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Energy efficient homes: Windows and skylights
Cantrell, Randall
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FCS3276 Energy Ecient Homes: Windows and Skylights 1 Kathleen C. Ruppert, Wendell A. Porter, Randall A. Cantrell, and Hyun-Jeong Lee 2 1. This document is FCS3276, one of the Energy Ecient Homes series of the Department of Family, Youth, and Community Sciences, UF/IFAS Extension. Original publication date June 2008. Revised July 2015. This material was initially prepared with the support of the Department of Environmental Protection, Florida Energy Oce, which is now the Oce of Energy, Florida Department of Agriculture and Consumer Services. The revised versions were completed with the support of the Florida Energy Systems Consortium ( ). Any opinions, ndings, conclusions, or recommendations expressed herein are those of the author(s) and do not necessarily reect the views of the sponsoring organizations. Visit the EDIS website at 2. Kathleen C. Ruppert, Extension scientist, Program for Resource Ecient Communities; Wendell A. Porter, lecturer and P.E., Department of Agricultural and Biological Engineering; Randall A. Cantrell, assistant professor, Department of Family, Youth, and Community Sciences; and Hyun-Jeong Lee, former assistant professor, Department of Family, Youth and Community Sciences; UF/IFAS Extension, Gainesville, FL 32611. The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or aliations. For more information on obtaining other UF/IFAS Extension publications, contact your countys UF/IFAS Extension oce. U.S. Department of Agriculture, UF/IFAS Extension Service, University of Florida, IFAS, Florida A & M University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Nick T. Place, dean for UF/IFAS Extension. Quick Facts For New Window and Skylight Purchases: All ENERGY STARcertied windows bear the National Fenestration Rating Council (NFRC) label. Using the NFRC label to compare energy eciency is the best way to ensure that your window investment dollars get the best return in the long run. For windows receiv ing direct sunlight, look for ENERGY STARcertied windows for the southern climate zone, which will have a solar heat gain coecient (SHGC) of 0.25 or lower and a U-Factor of 0.40 or less for the whole window; SHGC of 0.28 or lower and a U-Factor of 0.60 or less for skylights. ENERGY STAR has a Most Ecient designation label recognizing products that deliver cutting-edge energy eciency along with the latest in technological innova tion. Windows are one of several appliances/devices that have units meeting these criteria. If interested, begin your search at For Existing Windows: Using solar screens, shutters, or awnings to shade existing windows can signicantly reduce your homes solar heat gain. Reducing air leakage can signicantly improve the e ciency of existing windows. Check caulking and weatherstripping around all openings to keep conditioned air in and un-conditioned air out of the home. Terms to Help You Get Started Air Leakage (AL) A rating that expresses the amount of air inltration through cracks in the whole window Fenestration Any opening in a building, including windows, skylights, and doors Glazing Glass in a window or door, or the act or process of tting with glass National Fenestration Rating Council (NFRC) A non-prot organization that administers the only uniform, independent rating and labeling system for the energy performance of windows, doors, skylights, and attachment products Solar Heat Gain Coecient (SHGC) Measures the amount of the suns heat admitted by the window, both directly and absorbed (and subsequently released into your home)


2 Energy Ecient Homes: Windows and Skylights ermal Envelope e buildings exterior shell that includes the walls, foundation, oors, ceiling, windows, doors, and roof; generally considered the boundary between the indoor cooled or heated area and the outdoors U-Factor Measure of the rate at which a window conducts non-solar heat (as from your heating system) R-Value Refers to a window assemblys resistance to heat ow; R-Value is the inverse of U-Factor (U=1/R), i.e., if the R-value of a window assembly is 2, the U-Factor is 0.5. Visible Transmittance (VT) Measures how much light comes through a product Whole Window Includes the glazing (glass), frame, sash, and any operable elements Overview Most people think of a windows function as simply to allow light and air into the home. But when you think about it, a window in a Florida home has a rather complicated job: it must allow the suns light to pass through, but not the suns heat; it must keep cool conditioned air inside, but not allow condensation to form on the surface of the glass; it must not allow hot, humid outside air to leak into the home in summer, nor allow warm conditioned air to leak out of the house in winter; and, as Hurricane Andrew and other signicant storms have demonstrated, windows are tradi tionally a key point of entry for storm-force winds, hail, rain, and wind-borne debris, so ideally a Florida window must also be impact-resistant. What are the primary concerns for Florida home envelopes with a lot of glass? ough glass is necessary for aesthetics, ventilation, light, and safety, it is a major cause of energy loss through your homes shell (oen referred to as thermal envelope). In the cold season, it is desirable to use solar heat to warm rooms in a home. However, because the warm season in Florida is much longer than the cold season, excessive solar heat gain is the biggest concern for our climate. Additionally, glass is easier to break than other parts of the home in the event of hurricanes and tornadoes. Energy-ecient windows, however, allow you to use more glass without additional