distance between them, free from any shading. The test facility was provided with a computer-controlled data acquisition system with 20 data channels. The system consisted of a data recorder connected to a modem for data remote downloading. In addition to the temperature measurements inside and outside the offices, relative humidity, wind speed and solar radiation were also continuously measured and recorded.

16.5

Benefits of applying thermal energy storage

16.5.1 Thermal comfort in summer season For the sake of clarity, only the results obtained over a seven-day period in January 2005 are presented here for discussion. Figure 16.5 shows measurements of solar radiation and wind speed in Auckland, New Zealand. The solar radiation exceeds 1000 W/m2 during the peak sunshine hours on all days. A large portion of New Zealand has at least 2000 hours of sunshine a year. The two offices were not supplied with any active cooling in order to Solar radiation Wind speed

10 9

1100 1000 900 800

8

700 600 500 400

5

300 200 100 0

2

7 6

4 3

1 0

12

24 36 48

0 60 72 84 96 108 120 132 144 156 168 Time (hour)

16.5 Summer readings of solar radiation and wind speed.


Wind speed (m/s)

Solar radiation (W/m2)

1400 1300 1200


391

accurately compare the performance of ordinary gypsum wallboards and PCMGW and their effects on the fluctuation of the indoor air temperatures in a real, albeit somewhat controlled, situation. Figure 16.6 shows that, in the PCM room, as the indoor air temperature rises passively to within the solid–liquid phase change temperature, the PCM begins to melt by absorbing heat from the room and storing it principally as a latent heat of melting. Thus, the PCMGW acts as a cooling medium. On the other hand, in the ORD room, the temperature rises much more steeply and reaches a higher level because only sensible heat storage is available. When the indoor air temperature falls below the PCM transition temperature, the PCM begins to solidify in the PCMGW, partially or completely, releasing the stored latent heat. The thermal behaviour of the wallboards affects significantly the indoor air temperatures of the room over the seven-day period. The temperatures in the PCM room rises at a lower rate during the day compared to that of the ORD room. At night, the temperature in the PCM room is higher than that in the ORD room. The ORD room maintained a wider range of weekly-averaged indoor air temperature compared to the PCM room, as shown in Table 16.1. It appears that the need for mechanically assisted cooling in the summer season in Auckland could be eliminated or reduced to a large extent through the use of PCMGW. The weekly-averaged indoor air temperature in the ORD room varied by 10.6 °C, while the corresponding variation was only 5.1 °C in the PCM room. Thus, the stored thermal energy in the form of latent heat has effectively 30

Ambient air temp (°C) ORD indoor air temp (°C) PCM indoor air temp (°C)

28

Temperature (°C)

26 24 22 20 18 16 14 12 10

0

12

24

36

48

60

72 84 96 108 120 132 144 156 168 Time (hour)

16.6 Summer ambient and indoor temperatures in the two rooms.


392

Materials for energy efficiency and thermal comfort in buildings Table 16.1 A comparison of measured weekly-averaged indoor air temperatures in the test rooms ORD room

DTORD

PCM room

DTPCM

28.1 °C–17.5 °C

10.6 °C

25.7 °C–20.6 °C

5.1 °C

reduced the variation of the indoor air temperature within the PCM room. The general notion is that if PCM is integrated into the walls and ceilings, it will act as a thermal reservoir and prevent excessive rise or fall of the indoor air temperature. As a result, it leads to a reduction in the overheating hours and hence in the cooling loads in summer, and yet provides a comfortable indoor atmosphere.

16.5.2 Peak-load shifting in winter season On sunny days in winter, the use of PCM in the wall and ceilings of buildings will allow the capture of solar radiation during the day for later use at night. However, on predominantly cloudy or foggy days, the indoor air temperature in both rooms did not reach the melting range of the selected PCM. Consequently, there was no stored energy and hence no major benefit was observed from using the PCMGW. When an internal heating system is supplied, there is potential for use of the high storage density of the PCM in the walls and ceiling for electrical peak-load shifting. This will allow some reduction in the cost of electricity by shifting electrical heating (in winter) and cooling (in summer) demands to periods when electricity prices are lower, for instance during the night. Thus the main application of thermal energy storage in winter would be to capture solar energy and reduce electrical demand charges by peak-load shifting. The indoor and ambient temperatures were monitored, collected and analysed during the month of July 2006, which is usually the coldest month in Auckland. For the sake of clarity, only the results obtained over two specific days (18 and 19 July), which are representative of typical cloudy and cold days in Auckland, are presented. Figure 16.7 shows the measured solar radiation and wind speed. A 5-fin oil radiator electrical heater with a power setting of 1 kW was used in each room. The heaters were programmed to turn on from 1:00 am to 7:00 am (off-peak period) every night, using a digital timer. In the PCM room, when the indoor air temperature rose to within the solid–liquid phase change temperature, the PCM melted in the PCMGW by absorbing heat from the room. At 7 am the heaters in both test rooms were turned off. Subsequently, the indoor air temperature fell below the PCM transition temperature, the PCM solidified in the PCMGW, completely or partially, and the stored latent heat was released. The heating load needed during the day


Phase change materials for energy storage and thermal 450

393 10

Sunshine Wind speed

400

9 8

Solar radiation (W/m2)

350

7

Wind speed (m/s)

300

6

250

5

200

4

150

3

100

2

50

1

0

0

4

8

12

16

20 24 28 Time (hour)

32

36

40

44

48

0

16.7 Winter readings of solar radiation and wind speed. 28 26 24

Temperature (°C)

22 20 18 16 14 12 10 8

Ambient air temp. ORD indoor air temp. PCM indoor air temp.

