2011, Volume 3
COMMERCIAL
DAYLIGHTING • Waterproofing Sandwich Slabs
• State-of-the-Art Building Envelope
Subscribe to our e-Newsletter at www.ARWmag.com
Announcing
THE NEXT GENERATION in TPO roofing.
GAF scientists have developed EverGuard ® Extreme™ TPO... designed for solar and other high heat applications...
EverGuard ® Extreme™ TPO is the next generation in TPO membranes—with the best guarantee in the industry!
Solar panels (which are dark in color by nature) can create areas of high temperatures up to 190°F on adjacent roof surfaces. Other highly reflective architectural structures (such as mirrored glass) can act as solar magnifiers and concentrate sunlight onto nearby roof surfaces. This concentrated sunlight can be far more intense than ambient sunlight — leading to premature membrane failure.
GAF scientists have created the next generation in TPO membranes, built to handle the extreme demands that new rooftop applications can place on roofing membranes. EverGuard® Extreme™ TPO uses proprietary stabilizers and UV absorbers to achieve weathering performance far beyond current standards. That means more protection for your customers — and for you.
Not eligible for Well Roof™ Advantage Guarantee extension. See guarantee for complete coverage and restrictions ©2011 GAF Materials Corporation 1/11
2011, Vol. 3 2011, Volume 3
COMMERCIAL
DAYLIGHTING
13
s 7ATERPROOlNG 3ANDWICH3LABS
s 3TATE OF THE !RT "UILDING%NVELOPE
Subscribe to our e-Newsletter at www.ARWmag.com
About the Cover: Daylighting through a building’s roofing system has taken on debate from an energy perspective, pitting thermal efficiency of a system against electric light energy reduction provided by the system. Photo courtesy of Sunoptics, An Acuity Brands Company.
Features State-of-the-Art Building Envelope System . . . . . . . . . . . . . . . . . . . .
13
Commercial Daylighting: Thermal Loss vs. Electrical Energy Savings . . . . . . . .
14 Architect Profile: Jason F. McLennan . . . . 18
18
Topping Off the Renovation of a Downtown Landmark . . . . . . . . . . . .
23
Mansard Roof Poses Multiple Challenges. . . . . . . . . . . . . . . . . . . . .
26 Durability, Longevity, Sustainability . . . . . 29
Columns
23
26
Editor's Note . . . . . . . . . . . . . . . . . . . . . . . . . . Best Practices: Waterproofing Sandwich Slabs . . . . . . . . . . .
4
33
Departments 8 Details: Roofing . . . . . . . . . . . . . . . . . . . . 10 Details: Waterproofing . . . . . . . . . . . . . . 12 Advertiser Index . . . . . . . . . . . . . . . . . . . 34 Roofing and Waterproofing Codes . . . . . . .
29 www.arwmag.com
2011, Vol. 3
3
{
check out these stories online @
www.ARWmag.com
A Publication Of BNP Media 2401 W. Big Beaver, Suite 700, Troy, MI 48084 Phone: 248.362.3700 Fax: 248.362.5103 Web site: www.ARWmag.com
Group Publisher Jill Bloom 313.570.7157
[email protected] Associate Publisher Liz Obloy 248.244.6423
[email protected] Editorial Editorial Director . . . . . . . . . . . . . John D’ Annunzio 248.936.8744
[email protected] Editor . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Chris King 248.244.6497
[email protected] Associate Editor . . . . . . . . . . . . . . . . . . . .Tom Watts 248.244.1738
[email protected] Senior Art Director . . . . . . . . . . . . . . . . Scott Hilling 248.244.1276
[email protected] Sales Sales Manager . . . . . . . . . . . . . . . . . Marcia L. Wright 415.793.2381
[email protected] Advertising Sales Coordinator . . . . . . Kristina Lorio 248.786.1579
[email protected] PROJECT PROFILE Roof System Keeps Your Favorite Breakfast Cereals From Getting Soggy HISTORIC RENOVATION Re-Creating a Classic in Detroit RETRACTABLE ROOFS Open Up to Sustainable Design THE BUILDING ENVELOPE Colorful Panels Envelop Humane Society PREVENTING TROUBLE IN PARADISE Honolulu Re-Roofing Project Solves Difficult Dilemma ‘ECO-TOWN’ BECOMES A REALITY Solar and Vegetative Roofs Help Town Go Green PEARL OF WISDOM The Pearl’s Unique Design Optimizes Passive Solar Collection
Production Production Manager . . . . . . . . . . . . . Jennifer Nagel 248.244.1282
[email protected] Circulation Audience Development Manager . . . Jill Buchowski 248.786.1593
[email protected] Marketing Reprints . . . . . . . . . . . . . . . . . . . . . . . . Kristina Lorio 248.786.1579
[email protected] Marketing Manager . . . . . . . . . . . . . . . Paul Dykstra 248.786.1677
[email protected] BNP Custom Media Group Business Development Manager . . . Christopher Wilson 248.244.8264
