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What’s New Selecting Windows in Building for Energy Efficiency Energy New window technologies have increased energy Efficiency benefits and comfort, and have provided more practical options for consumers. This selection guide will help homeowners, architects, and builders take advantage of the expanding window market. The guide contains three sections: an explanation of energy-related window characteristics, a discussion of window energy U.S. Department of Energy performance ratings, and a convenient checklist for window selection. electing the right window for a unintentional (infiltration). (See the S specific home invariably requires tradeoffs between dif- ferent energy performance features, Window Energy Glossary for expla- nations of these terms.) and with other non-energy issues. An Insulating Value INSIDE understanding of some basic energy The non-solar heat flow through a concepts is therefore essential to window is a result of the temperature Solar Control 6 choosing appropriate windows and difference between the indoors and skylights. As illustrated on the fol- outdoors. Windows lose heat to the Window Energy lowing page, three major types of outside during the heating season and Rating and Labeling 10 energy flow occur through windows: gain heat from the outside during the (1) non-solar heat losses and gains in cooling season, adding to the energy Window Checklist 13 the form of conduction, convection, needs in a home. The effects of non- and radiation; (2) solar heat gains in solar heat flow are generally greater Window Energy the form of radiation; and (3) airflow, on heating needs than on cooling Glossary 15 both intentional (ventilation) and needs because indoor-outdoor tem- perature differences are greater dur- ing the heating season than during the cooling season in most regions of the United States. For any window product, the greater the temperature difference from inside to out, the greater the rate of heat flow. A U-factor is a measure of the rate of non-solar heat flow through a win- dow or skylight. (An R-value is a measure of the resistance of a win- dow or skylight to heat flow and is the reciprocal of a U-factor.) Lower U-factors (or higher R values), thus indicate reduced heat flow. U-factors allow consumers to compare the insulating properties of different win- dows and skylights. The insulating value of a single- pane window is due mainly to the The three major types of energy flow that occur through windows: (1) non- solar heat losses and gains in the form of conduction, con - vection, and radiation; (2) solar heat gains in the form of radiation; and (3) airflow, both intentional (ventilation) and unintentional (infiltration). Solar Radiation Convection and Conduction Thermal Radiation Infiltration thin films of still air on the interior and nologies aimed at decreasing U-factors. moving air on the exterior glazing sur- These technologies include low-emittance faces. The glazing itself doesn’t offer (low-E) coatings and gas fills. much resistance to heat flow. Additional A low-E coating is a microscopically panes markedly reduce the U-factor by thin, virtually invisible, metal or metallic creating still air spaces, which increase oxide coating deposited on a glazing sur- insulating value. face. The coating may be applied to one In addition to conventional double-pane or more of the glazing surfaces facing an windows, many manufacturers offer win- air space in a multiple-pane window, or to dows that incorporate relatively new tech- a thin plastic film inserted between panes. The coating limits radiative heat flow between panes by reflecting heat back into the home during cold weather and back to the outdoors during warm weath- er. This effect increases the insulating value of the window. Most window man- ufacturers now offer windows and sky- lights with low-E coatings. The spaces between windowpanes can be filled with gases that insulate better than air. Argon, krypton, sulfur hexafluo- ride, and carbon dioxide are among the High-performance windows make energy- efficient homes possible with greater freedom of design than in the past. gases used for this purpose. Gas fills add nounced influence on the U-factors of only a few dollars to the prices of most windows and skylights. As a result, frame windows and skylights. They are most and spacer options have also multiplied as effective when used in conjunction with manufacturers offer improved designs. low-E coatings. For these reasons, some Window frames can be made of alu- manufacturers have made gas fills stan- minum, steel, wood, vinyl, fiberglass, or dard in their low-E windows and sky- composites of these materials. Wood, lights. fiberglass, and vinyl frames are better The insulating value of an entire win- insulators than metal. Some aluminum dow can be very different from that of the frames are designed with internal thermal glazing alone. The whole-window U-fac- breaks, non-metal components that reduce tor includes the effects of the glazing, the heat flow through the frame. These ther- frame, and, if present, the