6 4

0

4

8

12

16

20 24 28 Time (hour)

32

36

40

44

48

16.8 Winter ambient and indoor temperatures in the test rooms.

in winter was reduced while the thermal load was met with the released heat from the PCM. Figure 16.8 shows that the variation in the indoor temperature was reduced significantly by the application of PCM. Figure 16.8 shows that thermal storage is often advantageous in facilities where there is a limited number of peak demand hours every day. Office


394


buildings with no heating demands overnight and high heating demands in the morning hours, exhibit the optimum profile for this application of thermal energy storage.

16.6 Computer simulation 16.6.1 Thermal building model It was necessary to conduct a computer simulation of the buildings so that the idea of using PCM could be tested in future at any location worldwide. SUNREL version 1.04 is technical software used for building energy simulations based on finite difference approaches to model active or passive building elements. It is an upgrade to SERIRES version 1.0 that was written under the guidance of the Solar Energy Research Institute (SERI), now the National Renewable Energy Laboratory (NREL) at Golden, CO, USA. The upgrade of SERIRES to SUNREL was completed by Colorado State University and NREL. SUNREL has been tested satisfactorily through experimentation using the procedure of the International Energy Association [18]. A development of thermal models for buildings is required to arrive at optimal design parameters especially with regard to the required thermal mass. It is well known that thermal models of buildings depend in a complex way upon many interrelated factors. But using an appropriate level of detail in the SUNREL program depends primarily on the nature of the desired outputs and the applications under study. The basic descriptive constructs provided by SUNREL for developing the thermal models will be created within the constraints of the program. Given the correct input parameters that cover different aspects of the building size, construction, and location, SUNREL is, therefore, able to internally convert them to a mathematical form suitable for numerical solutions. The SUNREL simulation software uses the concept of ‘thermal zone’ to define thermal properties necessary for the simulation of a specific area. The thermal zone is either a single room or a group of rooms. Usually, a building is represented as one or more thermal zones with thermal communications (heat flow) between them and with the ambient including solar radiation. The most common paths of thermal communications are windows and walls including those walls with special constructions such as layers of PCMs. The wall construction consists of up to 10 layers from inside to outside. The layers of the walls are usually composed of different building materials and insulations in the ‘ORD’ case, which does not contain PCM materials. In the ‘PCM’ case, the layers of the walls include PCM in addition to the building materials and insulations. Within the SUNREL program, the most important requirements are the building materials of the wall. Figure 16.9 shows the main configuration used to construct the walls and the ceiling in



395

Siding

Insulation

External side

Internal side

Gypsum boards

Wood

16.9 A schematic of hypothetical four layers of the walls in the ‘ORD’ case. Table 16.2 Thermo-physical properties of the mass types used in the ‘ORD’ case Mass name

Conductivity (W/m K)

Density (kg/m3)


Thickness (m)

Board Insulation Wood Siding

0.25 0.038 0.12 0.094

670 32 510 640

1.089 0.835 1.38 1.17

0.013 0.075 0.025 0.01

Table 16.3 Thermo-physical properties of the PCM used in the ‘PCM’ case Conductivity (W/m K)

Density (kg/m3)


Latent heat (kJ/kg)

Melting point (°C)

Thickness (m)

0.2

810

2.1

172

20

0.004

the simulations of the ‘ORD’ case. Four layers have been used to construct the walls and the ceiling. From the internal to the external side, the layers are gypsum wallboards, insulation, wood and siding. The physical properties of each material, used to make the wall construction, have been defined. The thermal conductivity, density, specific heat and thickness of each layer are listed in Table 16.2. The PCM type has been defined based on its heat of fusion and melting point in addition to the foregoing properties as listed in Table 16.3. Composite walls, such as a typical wood framed wall with stud and insulation may be modelled either as two separate walls belonging to the same exterior surface or two consecutive layers in one wall, as is the case here. In the PCM case, the same configuration of the construction of the walls has been used with the addition of a PCM layer that is placed between two


396


layers of the gypsum wallboards. The two layers of the gypsum wallboards are identical in all physical and thermal properties. The combined thickness of the two layers of the gypsum wallboards used in the PCM case is equal to that of the gypsum wallboards used in the ORD case.

16.6.2 Computer predictions using the SUNREL software SUNREL thermal simulations have some limitations and these are related to the methods of calculating the effect of actual thermal mass in the interior environment and random behaviours of real occupants. Thermal mass is not only present in the building structure, but also in occupants and furniture within the building. Although SUNREL deals with these details in a systematic manner and allows estimating these thermal masses in a certain way, there are generally large uncertainties in the description of the real situation in the simulation input files. Other uncertainties include the method SUNREL uses to calculate the effect of latent heat of the PCM that melts and solidifies at a fixed temperature, while commercial PCMs melt and solidify within a range of temperatures (generally ~5 °C or higher). Also, SUNREL deals with the PCM as a layer placed between multi-layers of building materials, which does not represent the actual situation when the PCM is impregnated and is uniformly distributed within the pores of the building materials. It is, therefore, important to examine the validity of the SUNREL model against real experimental measurements. This can be accomplished by making a comparison between the measured data of the real test rooms and simulation results of the thermal zones, representing the test room, for at least a oneday period, and this was selected randomly as 15 February 2006. The main climatic measurements of hourly solar radiation, ambient temperature and wind speed of that day were used as input data for SUNREL. The results of the simulations were compared with those measured for both thermal zones as shown in Fig. 16.10(a-b). It can be seen that the simulation results (Fig. 16.10(b)) of the indoor temperature in both the ORD case and the PCM case are in a reasonable agreement with the measured data (Fig. 16.10(a)). This agreement between the theoretical and measured indoor temperatures shows that the latent heat of the PCM is well accounted for by the SUNREL simulation program.