energy penalties, and impact-resistant glass can help address the issue of breakage due to wind-borne debris. What are energy ecient or high performance windows? How do I know which windows keep out the most heat (solar heat gain)? e National Fenestration Rating Council (NFRC) provides energy performance labels (Figure 1) on windows, doors, skylights, and window lms voluntarily tested by an independent third party for how well the product will perform the functions of cooling your home in the summer, heating your home in the winter, keeping out wind, and resisting condensation. e label also lists the manufacturer, describes the product, provides a source for more information, and includes a rating for how much light comes through the product. Basically, the NFRC label allows you to comparison shop. Keep in mind that the label is voluntary, so not all products will have it. A = U-Factor (also referred to as U-Value ) measures the amount of heat that escapes through the product. e lower the rating, the better the window is at preventing heat loss (as imagined, this is more of a concern in northern cli mates). NFRC certied products require U-Factor ratings. In Florida, always look for the whole window U-Factor and select windows with a U-Factor at least as low as 0.40. Figure 1. Sample NFRC label Credits: Courtesy of


3 Energy Ecient Homes: Windows and Skylights B = Solar Heat Gain Coecient (SHGC) measures how well a product blocks heat caused by sunlight. e SHGC is the fraction of incident solar radiation admitted through a window, both directly transmitted, and absorbed and subsequently released inward. SHGC is expressed as a number between 0 and 1. A window with an SHGC rating of 0.70 will allow 70% of the solar heat to pass through it. e lower a windows SHGC, the less solar heat it transmits. A high solar gain window will let in solar heat and trap it, increasing cooling load in the summer. In Florida, SHGC is the most important consideration in selecting windows that are exposed to direct sunlight. C = Visible Transmittance (VT) measures how much light comes through a product. e visible transmittance is an optical property that indicates the amount of visible light transmitted. VT is expressed as a number between 0 and 1. e higher the VT, the more visible light transmitted. Dark-tinted coatings can signicantly reduce visible transmittance, but it is possible to get windows with high VT (about 0.70 for the glass or 0.50 for the whole window is best for maximum daylight and view) and low SHGC greater amounts of light with less solar heat gain. D = Air Leakage (AL) is indicated by an air leakage rating, expressed as the equivalent cubic feet of air passing through a square foot of window area. Heat loss and gain occur by inltration through cracks in the window assembly. e lower the AL, the less air will pass through cracks in the window assembly. E = Condensation Resistance (CR) measures the ability of a product to resist the formation of condensation on the interior surface of that product. ough this rating cannot predict condensation, it can provide a credible method of comparing the potential of various products for condensa tion formation. CR is expressed as a number between 0 and 100. e higher the CR rating, the better that product is at resisting condensation formation. CR is an optional rating, however, and manufacturers can choose not to include it. F = Emissivity (or Low-E) measures the eciency in which a surface emits thermal energy. In case of Low-E windows, long wavelengths of light (or heat) are ltered out by a thin coating on the glass while short wavelengths of light (or visible light) are allowed to pass through). Sometimes I see ratings for windows, while other times just the glass rating is displayed whats the dierence? A glass ratinglogically enoughwill indicate how the glass (glazing) itself performs. But a window is made of more than glass. e framing material, the number of panes of glass and air spaces between the panes, any coatings applied to the glassall of these are components of the window and aect how the entire structure performs. e NFRC window rating indicates the overall performance and thus is a better guide in choosing one particular type of window product over another. Note that the NFRC label, located next to the ENERGY STAR label, is found on all ENERGY STARcertied windows for the southern climate zone 3 What types of framing materials are available? Typical framing materials include wood, vinyl, aluminum, berglass, and composites. Manufacturers oen combine materials to use the best features of each. For example, a variation of the wood-framed window is to clador coverthe exterior face of the wood frame with either vinyl or aluminum, creating a weather-resistant surface. Clad frames have lower maintenance requirements, while retain ing the attractive wood nish on the interior. Although great for safety, sound protection, and aesthetics, frames with high insulation value are not as important for most Florida homes, but the frame material should be thermally non-conductive, and the whole window will ideally have an AL (Air Leakage) rating of 0.30 or less. Learn more about thermal breaks, spacers, frame types and more at http:// What type of window should I consider when building a new Florida home? Limit windows facing east and west as these would be subject to the greatest amount of sun exposure. In general, the most ecient window designs start at a minimum of three feet o the oor and end a maximum of six to seven feet from the oor. is design allows people to see out of the window while sitting or standing, without excess glass area. Avoid designs that call for sheets of glass installed directly into the walls, as plain glass is not good for energy eciency.