insulating glass mally broken aluminum frames can resist spacer. (The spacer is the component in a heat flow considerably better than alu- window that separates glazing panes. It minum frames without thermal breaks. often reduces the insulating value at the Composite frames may use two or more glazing edges.) materials (e.g. aluminum-clad wood, Since a single-pane window with a vinyl-clad wood) to optimize their design metal frame has about the same overall U- and performance, and typically have insu- factor as a single glass pane alone, frame lating values intermediate between those and glazing edge effects were not of great of the materials comprising them. Frame concern before multiple-pane, low-E, and geometry, as well as material type, also gas-filled windows and skylights were strongly influences thermal performance widely used. With the recent expansion of properties. thermally improved glazing options Spacers can be made of aluminum, offered by manufacturers, frame and spac- steel, fiberglass, foam, or combinations of er properties now can have a more pro- these materials. Spacer thermal perfor- Aluminum Frame without thermal break (with conventional spacer) Alum. Frame with thermal break (with conventional spacer) Wood or Vinyl Frame (with insulated spacer) GLAZING TYPE U-FACTOR (Btu/hr-ft2-˚F) Single glass ----- 1.07 1.30 Double glass, 1/2-inch air space 0.48 0.62 0.81 Double glass, e = 0.20*, 1/2-inch air space 0.39 0.52 0.70 Double glass, e = 0.10*, Representative Window 1/2-inch air space 0.37 0.49 0.67 U-Factors *e is the emittance of the Double glass, e = 0.10*, low-E coated surface. 1/2-inch argon space 0.34 0.46 0.64 Values are for 3-foot-by-5- foot windows. U-factors vary Triple glass, e = 0.10 on two somewhat with window size. panes*, 1/2-inch argon spaces 0.23 0.36 0.53 Source: ASHRAE Handbook— Quadruple glass, Fundamentals, American Society of e = 0.10 on two panes*, Heating, Refrigerating, and Air- Conditioning Engineers, Inc., Atlanta, 1/4-inch krypton spaces 0.22 ----- ----- GA, 1993. Outdoor air temperature and indoor air rela- tive humidity combinations at which conden- sation will occur on the center of the glass for single, double, and triple glazings, some with low-E coatings and gas fills. On or above each curve, the conditions are right for condensation. Below each curve, condensa - tion will not occur on that glazing type as long as the glazing is exposed to room air circula - tion. Values are based on winter conditions: 70˚F indoor air temperature, 15 mph outdoor air velocity, and no incident solar radiation. Source: WINDOW 4.1 (a computer program for calculating the thermal and optical properties of windows), Lawrence Berkeley National Laboratory, Berkeley, CA, 1994. mance is as much a function of geometry Preventing Condensation as of composition. For example, some Air can hold varying amounts of water well-designed metal spacers insulate vapor or moisture. The warmer the air is, almost as well as foam. the more moisture it can hold. The The table on page 3 shows representa- amount of moisture in the air, expressed tive U-factors for window glazing, frame, as a percentage of the maximum amount and spacer combinations under winter the air could hold at a given temperature, design conditions. Due to their orientation is called its relative humidity. For health and their greater projected surface areas, and comfort, indoor air should contain domed and other shaped tilted and hori- some moisture. The relative humidity zontal skylights have significantly higher should generally be between 30% and U-factors than do vertical windows of 40% at normal room temperature. similar materials and opening sizes. The relative humidity of air can be increased by adding more moisture or by reducing the temperature. When the rela- tive humidity reaches 100%, the air can hold no more moisture, and water begins to condense from it. The temperature at which this condensation occurs is called the dew point temperature of the air. When moist air comes in contact with a cold surface in a home, it may be cooled to its dew point temperature, resulting in condensation on the surface. Single-glazed windows characteristical - ly suffer from water condensation prob - lems and the formation of frost on the inside surface of the glass in winter. Windows don’t cause condensation, but Recommendations for Selecting historically they have been the first and Window U-Factors most obvious place it occurs. This is When shopping for windows and sky- because windows generally have lower lights, pay close attention to whether the thermal resistances than insulated walls, U-factor listed by the manufacturer ceilings, and floors. As a result, their applies to the glazing only or to the entire inside temperatures are usually lower than unit. If it is for the glazing only, the over- those of other surfaces in a home during all U-factor may be considerably higher cold weather. If the air in a home is humid because of the frame and spacer effects. enough, water will condense from it when These effects increase with decreasing it is cooled at a window surface. total window