16.7

Conclusions

The study presented in this chapter examined the benefits of thermal energy storage in buildings by means of incorporating PCM throughout the building materials. Paraffin RT20, as a PCM, was successfully impregnated into gypsum wallboards to form PCMGW of a significant thermal energy storage


Phase change materials for energy storage and thermal 32

397

Ambient air ORD indoor air PCM indoor air

30 28

Temperature (°C)

26 24 22 20 18 16 14 12

0 2 4 6 8 10 12 14 16 18 20 22 24 0 2 4 6 8 10 12 14 16 18 20 22 24 Time (hour) Time (hour) (a) (b)

16.10 Profiles of temperature of the ORD and PCM cases: (a) experimental results, and (b) simulation results.

effect. These PCMGW walls act as a heat storage or heat sink for trapping heat from the sun during daylight hours for using it later at night. In summer, measured thermal performances of the full-scale room that was constructed using PCMGW as interior surfaces, showed a significant reduction in the daily fluctuation of indoor air temperature (~5.5 °C), providing thermal comfort and healthier interior spaces compared to the control room containing no PCMGW. In winter, this application of PCM storage in buildings can lead to a significant improvement in the energy efficiency of the buildings in terms of use of favourable off-peak rates. Similar observations were obtained through the computer simulation conducted using SUNREL software.

16.8

References

1. R. Vale, ‘The role of insulation and thermal mass in the design of zero-heating homes’, PCM2003 Workshop, The University of Auckland, New Zealand, 2003. 2. P. O. Fanger, ‘Strategies to avoid indoor climate complaints’, ICBEM 1987, Polytechnique Romandes Press, Lausanne, Switzerland. 3. I. O. Salyer, ‘Thermal energy storage’, The DOE Energy Storage Research Activities Conference, 1989, New Orleans, pp. 97–110. 4. D. W. Hawes, D. Banu, D. Feldman, ‘Latent heat storage in concrete’, Solar Energy Materials, 1989, pp. 335–348. 5. A. K. Athienitis, C. Liu, D. Hawes, D. Banu, D. Feldman, ‘Investigation of the thermal performance of a passive solar test-room with wall latent heat storage’, Building and Environment, 1997, pp. 405–410.


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6. P. Schossig, H. M. Hennin, ‘Encapsulated phase change materials integrated into construction materials’, Futurestock ‘2003’, Warsaw, Poland. 7. L. F. Cabeza, C. Castellón, M. Nogués, M. Medrano, R. Leppers, O. Zubillaga, ‘Use of microencapsulated PCM in concrete walls for energy savings’, Energy and Buildings, 2007, 39, 113–119. 8. A. M. Khudhair, M. M. Farid, ‘A review on energy conservation in building application with thermal storage by latent heat using phase change materials’, Energy Conversion & Management, 2004, pp. 263–275. 9. S. D. White, D. J. Cleland, R. Fraser, ‘Demand Side Response from HVAC&R’, AIRAH Energy Efficient Design Conference, November 2002, Sydney, Australia. 10. I. Dincer, ‘On Thermal Energy Storage Systems and Applications in Buildings’, Energy and Buildings, 34, 2002, pp. 377–388. 11. I. Dincer, M. A. Rosen, Thermal Energy Storage Systems and Applications, Wiley Inc., London, 2002. 12. A. Abhat, ‘Low temperature latent heat thermal energy storage; heat storage materials’, Solar Energy, 30, 1983, pp. 313–332. 13. H. G. Lorsh, K. W. Kauffman, J. C. Denton, ‘Thermal energy storage for heating and air conditioning: future energy production system’, Heat and Mass Transfer Processes, 1, 1976, pp. 69–85. 14. G. A. Lane, D. N. Glew, E. C. Clark, H. E. Rossow, S. W. Quigley, S. S. Drake, J. S. Best, ‘Heat of fusion system for solar energy storage subsystems for the heating and cooling of building’. Chalottesville, Virginia, USA, 1975. 15. W. R. Humphries, E. I. Griggs, ‘A designing handbook for phase change thermal control and energy storage devices’. NASA Technical Paper, 1977. 16. D. Feldman, M. M. Shapiro, ‘Fatty acids and their mixtures as phase-change materials for thermal energy storage’, Solar Energy Materials, 18, 1989, pp. 201–216. 17. M. C. Smith, M. M. Farid, A. J. Easteal, ‘Review of microencapsulated phase change materials for thermal energy storage applications’, IIR/IRHACE Conference, 16–18 February 2006, Auckland, New Zealand. 18. M. Deru, ‘BESTEST Results and SUNREL’, 1997, NREL, Version 1.0, Golden, CO, USA.


17

Porous materials for direct and indirect evaporative cooling in buildings

X. Z h a o, De Montfort University, UK

Abstract: This chapter investigates several types of porous materials that have potential to be used as heat and mass transfer media in indirect evaporative cooling systems, namely metals, fibres, ceramics, zeolite and carbon. The investigation identifies the most suitable material and structure. The magnitude of heat/mass transfer rates in relation to air conditioning applications was analysed, and the results showed that thermal properties of the materials, i.e., thermal conductivity and water retaining capacity (porosity), have little impact on system heat/mass transfer, and therefore, these two parameters have low importance in material selection. Instead, shape formation/holding ability, durability, compatibility with waterproof coating, contamination risk and cost are the most important concerns when selecting materials. Each material type was analysed based on the above criteria and the preferable structure and configuration illustrated. Comparing the different material types indicated that wicked (sintered, meshed, grooved or whiskered) metal plates (copper or aluminium) are the most suitable structure and material. Wicked aluminium sheet is much cheaper than copper and therefore more suitable for this application. Other potential applications of porous materials in building are also discussed. Key words: heat and mass exchange, material, thermal conductivity, porosity, shape formation, durability, compatibility, contamination risk, cost.