4 Energy Ecient Homes: Windows and Skylights Are window designs tested for durability? e American Architectural Manufacturers Association (AAMA), Window and Door Manufacturers Association (WDMA), and Keystone Certications, Inc., conduct voluntary design pressure tests for structural pressure, air inltration, water penetration, and operational force. For those living in hurricane-prone areas, purchasing new win dows with high performance ratings in these rigorous tests is a good idea. (Contact your local government building permit oce for assistance in choosing windows specic for your location.) Some Florida insurance companies may give a reduction in hurricane insurance premiums for windows that have been tested for impact resistance. Impact resis tance is particularly important because if a window breaks, providing an opening into the home, pressure can build up inside, increasing the chance of the roof blowing o and/or signicant water damage to the structure. How can I make my existing windows perform better? Windows should shut tightly when closed. Add shutters, awnings, and/or solar screens. Check caulking and weatherstripping on windows at least annually. Damaged weatherstripping and caulking should be completely removed and then replaced with new materials. e U.S. Department of Energy has a lot of information on recommended uses, etc., for dierent types of caulking and weather-stripping at and http:// Carefully read all product labels because some products will fail when subjected to certain or extreme conditions. How do I know if my windows need caulking and weatherstripping? When looking for dras around your windows, check the outside for any sign of caulking that has cracked or peeled. A simple dra-checker, made with a piece of tissue paper taped to a metal clothes hanger, can detect even small breezes inside your home. While inside the home on a windy day, hold the dra-checker still at several places around the window and frame to detect leaks. If the tissue paper moves, then you know there is a leak allowing conditioned air to escape and unconditioned air to enter. Try this dra-checker technique around doors as well. You and a friend or relative can also try the Can you see me now ? trick. 4 At night, turn o all of the lights and shine a ashlight from the outside around exterior window and door frames. If your friend or relative sees light coming inside through cracks or crevices in or around those frames youre examining, then you know there are areas for air to get in or get out. What should I know about shading devices? Outside treatments keep out more heat than indoor treatments and save you the most money. Remember, you are trying to stop heat from entering your homes thermal envelope. Heat that stays outside the home in the rst place doesnt have to be cooled down! Shutters, awnings, and solar screens are good choices for shading. A solar screen is similar to the familiar insect screen mounted within a window casement. Quality solar screens block the suns heat and glare while oering good visibility from within the room. Solar lms range from tints, which reduce light entering the home, to transparent lms, which reduce solar radiation while still allowing light to pass through. Currently, window lms are not eligible for the ENERGY STAR label but the NFRC independently certies product energy performance. e NFRC makes these ratings available on its website at More information on window lms can be found through the International Window Film Association at What about skylights? Skylights are generally not recommended in Florida because they are vulnerable during hurricanes and require aggressive shading devices to protect against intense sun. However, natural daylighting is the most desirable and ecient form of light. erefore, if you do want an ENERGY STARcertied skylight, make sure that it has the NFRC and ENERGY STAR label for the southern climate zone with an SHGC less than or equal to .28 and a U-Factor of 0.60 or less. Skylights do not have to be very large to get the desired eect, so select the smallest available size. e sha should be insulated to conform to the Florida Building Code Proper sealing and caulking are imperative to maintaining the roofs integrity. If skylights are present in an existing home, consider adding window tint or a remote-control shade to reduce heat gain if you feel heat entering the room through the skylight.