area. Compare different Condensation is most often thought of as window types or makes by their total U- a cold climate winter problem. However, factors, which are best obtained from in hot, humid weather, moisture can con- NFRC labels. (See Window Energy dense on the outside surface of a poorly Rating and Labeling, page 10.) New win- insulated window in an air-conditioned dow energy ratings and the RESFEN building. computer program can be used to estimate Left unchecked, condensation can dam- the relative energy usage associated with a age window frames, sills, and interior particular window type and U-factor. shades. Water can deteriorate the sur- Avoid aluminum-frame windows with- rounding paint, wallpaper, plasterboard, out thermal breaks if possible. Even in and furnishings. In severe cases, it can milder climates, these windows tend to seep into adjoining walls, causing damage have low inside surface temperatures dur- to the insulation and framing. ing the heating season, giving rise to con- The indoor air coming in contact with densation problems. Aluminum-frame energy-efficient windows is less likely to windows with properly designed thermal be cooled to its dew point temperature breaks can be used in moderate climates. because the inside surface temperatures Wood, vinyl, and fiberglass are the best remain higher during cold weather than do frame materials for maximum insulating those of windows with single glazing, tra- value. ditional metal spacers, and metal frames. Single-pane windows are impractical in The figure on page 4 illustrates condi- heating-dominated climates. In these tions under which condensation will form regions, multiple-pane, low-E, and gas- on the center of the glass of five glazing filled window configurations are advis- types with widely varied U-factors. The able. In most climates, glazings with low- graph shows clearly that the risk of con- E coatings and gas fills will be a choice densation at the center of the glass is that provides significant energy savings in reduced as the insulating value of the a cost-effective product. Low-E and gas glass increases. Even at an outdoor air fills have now become a common option temperature of -30˚F, the indoor air rela- for many manufacturers, which reduces tive humidity must be nearly 70% before their added cost. The resultant total win- condensation will form on the triple glaz- dow U-factor should be 0.5 or lower and ing with two low-E coatings and a gas fill. preferably below 0.4 for maximum energy On the other hand, at an outdoor tempera- savings. Consumers should select win- ture of 10˚F, condensation will form on dows with long warranty periods, which the single glazing at an indoor relative indicate sound window design and con- humidity of only 18%. struction, and a reduced probability of Condensation is even more likely to insulating glass seal failure or gas leak- occur at window spacers and frames, age, which would reduce performance. which are usually less insulating than the Remember that lower window and sky- corresponding glazings. With so many light U-factors mean less energy con- insulating glazing types available, efforts sumption, lower utility bills, and greater to prevent condensation have shifted comfort in the living space. toward the development of better insulat- ing spacers and frames. Daily total solar heat gain through 1/8-inch clear sin - gle glass for various win - dow orientations on very clear days at 40˚N lati - tude (for example, Columbus, Ohio, and Boulder, Colorado). Source: ASHRAE Handbook— Fundamentals, American Society of Heating, Refrigerating, and Air- Conditioning Engineers, Inc., Atlanta, GA, 1993. Solar Control 40˚N latitude. South-facing windows Solar transmission through windows and allow the greatest and potentially most skylights can provide free heating during beneficial solar heat gain during the heat- the heating season, but it can cause a ing season, while admitting relatively lit- home to overheat during the cooling sea- tle of the solar heat that contributes to son. Depending upon orientation, shading cooling requirements during the cooling and climate, solar-induced cooling costs season. The reverse is true for skylights can be greater than heating benefits in and east- and west-facing windows. North many regions of the United States. In fact, exposures transmit only minimal solar solar transmission through windows and heat at any time. The ultimate importance skylights may account for 30% or more of of these climatic and orientation effects the cooling requirements in a residence in will depend on the type of glazing under some climates. consideration. Because the sun’s position in the sky The Solar Heat Gain Coefficient changes throughout the day and from one (SHGC) is a measure of the rate of solar season to another, window orientation has heat flowing through a window or sky- a strong bearing on solar heat gain. The light. (AShading Coefficient (SC) is the figure above shows the solar heat gain previous standard indicator of a window’s through 1/8-inch clear single glass for var- shading ability and for simple glazings is ious