17.1

Introduction

17.1.1 Types of evaporative cooling systems Evaporative cooling utilizes the latent heat of water evaporation to cool air, either directly or indirectly. Direct evaporative cooling A direct evaporative cooling process is shown schematically in Fig. 17.1. A porous medium is made into thin layers which are arranged in parallel. The process air flows across the surfaces of the layers at a particular velocity, which causes water inside the voids of the porous layers to evaporate. In this process, the energy (enthalpy) in the air remains the same as no external energy is supplied, so the heat required for evaporation must come from the 399 © Woodhead Publishing Limited, 2010

400

Materials for energy efficiency and thermal comfort in buildings Wet channel

Air flow

17.1 Schematic of a direct evaporative cooling process.

Wet channel

Dry channel Working

Product

Wet channel

Dry channel

17.2 Schematic of an indirect evaporative cooling process.

air flow. As a result, warm dry air is changed to cool moist air in a quasiadiabatic process. The cooling effectiveness of this type of direct evaporative system ranges from 70% to 95% in relation to the incoming air’s wet-bulb temperature [1, 2]. However, it adds moisture to the air, making it only suitable for use in hot, dry climates or for spaces requiring humidification. Indirect evaporative cooling Indirect evaporative cooling lowers the air temperature without adding moisture to the air, making it more attractive than direct cooling, and is shown schematically in Fig. 17.2. In an indirect evaporative air cooler, primary (product) air passes over the dry side of a heat exchanging wall, while secondary (working) air passes over its opposite, wet side. The wet side absorbs heat from, and therefore cools, the product air through evaporation of water, while the latent heat of evaporation is transferred to the working, wet side air. Under ideal operating conditions, i.e., the product air travels in a counter flow to the working air and the two airstreams have a good balance of flow rates and an infinite contact area, the product air temperature on the


Porous materials for direct and indirect evaporative cooling

401

dry side of the heat exchanger would reach the wet-bulb temperature of the incoming working air, and the temperature of the working air on the wet side of the sheet would increase from its incoming dry-bulb temperature to the incoming product air dry-bulb temperature and be saturated. However, practical systems are far from this ideal. It has been suggested that only 50% of the incoming working air wet-bulb temperature can be achieved for a typical indirect evaporative cooling system [1, 3]. In recent years, an innovative indirect evaporative cooling process known as the dew point process has emerged (Fig. 17.3). This system results in greater temperature reductions than other methods of evaporative cooling, and allows the supply air to be cooled to a level below the wet bulb and above the dew point of the inlet air. It employs a perforated cross-flow heat and mass exchanger, and achieves a dew point effectiveness of 60 to 70%, and wet bulb effectiveness of greater than 100% [4]. The product air travels 1 2 Product channel

dp

3 Working

3¢

3¢

wet

channel

3¢

3¢

1 Working

dry

channel

Saturation line

1

3

dew: dew point 2: product air 3¢

3≤

Absolute humidity

1

3≤

3: working air 3≤ 3≤ 3≤

dew 3¢

3¢ 2 3¢

3¢

3¢

1

Dry-bulb temperature

17.3 Schematic of a dew point cooling system and its air treatment process in a psychrometric chart.


402


along a dry channel and its temperature is lowered from state 1 to 2 due to the heat transfer occurring between the dry channel and its adjacent wet channel, where water is saturated around the wall, causing a temperature difference between the dry and wet channels. In the meantime, working air at initial state 1 travels along the working dry channel but is gradually diverted into the adjacent wet channel. In the working air dry channel, the working air is also cooled, and at the turning point to the wet channel, its temperature is lowered to state 3, which varies with the location of the point along the travel path. When the working air turns into the adjacent wet channel, it flows backwards along the wet channel and quickly becomes saturated by the vapour evaporated from the wall, changing from state 3¢ to 3≤. It also receives the sensible heat from the dry channel, thus leading to a further change, from 3≤ to 3. A new type of polygonal exchanger for dew point cooling has been developed recently by the author [5]. A preliminary study indicated that the new exchanger could achieve an enhanced dew point effectiveness of up to 85%, which permits higher cooling volumes. Advances in dew point cooling technology open up the possibility of using evaporative cooling for air conditioning in buildings. A diagram of a polygonal exchanger is shown in Fig. 17.4.

17.1.2 Materials used for evaporative cooling Both direct and indirect evaporative cooling require a porous medium to allow heat and mass transfer between the water in the medium and the passing air, or between dry (product) and wet (working) air. The medium is formed into a plate and the air flows across it. The properties of the porous medium are important as these affect the cooling efficiency and performance of the evaporative cooling system. A range of porous materials can be used for this purpose, including metal, fibre, ceramics, zeolite and carbon. Metal materials used include metal foams, metal wools, sintered metals and wicked metal plates/tubes; fibres include paperboard and cloth (wood or glass); ceramics include SiC/SiC composites, zirconia ceramic, zirconia toughened aluminium, Al2O3, and aluminium nitride and polystyrene composites; zeolites include porous ceramics, molecular sieves and synthetic polymers; and carbon materials include carbon–carbon composites and activated carbon. Materials used for evaporative cooling should have high thermal conductivity and large moisture retaining ability, to allow a large amount of heat to be conducted from the air to the inside of the wall (for direct evaporative cooling), or from the dry side of the wall to the wet side, and an adequate amount of water to be retained within the wall. They should also be relatively inexpensive, suitable for shaping, easy to clean and replace, and resistant to bacterial growth on the wet surface.


Porous materials for direct and indirect evaporative cooling Hole

403

Sheet Working (discharged) air

Product (cooled) air

Wet

Dry channel air quid

channel air quid

Intake (product + working) air

Working (cooling air)

Working (discharged) air

Hole Extended Product (cooled) air

Sheet

Working (cooling) air Intake (product + working) air

17.4 The polygonal stack exchanger configuration.

Various materials were investigated to determine their suitability for evaporative cooling systems. The heat/mass transfer rates in relation to an air conditioning application were analysed, and the results used to judge whether the materials demonstrated viable heat and moisture transfer. A number of criteria, including thermal conductivity, water retaining capability, shape formation/holding ability, durability, compatibility with waterproof coating materials, contamination risk and cost, were applied in the search for a material that offered enhanced cooling effects with reasonable durability, low contamination risk and acceptable costs. The results are presented below, and the reader is also referred to the author’s paper published in the journal Building and Environment [6].