[email protected] Corporate Directors Publishing: John R. Schrei Corporate Strategy: Rita M. Foumia Marketing: Ariane Claire Production: Vincent M. Miconi Finance: Lisa L. Paulus Creative: Michael T. Powell Directories: Nikki Smith Human Resources: Marlene J. Witthoft Information Technology: Scott Krywko Clear Seas Research: Beth A. Surowiec
BNP Media Helps People Succeed in Business with Superior Information
4
2011, Vol. 3
{ Editor'sNote
E
conomic rough spots can be driving forces for change. In the 1970s, skyrocketing fuel costs forced car makers to increase mileage and builders to tighten homes and increase insulation to save energy. Cost was the major driver, but the overall effects on the environment were inextricably intertwined. Better car mileage and less electricity and natural gas consumption conserved natural resources and minimized pollution. In the ensuing decades, the push to lessen the impact of buildings on the environment continued to grow, culminating in the sustainability movement. The terms “sustainable” and “sustainability” are subject to varying definitions, and these definitions are continuing to evolve. The National Roofing Contractors Association (NRCA) defines a “sustainable building” this way: “A building that provides the specified building performance requirements while minimizing disturbance to and improving the functioning of local, regional and global ecosystems both during and after specified service life. A sustainable building optimizes efficiencies in resource management and operational performance and minimizes risks to human health and the environment.” Facets of sustainability in the construction process include recycling, a longer life cycle, lower energy costs, water conservation, reduced storm water runoff, reducing the urban heat island effect, reducing global warming, minimizing ozone depletion, reducing toxic emissions and minimizing the amount of energy used to transport materials. The U.S.Green Building Council (www.usgbc.org) has incorporated these factors and more into its Leadership in Energy and Environmental Design (LEED) program, an internationally-recognized green building certification system. All too often, sustainable products and systems were perceived to have a “green penalty” — that is, the more environmentally friendly options had some sort of shortcoming when compared to their counterparts, such as higher cost or diminished performance. In this tough economy, when building owners are squeezing every penny out of their budgets, there can’t be such a thing as a green penalty. Truly sustainable systems have to offer performance that is at least as good if not better than other options. They also have to offer the lowest life-cycle costs. Roofs used to be taken for granted — they were “out of sight, out of mind.” Those days are over. Building owners and investors are looking for long-term value from their construction assets, and it’s up to industry professionals to educate them about life-cycle costs. If you can offer the best roof system for the building, save the owner money, and help the environment, that’s a win-win for everyone — including our grandkids.
the ARW team
{
Redefining Sustainability
jill BLOOM Group Publisher
liz OBLOY Associate Publisher
chris KING Editor
john D’ANNUNZIO Editorial Director
tom WATTS Associate Editor
marcia WRIGHT Sales Manager
Thank you to our sponsors:
6
2011, Vol. 3
www.arwmag.com
{ Roofing&WaterproofingCodes
RoofingCode: Section 1507.2.9.3
Drip Edge Section 1507.2.9.3 Drip edge. Provide drip edge at eaves and gables of shingle roofs. Overlap to be a minimum of 2 inches (51 mm). Eave drip edges shall extend 0.25 inch (6.4 mm) below sheathing and extend back on the roof a minimum of 2 inches (51 mm). Drip edge shall be mechanically fastened a maximum of 12 inches (305 mm) o.c.
Code Interpretation This code implies that all steep-slope shingle roof systems require the application of a drip edge metal at the gables and the eaves. The intent of the drip edge metal is to secure the underlayment and to protect against moisture from infiltrating the system at these vulnerable points. The drip edge is typically fabricated from aluminum or galvanized metal in an L-shape configuration that is secured to the deck and down the face of the eave or gable edge. The code specifically states that the drip edge shall extend a minimum of ¼ inch (6.4 mm) down the eave and a minimum of 2 inches (51 mm) over the deck. The metal shall overlap a minimum of 2 inches (51 mm) and should be fastened with the appropriate fasteners every 12 inches (305 mm) on center.