window orientations on very clear approximately equal to the solar heat gain days in the heating and cooling seasons at coefficient multiplied by 1.15.) Solar heat gain coefficients allow consumers to com- pare the solar heat gain properties of dif- ferent windows and skylights. The solar heat gain coefficient accounts for both the transmissive glazing element, as well as the opaque frame and sash. Additional glazing layers provide more barriers to solar radiation, thus reducing the solar heat gain coefficient of a win- dow. Tinted glazings, such as bronze and green, provide lower solar heat gain coef- The NFRC label provides useful informa - tion—such as the window’s U-Factor, Solar Heat Gain Coefficient, Visible Light Transmittance, and Air Leakage rating—to help you choose wisely. ficients than does clear glass. Low-E coat- or space. Mirror-like reflective glazings ings can be engineered to reduce window are commonly used in office buildings, solar heat gain coefficients by rejecting but only occasionally chosen for resi- more of the incident solar radiation. dences. While they may have very low Spectrally selective glazings, including solar heat gain coefficients, they block so some low-E coated glazings with low much of the light and view that they are solar heat gain coefficients and new light not normally desirable in homes. blue and light blue-green tinted glazings, The table below shows representative block out much of the sun’s heat while solar heat gain coefficients and visible maintaining higher visible transmittances transmittances for glazings with typical and more neutral colors than more heavily wood or vinyl frames and aluminum tinted bronze and grey glazings. High- spacers. Aluminum-frame windows of transmittance, low-E coatings, used in comparable size and glazing type general- conjunction with a tinted outer glass layer, ly have slightly higher solar heat gain also reduce solar heat gain by preventing coefficients because of their thinner the absorbed heat from reaching the interi- frames and greater glazing areas. Solar Heat Gain Coefficient Visible Transmittance glass total glass total GLAZING TYPE only window only window Single glass, clear 0.90 0.66 0.86 0.66 Single glass, bronze tint 0.68 0.50 0.73 0.56 Single glass, green tint 0.82 0.60 0.71 0.55 Single glass, clear, solar control 0.25 0.18 0.40 0.33 retrofit film Double glass, clear, 1/2-inch air space 0.81 0.59 0.76 0.59 Double glass,bronze tint 0.62 0.45 0.62 0.49 Representative Window outer pane, 1/2-inch air space Solar Heat Gain Coefficients and Visible Double glass, green tint 0.75 0.54 0.60 0.47 Transmittances outer pane, 1/2-inch air space Values are for a 3-foot-by- 5-foot horizontal slider Double glass, clear, 0.76 0.55 0.65 0.51 window with wood or vinyl e = 0.20*, 1/2-inch air space frames and aluminum spacers. Solar heat gain Double glass, 0.45 0.33 0.35 0.29 coefficients vary somewhat “Southern” low-E, e=0.08*, with window size. on tint, 1/2 inch air space *e is the emittance of the low- E coated surface; emittance Double glass, spectrally selective, 0.72 0.52 0.40 0.33 will vary slightly with specific e = 0.04*, 1/2-inch argon space products. Triple glass, clear, 0.68 0.50 0.49 0.39 Source: WINDOW 4.1 (a computer e = 0.08 on two panes*, program for calculating the ther - mal and optical properties of win - 3/8 to 1/2-inch air or argon spaces dows), Lawrence Berkeley National Laboratory, Berkeley, CA, 1994. Ultraviolet Protection tion levels at that time on south-facing Ultraviolet radiation is the main compo- windows, especially those with adequate nent of sunlight that can fade and damage roof overhangs. drapes, carpets, furniture, and paintings Skylights and east- and west-oriented when transmitted through windows and windows may warrant lower solar heat skylights. Efforts to produce window gain coefficients since they transmit the glazings that transmit less ultraviolet most solar heat during cooling periods. In energy have met with some success. In most climates, there is not much point in general, windows and skylights with plas- spending more money to obtain lower tic glazing layers or low-E coatings solar heat gain coefficients for north-fac- reduce ultraviolet transmission. Even ing windows. without any ultraviolet radiation, sunlight In hot, sunny climates, select windows can still cause fading of fabrics and other with spectrally selective glazings to pro- furnishings. vide low solar heat gain coefficients with- out loss of light. Darker tinted glazings Recommendations for also provide lower solar heat gain coeffi- Solar Control cients, but they will yield somewhat decreased outdoor visibility, particularly Consumers should consider two aspects at night. Where glare is a concern, this of window selection to control solar effect may be desired, but under other gain—the selection of the window itself conditions it may not. In climates where and the choice of interior or exterior cooling loads are large, look for windows shading devices. Traditional windows with a SHGC of 0.4 or less. To maintain with clear glass required the use of shad- good light transmittance