404

17.2


Assessing the capacities of evaporative cooling systems and the associated requirements in materials

The heat and mass transfer within an indirect evaporative cooling system is shown schematically in Fig. 17.5, and the process indicated by the psychrometric chart is shown in Fig. 17.6. When the product air (P) flows across the dry channel, it loses heat through the wall due to the temperature difference between the dry and wet sides. As a result, the temperature of the product air falls by a few degrees and its state changes from p1 to p2. In the meantime, water placed on the wet side of the wall evaporates by absorbing the sensible heat from air in the dry/wet channels and the vapour generated is removed by the airstreams across the wet channel. As a result,

W2

W1

P2

P1

17.5 Indication of heat and moisture transfer in an indirect evaporative cooling system.

W1

Absolute humidity

Saturation line

W2

P2 W1

P1

Dry-bulb temperature

17.6 Psychrometric diagram indicating the heat and moisture transfer in an evaporative cooling system.



405

the moisture content of the working air rises gradually until a saturated state is achieved (W1 to W1,w). Thereafter, the working air continues to attract moisture and its temperature rises, while its state moves along the saturation line (from W1,w to W2). The cooling efficiency of the indirect evaporative cooling system is defined as:

h=

T p1 – T p2 T p1 – Tw11,w

17.1

The cooling efficiency will depend largely on the logarithmic average temperature difference (DTm) between the two airstreams. The larger the DTm, the higher the cooling efficiency will be. The definition of DTm can be expressed as follows: DTm =

DTmax – DTmin DT ln max DTmin

17.2

at any position in the channels, heat transfer from the dry airstream to the wet surface of the wall can be expressed as: q = Us(Tp – Tw,s)

17.3

1 1+d h k

17.4

Us =

Combining Eq. 17.3 and Eq. 17.4 yields: q=

T p – Tw,s 1+d h k

17.5

as mentioned previously, the heat from the dry side is partly used for vaporising the water on the wet side of the wall, and the rest for increasing the temperature of the working air in the wet channel. however, if the heat from the dry side is used completely in vaporising the water on the wet side of the wall with no sensible heat transfer between the product and working airstreams, the system will achieve its maximum cooling efficiency, as the logarithmic average temperature difference between the two airstreams reaches its highest value. In this case, the working air temperature will remain constant along its pathway, which is equal to the wet side surface temperature (tw = tw,s), but its moisture content will rise along the flow path. For this particular circumstance, the evaporation rate on the wet side surface can be expressed as:


406


w = q /g =

T p – Tw,s Ê 1 dˆ gÁ + ˜ Ë h k¯

17.6

It can be seen from Eq. 17.6 that both air flow conditions (h, Tp, Tw,s) and material thermal properties (k, d) affect the cooling performance of the system. In terms of air flow, a greater temperature difference between the airstreams in the dry and wet channels would enhance heat/mass transfer in the system, as would increasing the speeds of air flow in both dry and wet channels. In terms of material thermal properties, water retaining capability (Wr), thermal conductivity (k) and wall thickness (d) are potential factors that may impact system performance. Since heat/mass exchange walls are usually thin, with thickness (d) ranging from 0.1 to 0.5 mm, their thermal resistance (d/k) is low, although the value of thermal conductivity (k) may vary widely (between 5 and 300 W/m·°C). Given this, water retaining ability (Wr) is important for cooling effectiveness. Insufficient water retention would reduce the amount of evaporation and so reduce cooling efficiency, while surplus water retention would result in water accumulating on the wet side of the material, which would retard heat/mass transfer. In air conditioning applications, the wet surface temperature, Tw,s, could be taken as the inlet working air wet-bulb temperature (the lowest possible level), which is around 15 °C for the UK climate [7]. The product air temperature, Tp, could be as high as 35 °C. Thus the Tp – Tw,s would be about 20 °C or less, and h is usually less than 20 W/m2·°C for a heat exchanger [8]. The latent heat of water vaporisation, g, could be expressed as: g = 2500 – 2.387Twb

17.7

where Twb is the wet-bulb temperature of the contact surface above the water film. If it was 15 °C, g would be 2464 J/g. Substituting these values into Eq. 17.6 yields, w=

20

dˆ Ê 2464Á 0.05 + ˜ k¯ Ë

=

0.0081 dˆ Ê ÁË 0.05 + k˜¯

17.8

The thickness of the wall, d, is in the range of 0.1 to 0.5 mm, while thermal conductivity, k, takes the weighted average of the k values of the materials and the water, owing to the porous structure. Thus k can be written as [9]: k = rkwater + (1 – r)kmaterials

17.9

where the k value of the water is about 0.6 W/m·°C. For commonly used wall materials, such as metal, fibre, ceramics, zeolite and carbon, k values



407

range from 0.3 to 300 W/m·°C, and porosity varies between 20% and 90%, as shown in Tables 17.1 to 17.4. Substituting these values into Eq. 17.9 yields the k value of the watersaturated wall, which is in the range of 0.57 (ceramics with 90% porosity) to 270.06 W/m·°C (metal with 20% porosity). Assuming a wall thickness of 0.5 mm, then the yield value of d/k would be in the range 0.88 ¥ 10–3 to 1.85 ¥ 10–6 m2·°C/W. Substituting this into Eq. 17.8, the evaporation rate w can then be obtained, which is in the range 0.1592–0.16194 g/m2·s, i.e., 0.57312–0.58298 l/m2 · h. If these figures are applied to Eq. 17.6, the heat transfer rate across the wall is then obtained, which in this example would be about 392–399 W/m2. Assuming the void space is completely filled with water, the water retaining volume of a 0.5 mm thick wall would be in the range 0.1–0.45 litre, which is able to support 0.3–1 h of evaporation in an air conditioning application. The above analysis shows that the thermal conductivity and water retaining capability (porosity) of the material have little impact on the magnitude of Table 17.1 Thermal conductivity of selected metals and alloys Metals