WaterproofingCode: Section 1807.3.1
Floors 1807.3.1 Floors. Floors required to be waterproofed shall be of concrete and designed and constructed to withstand the hydrostatic pressures to which the floors will be subjected.
Code Interpretation This section implies that all floors that require waterproofing shall be constructed of concrete and shall be of structural integrity to withstand hydrostatic pressure. The waterproofing applied to the floors shall consist of one of the following membranes: • Rubberized asphalt • Butyl rubber • Fully adhered/HDPE • 6-mil-plus polyvinyl chloride (PVC) The membrane joints shall be lapped a minimum of six (6) inches and shall be applied in accordance with the membrane manufacturer’s latest printed requirements.
8
2011, Vol. 3
www.arwmag.com
GREEN ROOFING SYSTEMS & ACCESSORIES
HARNESS THE POWER OF THE SUN.
Firestone is committed to environmentally responsible products that take advantage of natural sunlight. The SkyScape™ Vegetative Roof System is a living roof covering that insulates and adds beauty to your building. Add decorative walkways with lightweight SkyPaver™ roof pavers. Brighten the inside with the innovative SunWave™ Daylighting System. More than a skylight, the unique design allows greater visible light transmittance and distribution, reducing the use of indoor electrical lighting by up to 70% during daylight hours. SkyScape Vegetative Roof System with SkyPaver Composite Pavers
SunWave Daylighting System
7KH)LUHVWRQHSRUWIROLRRIJUHHQURRÀQJSURGXFWVDQGDFFHVVRULHVSXWVWKHSRZHURIWKHVXQWRZRUNIRU\RX WRUHGXFHHQHUJ\ELOOVDQGPDNH\RXUURRIDPRUHLQYLWLQJDQGHQYLURQPHQWDOO\UHVSRQVLEOHHQYLURQPHQW ³5LD]+DVDQ$FFHVVRULHV3URGXFW0DQDJHU Visit us at the following shows: Greenbuild, booth #5742S 2FWREHU7RURQWR METALCON, booth #1015 2FWREHU'DOODV ÀUHVWRQHESFRFRPURRÀQJJUHHQURRÀQJ²
{ Details: Roofing Roof Curbs
T
he most important aspects of curb details are that they are designed at the proper height and there is sufficient termination of the f lashing material. The f lashing material shall always be fully adhered to the curb substrate, even in loose-laid and mechanically attached systems. The proper curb height should be a minimum of 8 inches above the completed roof surface to eliminate the potential of moisture entering the top of the curb area.
The flashing should be properly terminated — with a fastened termination bar or secured counter flashing — to eliminate the potential of flashing slump or slippage. This is particularly important on use of the heavier modified bitumen flashing sheets. Flashings should always be additional material applications — separate from the membrane application — and should fully extend from the top of the curb out onto the roof surface a minimum of 4 inches. Application should be in accordance with the membrane manufacturer’s requirements.
NOTES: FLEX FLASH F MAY BE USED AT ALL CONCRETE-TO-CONCRETE AND CONCRETE-TOCONCRETE BLOCK TRANSITIONS. FLEX-FLASH UN MUST BE USED AT ALL OTHER TRANSITIONS (i.e., CONC.-TO-GYP. BOARD, GYP. BOARD-TO-GYP. BOARD, ETC.).
Q Curb details courtesy of American Hydrotech, Inc.
10
2011, Vol. 3
www.arwmag.com
FICIENT
TM
EXPAND YOUR INSPIRATION Inspiration comes in many forms. Expanding inspiration? That’s where we come in. Eco-ficient™ insulated metal panels provide beauty, integration and efficiency. Not only do these panels come in a variety of profiles, colors and textures, but they also integrate well with other exterior finish systems and offer sustainability attributes significant for improving a building’s energy efficiency. Look to Eco-ficient™ insulated panels from MBCI for your inspiration. Visit us at www.mbci.com/ARWeco to learn more.
years of
Thank You to Our Customers for 35 Years of Support Houston, TX (Corporate) 877-713-6224 | Adel, GA 888-446-6224 | Atlanta, GA 877-512-6224 | Atwater, CA 800-829-9324 | Dallas, TX 800-653-6224 | Indianapolis, IN 800-735-6224 Jackson, MS 800-622-4136 | Lubbock, TX 800-758-6224 | Memphis, TN 800-206-6224 | Oklahoma City, OK 800-597-6224 | Omaha, NE 800-458-6224 | Phoenix, AZ 888-533-6224 Richmond, VA 800-729-6224 | Rome, NY 800-559-6224 | Salt Lake City, UT 800-874-2404 | San Antonio, TX 800-598-6224
www.mbci.com/ARWeco
{ Details: Waterproofing Treating Joints and Cracks
Q General concrete crack and joint treatment details courtesy of American Hydrotech, Inc.