and visibility, ing devices to obtain adequate perfor- select windows whose glazings have visi- mance, especially when the orientation ble transmittance of 0.6 or higher. admitted substantial sunlight in summer. In some hot climates, where winters However, modern high-performance win- are mild, it might seem reasonable to dows can do such a good job of control- select a single-glazed window with a low ling sunlight that the importance of these Solar Heat Gain Coefficient rather than a shading systems is reduced. more typical double glazing. However, Window Solar Heat Gain Coefficients single glazings have a more limited range should ideally be selected according to of solar control (even if laminated glass orientation, but it may not always be and glue-on plastic films are considered), practical to do so. If south exposures are so a double-glazed window with appro- to admit beneficial solar heat during the priate glazing choice as noted above, may heating season, their Solar Heat Gain be the best overall solution, even in hot Coefficients should be high. These high climates. coefficients will not usually result in Exterior or interior shading devices— overheating problems during the cooling such as awnings, louvered screens, sun- season because of the lower solar radia- screens, venetian blinds, roller shades, and drapes—are essential for shading clear glass, and can complement and enhance the performance of windows with low Solar Heat Gain Coefficients. One advantage of many shading devices is that they can be adjusted to vary solar heat transmission with the time of day and season. But windows with “built-in” lower Solar Heat Gain Coefficients pro- Commercially available windows must meet numerous standards and build - ing regulations addressing condensa - tion resistance, sound control, main - tenance requirements, and overall durability of the unit. vide better visibility and require less man- and skylight frames, sash, and glazings. agement and maintenance in today’s busy This leakage can account for up to 10% households. of the energy usage in a home. The air- Exterior shading devices are more tightness of a window depends on both effective than interior devices in reducing the characteristics of the window—such solar heat gain because they block radia- as sash type and overall quality of win- tion before it passes through a window. dow construction—and the quality of the Light-colored shades are preferable to installation. Operable windows with com- dark ones because they reflect more, and pressing seals are generally more airtight absorb less, radiation. Horizontally orient- than purely sliding seals, because of the ed adjustable shading devices are appro- way the sash element seals against the priate for south-facing windows, while framing. vertically oriented adjustable devices are An air leakage rating is a standardized more effective for shading windows on measure of the rate of infiltration through east and west orientations. a window or skylight under specific envi- ronmental conditions. Air leakage ratings allow consumers to compare the airtight- ness of different windows and skylights Sash Type Effective as manufactured products; they do not Open Area* account for leakage between the installed product and the wall or roof. Lower air Casement 90% leakage ratings indicate greater airtight- Awning 75% ness. Hopper 45% Horizontal sliding 45% Airflow Recommendations Single-hung 45% In milder climates, or in spring and fall in Double-hung 45% more severe climates, operable windows can provide ventilation, improve comfort, Representative Window Ventilation Areas and reduce the need for air conditioning. *Effects of window screens are not Operable windows are often specified to included. meet building code requirements for emergency egress. Although operable Source: R.K. Vieira and K.G. Sheinkopf, Energy- windows are sometimes useful in house- Efficient Florida Home Building, FSEC-GP-33- hold areas with high moisture production, 88, Florida Solar Energy Center, Cape such as bathrooms, kitchens, and laundry Canaveral, FL, 1988. rooms, exhaust fans provide more reliable control throughout the year. Select windows with air leakage rat- Ventilation and Airtightness ings that meet or exceed standard industry Airflow through and around windows requirements of 0.37 cfm/ft 2 to minimize occurs by design as ventilation and inad- discomfort from uncontrolled infiltration. vertently as infiltration. The use of win- Select windows with even lower values dows for natural ventilation is as old as for particularly windy sites or harsh cli- architecture itself. Opening windows, par- mates. Check the seals between window ticularly on opposite sides of a living components for airtightness. To minimize space, can cool a home for free. The sash infiltration around installed windows, fol- type of a window influences the ventila- low the manufacturer’s installation proce- tion airflow rate through the window rela- dures carefully and seal and caulk joints tive to its size. Some common sash types and cracks. and their effective open areas for ventila- tion purposes are