Thermal conductivity (W/m·K)

Copper and copper alloys Aluminium and aluminium alloys

115.98–159.25 349.66–400 83.78–86.55 36.35–55.93 29.43–45.01 201.95–226.00 158.6–340.89 107.32–323.70 229 121–222

Copper-zinc-lead brasses Wrought copper Copper-tin-bronze alloys Wrought copper-aluminium alloys Wrought copper-nickel alloys Wrought copper suitable for tube Copper suitable for pipes Wrought high copper alloy Aluminium Aluminium alloys

Table 17.2 Thermal characteristics of selected fibres Fibres


Wood fibres 0.012–0.654 Latex with cement 0.28–0.52 Methylcellulose with cement 0.32–0.42 Silica fume 0.33–0.36 Methylcellulose + fibre + cement 0.28–0.44 Silica fume + methylcellulose 0.28–0.36 Natural random fibrous 0.02–0.13 materials (Porosity = 10–58%) (Sisal, ramie and jute) Cotton fibre 0.176–0.351 Glass fibre 0.277


Porosity (%) 54.7–63.6 (very high water penetrability) 1.10–2.32(±0.02) 2.07–2.12(±0.02) 2.97–3.14(±0.02) 3.33–3.97(±0.02) 3.14–4.36(±0.02) Porosity = 10–58% 40 2.05–2.3 (±0.02)

408


Table 17.3 Thermal conductivity of various ceramics Kinds of porous ceramics

Thermal conductivity (W/m · K)

SiC/SiC composites (conventional process) 0.08–15 (porosity = 33.0%) SiC/SiC composites (chemical vapour deposition) 18.5–32.5 (porosity = 33.0 ± 0.6%) SiC/SiC composites (polymer impregnation and pyrolysis) 19–50 (porosity = 32.4 ± 0.4%) Zirconia ceramics (porosity = 0–100%) 0–2.2 Zirconia toughened alumina (a) (porosity = 40%) 11.09 Zirconia toughened alumina (b) (porosity = 22%) 21.67 Zirconia toughened alumina (c) (porosity = 15%) 22.79 3.1 ZrO2 Al2O3 29 Aluminum nitride and polystyrene composites 20–240 (porosity = 0–40)

Table 17.4 Thermal conductivity and porosity of selected carbon fibres and carbon composites Various carbons materials


Porosity (%)

Carbon–carbon composites Activated carbons Natural graphite + carbon composites Mesophase pitchbased fibres Interplay continuous/spun hybrid carbon composites Carbons fibres (porosity = 2.37–9.31) 3D-Hi-Nicalon type S–CVI composites

55–320 0.2 1–32 300–1100 0.68–2.24

31.4–46.1, 6.3–9.1 80 ≥18 1.94 ± 0.1–29.9±0.5 30–40

175–200 15–108

2.37–9.31 34–40

heat and mass transfer rates, and are therefore not important considerations for material selection. The water retaining capacity of the material is greater than that needed to support the evaporation, so the wet surface of the wall will not dry out. Other criteria relevant to material selection include shape formation/holding ability, durability, compatibility with waterproof coatings, contamination risk and cost. Shape formation/holding ability describes the level of difficulty in shaping a material and whether it holds that shape effectively, and is evaluated by Young’s modulus, in Pascal (Pa). A moderate hardness is preferable, as hard material is difficult to shape, while soft material will not hold a shape. Durability refers to how long the material will work under saturated conditions. Compatibility indicates how the material adapts to other media, for instance substances used for waterproofing the material. This is particularly relevant for



409

indirect evaporative cooling, since the material will be wet on one side and dry on the other and so one side needs to be waterproofed to prevent water penetration. The waterproof coating could be made of the same material as the wall, or some other material which is compatible with the wall material. Contamination risks indicate the possibility of bacterial growth under wet operating conditions, and cost is an important factor when determining the type of material to use.

17.3

Comparative analyses of potential materials for evaporative cooling

17.3.1 Metal Traditional metal heat exchangers are made mainly of aluminium, copper and their alloys, which are shaped into an exchanger surface such as a plate or tube. To increase surface capillary force, and thus improve its ability to retain water for evaporative cooling, the surface on one or both sides of the sheet or tube is covered with a porous wick structure [10]. Several porous metal structures, wicked metal, metal foams and wools, will be described in this section. Table 17.1 gives the thermal conductivity of copper, aluminium and their alloys [11, 12]. The wick may be in the form of sintered particles, microscopic holes, meshes, grooves or whiskers, and is attached to the tube or sheet to hold water for evaporation. The porosity of the wick varies widely, from 20% to 90%, depending upon its construction, density and configuration. Fig. 17.7 shows a whiskered tube heat exchanger, which has a large volume of micro-cavities on the external surface of the tube [13]. The density of metal whiskers covering the tube determines the porosity of the tube surface. For water retention, this structure is usually better than making holes or grooves on the tube. Intensive evaporation process