T
here are two ways to waterproof joints and cracks in concrete for waterproofing applications. There is the traditional method of applying sealant in the joint or crack in accordance with proper sealant depth to width ratio of 2 to 1. For joints over 2 inches in width, an expansion joint material is required. Cracks over ¼ inch in width should be treated as control joints. Another method is to apply the waterproofing manufacturer’s materials over these locations. For joints less than 2 inches
12
2011, Vol. 3
in width and cracks less than ¼ inch in width, some waterproofing manufacturers require the application of their flashing or reinforced material, which is applied over the crack or joint. It is important that the concrete surface is properly cleaned off and free of all loose contaminants for adequate adhesion. The reinforcement material shall be applied in accordance with the manufacturer’s requirements for proper adhesion, which may require application in adhesives, self-adhered sheets or hot bitumen.
www.arwmag.com
{
CASE study Q The National Great Rivers Research and Education Center is a museum and a scientific facility that conducts research on the Mississippi, Missouri and Illinois rivers.
T
he National Great Rivers Research and Education Center serves as both a museum and a scientific research facility for individuals interested in how the ecosystem functions at the confluence of the Mississippi, Missouri and Illinois rivers. This state-of-the-art facility allows for exploration of the rivers, their impact on the surrounding communities, and the environment through which they flow. AAIC, Inc. of Collinsville, Ill., won the bid for the project’s design in 2006. “Before we got the project, we’d heard
WORLD-CLASS RESEARCH PROTECTED BY STATE-OF-THE-ART
CENTER
BUILDING ENVELOPE SYSTEM about it and knew it would be coming down the pipeline,” said site observer Erin Morris of AAIC. “It was a big deal.” It was an ambitious design from the start: a LEED-certified model of green construction, integrated renewable energy, and internal recycling systems. Science labs would pump water directly from the river into concrete channels in research stations, allowing scientists to conduct controlled experiments in an artificial environment. The external façade would blend into the surrounding natural environment, with locally quarried rock mimicking the natural limestone bluffs of the area. A sloping vegetated roof would include a patio section for visitors. AAIC contacted Foresight Services, Inc., a consulting firm, for assistance with the project’s details. Foresight Services, working closely with Carlisle Construction Materials, opted for a complete
www.arwmag.com
building envelope solution. Carlisle’s NVELOP® building envelope system provides a total system warranty for all the components, and tie-in details, that keep the building insulated and waterproofed. The uniqueness of the roof — from the shape to the drainage needs — required a nonstandard application. Once the concrete structural deck was complete, it was primed and a Carlisle modified base sheet was set in type III asphalt. The next layer consisted of Carlisle’s EPDM FleeceBACK® AFX-plus membrane, also set in asphalt, with overlaid pressure-sensitive field seams. Carlisle’s HV300 protection fabric was laid over the FleeceBACK membrane, followed by CCW’s MiraDrain 9800 drainage board to facilitate runoff. Insulation ranging from 4 inches to 3 feet thick sculpted the effect of rolling hills. Next, a 40-mil polypropylene root barrier with hot air-welded seams was installed over a drainage mat.
Carlisle growth media, applied at a minimum of 12 inches deep accommodates an assortment of prairie grasses meant to blend into grasses native to the region. The center practices what it preaches as one of the most technologically advanced green facilities in the entire world. “The building is pretty incredible,” said Morris. “There’s no connection to any sewage treatment. Our water reuse systems pipe and pump through the building to every type of fixture. We capture 100 percent of our storm water for landscaping, and washing our trucks and boats. All of our grey water finds a use – the only treated water you’ll find in the building is from the fountains and sinks. “It was my first LEED project,” said Morris, “and I’ve got to say — it really turned me green!” For more information, visit www.carlislesyntec.com.