shown in the table above. Casement windows are especially effective for ventilation because they tend to direct the greatest airflow into the liv- ing space when fully open. Infiltration is the uncontrolled leakage of air into a building from the exterior through joints and cracks around window Window Energy Rating and Labeling M any windows, skylights, and glazed doors now bear energy rat- ings or labels, similar to those Q How will designers and homeown - ers use these energy labels? being placed on household appliances, to assist consumers in selecting energy-effi- cient products. The labels have been A Energy labels will show a variety of product performance attributes, enabling designers to compare and developed by a non-profit group, the select products directly, based on National Fenestration Rating Council each project’s specific energy per- (NFRC). The following interview with formance needs. Until now, design- NFRC staff, provides homeowners, archi- ers have had to spend too much time tects, and builders with some important trying to understand a mixed bag of information on these new window energy rating techniques, test methods, and ratings. performance claims. A nationwide system for rating whole-product Q Why are energy ratings or labels important for windows and sky - lights? energy performance will not only give designers the energy informa- tion they seek, but will also permit direct product comparisons. A Fenestration—windows, skylights, and glazed doors—can account for over 25% of the heating and cooling Homeowners have faced a simi- lar dilemma. When selecting fenes- tration products for a remodeling energy bills in a typical home. project or new construction, home- Designers, builders, and homeown- owners have had no way to compare ers have never had a tool for deter- the energy performances of two mining or comparing the energy per- products directly. This difficulty has formances of fenestration products to been compounded by the different assist them in their purchase deci- energy rating techniques employed sions. Many manufacturers offer a by the various industry segments. variety of energy-efficient products, Window energy labels will enable but have not been able to demon- consumers to compare products strate their superiority through com- directly, regardless of glazing and parable performance ratings. frame type. Q How will the energy ratings be determined? A The energy ratings are determined using advanced computer tools developed in the United States and Canada, combined with standard- ized product performance testing. The WINDOW 4.1 program, devel- oped at Lawrence Berkeley National In evaluating the cost of new or replacement window units, the aesthet - ic, functional, and energy performance characteristics come together. To make a better decision, it is useful to think of the life-cycle cost of the unit. SM National Fenestration Rating Council 1300 Spring Street, Suite 120 NFRC Label Silver Spring, MD 20910 The Telephone: (301) 589-6372 Fax: (301) 589-0854 e-mail: NFRCUSA@aol.com When shopping for windows, look for the Web: http://www.nfrc.org National Fenestration Rating Council (NFRC) label as your guide to buying energy-efficient windows. The NFRC is a non-profit public/pri- vate collaboration that provides contractors and homeowners with standardized, unbiased meth- ods of comparing various brands and types of windows. Below is an example of an NFRC label. All the parts of the label are described, but the U-factor and Solar Heat Gain Coefficient, which rate the efficiency of the entire window (glass and frame), are the most important in helping choose the best window for your purposes. All labels have U-factors; Solar Heat Gain Coefficients and Visible Light Transmittance are now being added. Air leakage rat- ings and annual heating and cooling factors will be added in the near future. The NFRC insignia is This box contains the name of the your assurance that Independent Certification and this window has been Inspection Agency (IA) selected independently rated. by the window manufacturer. The U-factor is a measure of heat transmission due to a temperature dif- ference. The smaller Name of the window the U-factor, the manufacturer. less heat is trans- mitted. Description of the particular product The Solar Heat Gain to which this label is Coefficient is a attached. measure of the rate of solar heat flow through the window. Since ratings can vary depending on The Visible Light the typical size of Transmittance value windows in different is a measure of the building types, values fraction of visible are given for both light that passes residential and com- through the window. mercial applications. The Air Leakage rating is a measure of the rate of infiltra- tion through this particular window. Laboratory, is one of the fundamen- a result of this effort, consumers tal building blocks of the rating sys- across the country now have energy tem. This program and the FRAME rating labels on windows, skylights, program are used to calculate the U- and glazed doors analogous to those factors and Solar Heat Gain on automobiles, appliances, and