Extremely high surface area

Extremely high porosity

Extremely high cooling power

Metal profile and tube are unified

17.7 Heat exchanging copper tube with micro-structure surface.


410


In recent years, highly conductive foams based on copper or aluminium have been used to make heat exchangers. These foams have open cell structures that allow heat to be removed from or added to gases or liquids by letting them flow through the cooled or heated foam [14]. Owing to the open porous structure, this type of heat exchanger can contain plenty of water and so allow moisture transfer as well. The foams are produced using different methods, including melts, powders, sputtering and deposition [15]. Each method covers a characteristic range of density, cell size and cell topology, thus resulting in porosity ranging from 30% to 80%. At present, pore sizes ranging from 4.5–0.5 mm at constant porosity of 80% have been achieved. Porosity, shape information and construction expense are related to each other [15]. Figure 17.8 shows some commonly available metal foams [16]. Metal wools are the other type of porous metal used, and are made mainly from copper, aluminium and steel. Porosity varies, based on metal fibre length, fibre diameter and density, and ranges from 30% to 95%. The porosity of one kind of copper wool was found to be 0.95 and the thermal conductivity of this form of copper reduced to 1.0–2.7 W/m·K [17]. Figure 17.9 shows some commonly available metal wools [18]. If any of the above materials are used as the exchanger panel in an indirect evaporation cooling system, one side of the panel has to be waterproofed to prevent penetration of moisture. This can be achieved by attaching a thin solid film of the same material onto the porous metal. Analysing the materials using the method given in Section 17.2 above showed that the thermal conductivity and porosity of any metal structure are both high enough to allow the heat/mass transfer necessary for air conditioning applications. This means that any of the metal types described is suitable for use as the exchanger plate, and the thermal properties of the metals are the least important factors in material selection.

17.8 Metal foams.



411

17.9 Metal wools.

17.10 Fibre structure of hardwood.

In terms of hardness, copper and aluminium have Young’s moduli ranging from 70 to 140 GPa, giving them suitable shaping and shape-holding properties for the exchanger plate [19], [20]. Copper and aluminium are also durable and compatible with solid metal of the same material. In terms of risk of bacterial growth, metal plates with wick (sintered, meshed, grooved or whiskered) are better than foams and wools, as the pores on the wicked plates are exposed rather than concealed, facilitating cleaning or other hygiene treatments of the surface. In terms of cost, aluminium is much cheaper than copper [21].

17.3.2 Fibre Fibrous materials, such as paperboard, cloth, wood or glass fibre, and others, have relatively high penetrability and lower thermal conductivity and hardness. Figure 17.10 shows the structure of a hardwood fibre, and Table 17.2 presents the thermal conductivity of various fibrous materials [22]–[25]. It can be seen that the fibres have much lower thermal conductivity than metals, ranging from 0.01 to 0.3 W/m·K [26]. Solid fibres have lower thermal conductivity than porous ones [25]. Table 17.2 also gives the porosity of a number of commonly used fibre materials. Wooden fibre, natural random fibre and


412


glass fibre have the same level of porosity, ranging from 10 to 60%, while the others have a much lower level, ranging from 1 to 5%. The effective thermal conductivity of a fibrous material increases with the fibre length, and approaches a stable level when the fibre length is sufficiently long. The effective thermal conductivity decreases as porosity increases [27]. Although the thermal conductivities of fibres are much lower than those of metals, an analysis using the method given in Section 17.2 indicated that the fibres are still able to provide a heat transfer rate of 392–399 W/m2, which is sufficient for air conditioning applications. Fibre porosities are also sufficient to retain the water needed for moisture transfer. It would be preferable to choose a fibre with low porosity, as this kind of fibre has lower water retaining capacity, which would enhance sensible heat transfer. Most of the fibres presented in Table 17.2 are suitable for use as heat exchanger material, except for wood and cotton. In terms of hardness, most fibre materials are not strong enough to be used as exchanger plates. However, flax or wooden fibres are the exception, with Young’s moduli as high as 70 to 110 GPa in the longitudinal direction. The lifespan of most fibre exchangers is short, as the material is easily deformed or damaged when soaked with water, except for flax or wood fibre. Fibre is compatible with polyethylene, which is used to waterproof one side of the fibre to prevent water penetration [28]. Wet fibre surfaces are readily contaminated by bacteria, which is not good for evaporative cooling applications. However, fibres are extremely cheap [28], and so frequent replacement is affordable, potentially overcoming the difficulties of short lifespan and contamination risk.

17.3.3 Ceramics Porous ceramics have potential for use in evaporative cooling due to their strength, high thermal conductivity, waterproof nature and durability. Porous solids, such as extruded monoliths with parallel channels and thin walls, and made of various oxide and non-oxide ceramics, ceramic foams and metal structure, can perform both moisture retaining and heat exchanging activities. Figure 17.11 presents several foam-structured porous ceramics made by IKTS [29]. The thermal conductivity of porous ceramics depends upon composition, pore size and distribution, porosity and the manufacturing process. It varies from 0.1 to 240 W/m·K. Generally the thermal conductivity decreases as porosity increases [30]. Table 17.3 shows the thermal conductivity of a range of different ceramics [29], [31]–[33]. The water retaining capacity of ceramics increases with increasing porosity and pore size. One way to make porous ceramics is by mixing ground vermiculite and allophone at 600–800 °C [34].



413

17.11 Ceramic structure overview.

The thermal conductivities of ceramics are higher than those of fibres, but lower than those of metals. This is sufficient for the heat/mass transfer needed in an air conditioning application. The porosities of ceramics are also sufficient to retain the water needed for moisture transfer. A ceramic with low porosity is preferable, because it has lower water retaining capacity thus enhancing sensible heat transfer. Aluminium nitride and polystyrene composites can be made at low porosity levels, such as 1% or below, and therefore may be suitable for use in this application. A porous ceramic may be combined with a thin film made of the same material to prevent penetration of moisture when being used in indirect evaporation systems. In terms of hardness, most porous ceramics have Young’s moduli ranging from 50 to 400 GPa [35]–[38], making them suitable for use as exchanger plates. Porous ceramics are durable in wet conditions, and are compatible with solid film of the same material. Their porous structure is likely to be susceptible to bacterial growth when wet, and as the pores are concealed inside the structure, cleaning is difficult. Ceramics cost about twice as much as metal heat exchanger plates [39].