2011, Vol. 3
13
COMMERCIAL DAYLIGHTING
THERMAL LOSS VS ELECTRICAL ENERGY SAVINGS by Grant Grable
D
aylighting is the use of natural light as a main illumination source from a myriad of glazing devices in both vertical and horizontal positions within a building. The use of daylighting stems from the beginning of time and since the beginning there have always been similar concerns regarding proper design, light output, glare and even heat. The quality of light and the value to the human eye and visual acuity have never been argued. The light in the shade of a tree has been measured at 600 – 800 foot candles (6,000 – 8,000 lux), yet the eye is never more relaxed than when it’s introduced to these lighting levels. It is the light that our eyes were made for. However, over time, the value of daylighting through a building’s roofing system (often referred to as “toplighting” in the United States) has taken on debate from an energy perspective, pitting thermal efficiency of a system against electric light energy reduction provided by the system. Today, code bodies are torn between thermal efficiency mandates and the use of commissioned lighting control with properly diffused, high visable light transmission skylights as to which properties provide the greatest performance measurements for total building energy efficiency. This article was written to discuss these differences and to expose the need for varying standards in systems based on the design and energy efficiency prescriptive used.
14
2011, Vol. 3
Thermal Performance and Energy Savings In June 2008 the U.S. Department of Energy’s Building Technologies Program published a research paper titled “Commercial Building Toplighting: Energy Saving Potential and Potential Paths Forward,” which thoroughly investigated the energy savings benefits from toplighting systems in a multitude of climate conditions and locations throughout the United States. For this report, the U.S. DOE contracted with TIAX LLC that enlisted information and assistance from a breadth of respected lighting, controls and daylighting professionals around the country. The report was very thorough, looking into the multitude of reasons that building owners and operators of different functioning facilities should consider the use of skylights in conjunction with lighting control with a focus on total building energy savings and what the key factors were for a subsequent system. What the DOE identified is really at the crux of the argument. If a skylighting system is being designed for use in a building absent of lighting control, then thermal performance value should be the highest consideration. After all, any thermal value (whether in thermal loss or thermal gain) that deviates negatively from that of the building envelope and roofing system will provide a negative energy impact to the building either in increased cooling loads or increased heating loads to the building. However, what would be the primary motivator for including less thermally efficient building products through a perfectly well insulated roofing system? One could argue that if thermal
Insulation Values of Building Envelope Components The term “U-Value” may seem foreign to many in the roofing industry, as the usual measure for the insulation value of a product is described as “R-Value.” However, with skylights, U-Value is used to quantify heat loss through plane elements of the building envelope. U-value is defined as the overall thermal transmittance of a particular construction element (typically a window, door or skylight), including the effect of surface resistance. It depends upon the thickness and thermal conductivity of its component layers and, in the case of air cavities, the emissivity of the surfaces. As a measure of U-Value, the lower the number, the better the insulation value. Basically, U-Value is the converse of R-Value. There is an easy calculation that allows you to see what the R-Value of a window or skylight may be: 1 ÷ U-Value = R-Value Once you utilize this formula with different skylight systems, you will quickly realize that even some of the well-promoted thermally efficient skylights have a fraction of the R-Value that the building’s roofing system has. A skylight will never equal the insulation value of a roof and still allow for light to pass through it, so its only value is in the quality of light that passes through the product. After all, it is called a “skylight” not “sky-insulation.”
www.arwmag.com
Q Photo courtesy of Sunoptics, An Acuity Brands Company.
www.arwmag.com
Figure 1. Relative Savings/Losses, 4% SFR $0.35 Energy Cost Savings ($/yr./ft2)
performance were the main motivator and only energy savings opportunity, an energy professional would argue that all toplighting products should be removed from the facility as they will never equal the insulated value of the roofing system. However, globally you are now seeing mandates for thermal performance of toplighting systems rising with little to no mandate for performance around the light output properties let alone the key missing ingredient mandatory for any true energy savings through daylighting and that is commissioned lighting controls. In many cases around the world, what is happening is that building codes are being established with higher effective skylight-tofloor ratios to compensate for the reduced visible light transmission occurring from decreased values in solar heat gain (SHGC in United States or G-Value in Europe) as well as decreased conductance (U-value). For instance, the European Directive on the Energy Performance of Buildings—2002/91/EC under the 2006 Building Regulations Approved Document L2A — “Conservation of Fuel and Power in New Buildings other than Dwellings” — states that rooflights are limited to 20 percent of floor area and the area-weighted average U-value of all the rooflights must not exceed 2.2 W/m²K, while the U-value in an array must not exceed 3.3 W/m²K, providing the average U-value overall does not exceed 2.2 W/m²K. At these high
$0.30 $0.25 $0.20 $0.15
Lighting Total $0.20
Total $0.31
Heating Cooling
$0.10 $0.05 $0.00 -$0.05 Burlington
Phoenix
Q Figure 1: Relative savings/losses from lighting and HVAC for a big box retail store in Phoenix. Source: U.S. Department of Energy, Building Technologies Program – “Commercial Building Toplighting: Energy Savings Potential and Potential Paths Forward.” Prepared by TIAX LLC. June 2008.