Coefficients of windows. Air leakage insulation. and other energy performance attrib- utes are also being rated. Soon, homeowners will see two new rat- ings, the Fenestration Heating Rating Q Are these computer tools available to the public? (FHR) and Fenestration Cooling Rating (FCR), which provide a com- parative index of heating and cooling A Yes. These computer tools are avail- able through the National Fenestration Rating Council for use season energy use. The RESFEN by building energy professionals, program, developed at Berkeley Lab, engineers, architects, and others. can also be used to estimate the NFRC also provides detailed train- annual energy consumption and ing for manufacturers and design costs associated with a particular professionals in the proper use of window type and orientation in a these window-related computer specific geographic location, based tools. For more information on these on local utility costs. computer programs, contact: Q Who is responsible for implementing the window energy performance rat - ing and labeling program? National Fenestration Rating Council 1300 Spring Street, Suite 120 Silver Spring, MD 20910 Telephone: (301) 589-6372 Fax: (301) 589-0854 A The National Fenestration Rating Council has developed and is imple- menting this rating and labeling sys- e-mail: NFRCUSA@aol.com Web: http://www.nfrc.org tem. NFRC is a non-profit coalition of manufacturers, builders, state and federal energy officials, private and Q Where might I see NFRC labels ref - erenced or used? government laboratories, utilities, consumers, and others working together to develop a nationwide A Several state building codes and other organizations with an interest in promoting energy efficiency, such energy performance rating system as utilities, are already referencing that is fair, accurate, and credible. As NFRC ratings. The ratings are a pre- requisite for some special programs, such as low-interest financing to pur- chase energy-efficient windows. Look for labels on products dis- played in your local building materi- als supply store or window store. NFRC ratings are listed in the prod- uct literature, available from many window manufacturers, architects, or builders. Choosing a better-performing window to save on fuel costs will also improve comfort and performance in other areas. When shopping for windows, look for the NFRC Label as your guide. Window Checklist For Design, Specification, and Installation his checklist guides homeowners, architects, and builders in selecting residential T windows and skylights. Selecting the right window can be difficult because of the many factors involved and the great variations in climate, utility costs, and occu- pant needs. Check boxes are provided for marking entries during the selection or design process. Note that each entry below does not apply to all circumstances and that some general guidance may appear to be contradictory because all of the detailed conditions cannot be specified. Users should mark the items that apply to their particular needs. Other local sources of information for window selection are utilities, state and local building code officials, design professionals, and building materials suppliers. Insulating Value and Condensation If shading devices are to be used to sup - Resistance plement the use of high-performance win - t Look for NFRC U-factor ratings and dows, consider the following points: labels to guide window selection. t Select light-colored shading devices to t Select double-pane windows in all cli- minimize solar heat gains. mates where heating is needed. Select t Select exterior shading devices to mini- double- or triple-pane windows with mize the inward flow of absorbed solar low-E coatings and gas fills in cold cli- heat. mates to reduce heat losses and con- t Select interior shading devices to densation. reduce solar heat gains while providing t To reduce frame and edge heat losses for privacy and aesthetics, or when and condensation in all climates where exterior shading devices cannot be heating is needed, select windows with used. wood, vinyl, fiberglass, or properly t Select horizontally oriented shading designed, thermally broken aluminum devices for south-facing windows and frames. vertically oriented shading devices for t Use heavy drapes, thermal shades, or east- and west-facing windows. thermal shutters to provide additional t Specify overhangs, exterior awnings, or window insulation in cold climates. the planting of deciduous trees and shrubs to shade south-facing windows Solar Control and Ultraviolet during the summer while allowing ben- Protection eficial solar heat gains during the win- t Look for NFRC Solar Heat Gain ter. Coefficient ratings and labels to guide window selection. Daylight and View t Select windows with spectrally selec- t Look for NFRC Visible Light tive glazings (special tints or modified Transmittance ratings and labels to low-E coatings) to reduce solar heat guide window selection. gains (SHGC less than 0.4) while t Select window size, location, and glass maintaining high visible transmittance type to provide adequate