17.3.4 Zeolites Zeolites can be classified as natural and synthetic zeolites. Natural zeolites are aluminosilicate minerals. They occur in nature and are mined rather than synthesised. Zeolites have a very open crystalline lattice that allows molecules like water vapour to be held inside the crystal itself like an object in a cage. Particular atoms of an aluminosilicate determine the size of the openings between the ‘bars’ of the cage, which in turn governs the maximum size of the molecule that can be adsorbed into the structure. Synthetic zeolites, also called molecular sieves, are crystalline aluminosilicates manufactured in a thermal process. Controlling the temperature of the process and the composition of the ingredient materials allows close control of the structure and surface characteristics of the adsorbent. At a somewhat higher cost, this provides a much more uniform product than naturally occurring zeolites. Crystalline zeolites can be used for a wide variety of purposes including static and dynamic drying, ion exchange, and selective separations involving


414


gases and liquids. Industrial applications of zeolites are primarily as Linde Molecular Sieves (LMS) and Davison Microtraps, due to availability in quantity and cost. Synthetic zeolites are attractive for drying and separation owing to their affinity for water and other small diameter molecules, and also their ability to reject large diameter molecules [40]. The thermal conductivity of LMS is around 0.59 W/m·K, which is much smaller than that of metals or porous ceramics [40]. However, the figure is still high enough to provide adequate heat transfer for air conditioning applications. Linde Molecular Sieves have a high sorption capacity at low water vapour concentrations, and maintain their high sorption capacity at elevated relative humidity, which is in direct contrast to silica gel and activated alumina [41]. They have a porosity of 40–80%, but the absorption capacity is even higher due to water affinity characteristics. The water retaining potential is good enough to make LMS suitable for the moisture transfer requirements of air conditioning applications. Zeolite material used in the exchanging wall can be coated with polyethylene or wax [42, 43] on one side to prevent moisture penetration when being used in indirect evaporation systems. In terms of hardness, most zeolites are adequately strong for use as exchanger plates, having Young’s moduli ranging from 1 to 20 GPa [44]. Since the pores are internal, the zeolite structure is susceptible to bacterial growth when wet. In terms of cost, they are more expensive than ceramics [39].

17.3.5 Carbon Carbon fibres are important as reinforcement in composite materials because of their low density, high strength (up to 7 GPa) and tensile modulus up to 600 GPa. Carbon fibre is also useful in a wide variety of products due to its high electrical and thermal conductivity [45]. Activated carbons are often used for desiccants, carbon–carbon composites are commonly used in aerospace applications, and mesophase pitch-based carbon fibres with high preferred orientation have low density and high thermal conductivity (k), with a k value at room temperature up to 1120 W/m·°C. Figure 17.12 shows three types of carbon fibre structure [46–48]. Activated carbons have very low thermal conductivity (0.2 W/m·K), resulting in severe thermal limitations due to low overall adsorption and desorption kinetics as well as secondary reaction products or hazards in industrial applications. The in situ activation of various precursors within a consolidated expanded natural graphite matrix results in composites with high thermal conductivities (from 1 to 32 W/m·°C) and high effective adsorbent levels (80 wt%) [45]. The type of carbon fibres in the composites plays an



(a)

(b)

415

(c)

17.12 Three types of carbon fibre structures (a) laminated short fibre felt; (b) laminated carbon cloth felt; (c) needle picked long fibre felt.

important role in thermal conduction behaviour and the highly graphitic flat-layered structured fibres conduct in a direction parallel to the fibre axis [46]. Table 17.4 presents the thermal conductivity and porosity of selected carbon fibre materials [47–55]. Analysis using the method given in Section 17.2 indicates that the thermal properties of carbon materials are good enough to transfer both sensible and latent heat in air conditioning applications. A carbon material with low porosity would be preferable, as the resultant lower water retaining capacity would enable enhanced sensible heat transfer. Carbon fibres or mesophase pitch-based fibres could be made at low porosity levels, such as 1% or below, and therefore may be suitable for use in this application. Carbon fibres used in a heat exchanging wall can also be coated with polyethylene or wax on one side to prevent moisture penetration when being used in indirect evaporation systems [56, 57]. In terms of stiffness, carbon fibres are strong enough to be formed into heat/mass transfer elements, having Young’s moduli ranging from 1 to 220 GPa [44]. Since the pores are internal, a carbon fibre structure is likely to be susceptible to bacterial growth when wet for a long time. In terms of cost, carbon fibre materials have similar prices to metals [39].

17.3.6 Comparing the materials Table 17.5 summarises the performance of the materials described above in terms of thermal conductivity, porosity, hardness, compatibility with coating materials, contamination risk and cost. All the materials described have thermal properties suitable for the heat/ mass transfer required in air conditioning applications (up to 390 W/m2), so it is unnecessary to consider the impact of thermal conductivity and porosity on thermal performance when selecting one of these materials for a heat/mass exchanger. Thus the most important factors to be considered are hardness, compatibility with coating materials, contamination risk and cost. Metal and ceramics are better choices for hardness (shaping and shape-retaining ability), followed by zeolite, carbon and fibre. All the selected materials are compatible


416


Index Material type

Thermal Porosity (%) Hardness Compatibility Contamination conductivity (shaping ability) with coating risk (W/m·K)

Metal High 20–90 High Fibre Low 1–60 Low Ceramic Variable 1–80 High Zeolite Low 40–80 Medium Carbon Variable Variable Medium

Compatible with the solid metal Compatible to polyethylene Compatible with the solid metal Compatible to polyethylene or wax Compatible to polyethylene or wax

Cost (£) per sheet [39] 100 ¥ 100 ¥ 0.5 (mm ¥ mm ¥ mm)

Low (sintered metal) High

30–100

High

150–250

High

150–250

High

30–80

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