5 Energy Ecient Homes: Windows and Skylights What about impact-resistant glass? Impact-resistant glass is available as laminated glass and is also considered a type of safety glazing. Laminated glass consists of two or more panes of clear glass bonded together with clear plastic-like lm (usually polyvinyl butyral) sandwiched between the two. Ranging in thickness from .015 to .090, this inner lling can be ordered in various colored tints to help reduce ultraviolet damage in the home, and it tends to hold the glass together upon impact. If cracked or broken, the glass fragments tend to adhere to the plastic interlayer thus preventing water, wind, or windborne debris from entering the structure. Note that the frames for laminated glass are generally heavier than for regular residential windows, because, though an impact may not break through the layered glass, its frame must be able to bear the transferred force of a strong impact. When undergoing testing, the window is tested as a unit that includes the glass, frame, attachment hardware, and the installation method. Products designed to protect your homes openings must be both tested and approved for design, pressure, and impact. ere are testing standards set forth by the American Society for Testing & Materials (ASTM) and others that must be met before a window is certied as being impact-resistant. What about tempered glass? Tempered glass windows and impact-resistant windows are not the same. Tempered glass is one type of heat-treated glass in which the glass is rst heated, and then the surface is rapidly cooled. is treatment results in the center of the glass remaining relatively hot compared to the surface. As the center thickness cools, it compresses the surfaces and edges. When tempered glass does break, it fractures into small, relatively harmless fragments. is phenomenon, oen referred to as dicing, greatly reduces the likelihood of injury to people or pets. e Florida Building Code requires safety glazing mate rial, such as tempered glass, where there is a reasonable likelihood of exposure to human impact, such as indoor assembly areas, bathtub or hot tub enclosures, railings of glass adjacent to stairways, etc. Safety glazing materials are designed to reduce or minimize the likelihood of cutting and piercing injuries when broken by human contact. Keep in mind that all glass other than impact-resistant window glass needs to be protected during severe wind events, even if tempered, reinforced, or insulated. Special Note on Window Replacement in Pre-1978 Homes If replacing windows in pre-1978 housing, be aware that the paint in window frames and nearby areas may contain lead. If this is true for your home, follow all safety precautions to prevent lead poisoning through dust and lead paint chips. In eect since April 2010, contractors performing renova tion, repair, and painting projects that disturb lead-based paint in homes built before 1978 must be certied and follow specic work practices to prevent lead contamina tion. Learn more about this at renovation-repair-and-painting-program The Financial Side of Windows Simulation programs like RESFEN ( soware ), a PC program for calculating the heating and cooling energy use of windows in residential buildings, let you compare window performance options by calculating utility rates for your climate, house design options, and window design options. An online site developed by the non-prot Ecient Windows Collaborative ( http://www. ) oers a less detailed way to compare new and replacement window choices. When looking for upfront nancial incentives related to renewable energy and energy eciency, visit the Database of State Incentives for Renewable & Eciency (DSIRE) website at and enter your zip code. You may also want to check with your local utility provider for any window-related incentive programs. Notes 3. To be eligible for ENERGY STAR, products must be rated, certied, and labeled for both U-Factor and Solar Heat Gain Coecient (SHGC), in accordance with the procedures of the National Fenestration Rating Council (NFRC) for the specic climate zone (see http://www.en for more information). Currently ENERGY STARcerti ed windows for the Southern Climate Zone, which is all of Florida, must have a U-Factor of 0.40 or less and a SHGC of 0.25 or less. 4. Term coined by T. Prescott, University of Florida.


6 Energy Ecient Homes: Windows and Skylights References and Resources American Council for an Energy-Ecient Economy. Windows and Doors. Retrieved May 6, 2015, from http:// American Architectural Manufacturers Association. (n.d.). Welcome Homeowners. Retrieved May 6, 2015, from http:// American Society for Testing and Materials. Retrieved May 6, 2015, from Ecient Windows Collaborative. (n.d.). Window Selection Tool. Retrieved May 6, 2015, from http://www.ecientwin . (2015, January). Selecting Energy Ecient New Windows in Florida. Retrieved May 6, 2015, from http:// Federal Alliance for Safe Homes. (n.d.). Retrieved May 6, 2015, from International Code Council. (2014). Florida Building Code 5th Edition: Residential Retrieved from http://codes.iccsafe. org/app/book/toc/2014/Florida/Residential%20Code/ index.html International Window Film Association. (n.d.). Retrieved May 7, 2015, from Lawrence Berkely National Laboratory, Building Technolo gies Department. (n.d.). Retrieved May 7, 2015, from http:// Miller, C., Sullivan, J., & S. Ahrentzen. (2012). Energy Ecient Building Construction in Florida ISBN: 97809852487-0-3. Gainesville: University of Florida. National Fenestration Rating Council. (n.d.). Retrieved May 6, 2015, from U.S. Department of Energy. (n.d.). Database of State Incen tives for Renewables & Eciency (DSIRE) Retrieved May 6, 2015, from U.S. Department of Energy. (2012, May 7-a). Caulking. Retrieved May 6, 2015, from articles/caulking U.S. Department of Energy. (2012, May 7-b). Weatherstrip ping. Retrieved May 6, 2015, from energysaver/articles/weatherstripping U.S. Environmental Protection Agency. (n.d.). Renovation, Repair and Painting Program. Retrieved May 6, 2015, from renovation-repair-and-painting-program U. S. Environmental Protection Agency and U.S. Depart ment of Energy. (n.d.). ENERGY STAR. Retrieved May 6, 2015, from Wachtel, Larry. (n.d.). Clearing Up Low-E Confusion. Retrieved May 13, 2015, from about-windows/240-clearing-up-low-e-confusion