skylight-to-floor ratios (SFRs) between 10 percent to 20 percent have shown, the increase to thermal mandates has driven the increase to SFRs to be able to meet light output needs for the space. However, extensive studies have been performed on the value that lighting control mandates along with toplighting have done to minimize SFRs for a space, which decreases thermal impact and in many cases, even reduces air conditioning in the process. In section 4.6 of the U.S. DOE commercial toplighting report, it was found that the
primary factors influencing the economics of toplighting are climate and building type (primarily due to lighting power density, schedule, and light well needs); appropriate skylight technology selection is also crucial. The key performance attributes of a skylight for daylighting are good diffusing properties to aid in light distribution and avoid glare and high visible transmittance (VT). Of special note in this report is that the DOE stated, “Other desirable properties that are much less important for daylighting applications are low solar-heat-
2011, Vol. 3
15
Commercial Daylighting
Q Figure 2: Annual primary energy savings by end use (lighting, cooling, and heating) as a function of SFR in Burlington, Vt. Figure 3: Annual primary energy savings by end use (lighting, cooling, and heating) as a function of SFR in Phoenix. Figure 4: Annual primary energy savings technical potential, by climate zone and building type. The source for the figures shown here is the U.S. Department of Energy, Building Technologies Program – “Commercial Building Toplighting: Energy Savings Potential and Potential Paths Forward.” Prepared by TIAX LLC. June 2008.
Figure 2. Big Box Retail - Burlington 50.00
Primary Energy Savings (kBtu/yr/ft2)
40.00 30.00 Lighting 20.00
Total
10.00
Cooling Heating
0.00 -10.00
extreme climate conditions in the United States. The area with the lowest amount of solar load and highest heating degree figure in the United States was determined to be Burlington, Vt. The area with as the highest amount of solar load and highest cooling degree figure was determined to be Phoenix, Ariz. The report documented the results in total building energy savings from lighting, cooling and heating based on several building types and illustrated the findings in the report, re-created here as Figures 2 and 3. What you will find Figures 2 and 3 is that light energy savings truly does compensate for any thermal loss or gain even in extreme climates and, in fact, is the main driving factor to any energy savings available for the space.
-20.00 0%
1%
2%
3%
4%
5%
6%
7%
8%
9%
10%
Skylight to Floor Ratio Figure 3. Big Box Retail - Phoenix 60.00
Primary Energy Savings (kBtu/yr/ft2)
50.00 40.00 30.00
Lighting
20.00
Total
10.00
Cooling
0.00
Heating
-10.00 -20.00 -30.00 0%
1%
2%
3%
4%
5%
6%
7%
8%
9%
10%
Skylight to Floor Ratio
16
2011, Vol. 3
Figure 4
160 Primary Energy Savings Potential (Trillion Btu./yr.)
gain coefficient (SHGC in U.S., G-Value in Europe Standards) and conductance (U-Value). The DOE report illustrates the rationale for this prioritization in a graph, re-created here as Figure 1. The report also stated: “Reduced lighting energy use ranks as, by far, the greatest factor in the annual savings at economically optimum SFRs. The reduction in lighting energy use is directly related to VT, i.e. the higher the VT, the lower the total skylight area needed to achieve a given lighting energy savings. However total skylight area reduces cost and energy losses.” The report goes on to identify that, “To further minimize energy losses, in most climates, the SHGC and U-value of the skylight should be as low as possible. However, because heating and cooling energy losses are small relative to lighting energy savings, if reducing SHGC (G-value) or U-value results in any significant reduction in visible transmission (while maintaining high diffusion, 90%