daylight lev- (glass transmittance greater than 0.6). els in each space. t Select tinted windows to reduce solar t Select windows with high visible trans- heat gains and control glare by lower- mittances (greater than 50%) to maxi- ing visible transmittance. mize outward visibility. t Select special glazings (with plastic t Specify window sizes and positions in layers or low-E coatings) to reduce walls to take advantage of desirable ultraviolet transmission in rooms with views. materials subject to fading. (If this is a critical concern, consult expert assis- t Position windows away from bright tance.) external surfaces that create glare. Ventilation and Airtightness t Select laminated glass or tempered t Select operable windows for rooms requir- glass with screens for skylights and for ing substantial ventilation during mild windows near doors or close to the weather and to meet building code egress floor. requirements. t Select windows with locks or latches t Select casement or awning windows to that can be easily opened from the inte- maximize effective ventilation area. rior but cannot be opened from the exterior. t Select awning windows to better exclude precipitation while ventilating. Maintenance, Durability, and t Position operable windows in opposite Lifetime walls of living spaces to maximize cross- ventilation. t Check warranties for indication of durability and lifetime before selecting t Select fixed windows or windows with windows and skylights. compression seals to minimize infiltration. t Check the quality of window construc- t Select windows and skylights with contin- tion. uous edge seals to minimize infiltration. t Use protective paints, stains, or t Seal and caulk around window and sky- sealants on wood window and skylight light frames and sash to reduce infiltration. frames or select clad wood products. Follow the manufacturer’s installation instructions. t Follow the manufacturer’s instructions to maintain glazing, sash, frame, and hardware in good repair. Sound Control t Position windows away from external Installation sources of extreme noise. t Check all applicable building codes t Select double- or triple-pane windows before installing windows and sky- with panes of unequal thickness, laminated lights. glass, or gas fills to minimize noise from the exterior. t Follow the manufacturer’s installation instructions carefully. Privacy, Safety, and Security Economics t Select interior shading devices that obscure direct view for additional privacy. t Consider the relative effects on utility bills when selecting windows and sky- t Check building codes on fire, wind-load- lights. Contact the NFRC (see Window ing, and seismic safety before selecting Energy Rating and Labeling, page 10) and positioning windows and skylights. or consult energy specialists or utility representatives for estimates of the energy and cost savings provided by energy-efficient windows and sky- lights. t Consider the effects on the resale value of a home when selecting windows and skylights. t Check local, state, and federal energy efficiency programs and utility energy conservation programs for economic incentives for installing energy-effi- cient windows and skylights. New materials and improvements to all parts of the window assembly have contributed to their high performance. These double-glazed windows with vinyl frames have a high insulating value. Window Energy Glossary Air Leakage Rating: A measure of the rate person’s body can lose heat to a cold win- of infiltration around a window or skylight dow or skylight surface in a similar way. in the presence of a strong wind. It is expressed in units of cubic feet per minute R-Value: A measure of the resistance of a per square foot (cfm/ft 2) of window area material or assembly to heat flow. It is the or cubic feet per minute per foot (cfm/ft) inverse of the U-factor (R = 1/U) and is of window perimeter length. The lower a expressed in units of hr-ft 2-˚F/Btu. A high window’s air leakage rating, the better its window R-value, has a greater resistance airtightness. to heat flow and a higher insulating value. Conduction: The flow of heat through a Shading Coefficient (SC): A measure of solid material, such as glass or wood, and the ability of a window or skylight to from one material to another in an assem- transmit solar heat, relative to that ability bly, such as a window, through direct con- for 1/8-inch clear, double-strength, single tact. glass. It is equal to the Solar Heat Gain Coefficient multiplied by 1.15 and is Convection: The flow of heat through a expressed as a number without units circulating gas or liquid, such as the air in between 0 and 1. A window with a lower a room or the air or gas between window- Shading Coefficient transmits less solar panes. heat, and provides better shading. Fenestration: A window or skylight and Solar Heat Gain Coefficient (SHGC): its associated interior or exterior elements, The fraction of solar radiation admitted such as shades or blinds. The placement of through a window or skylight, both window openings in a building wall is one directly transmitted, and absorbed and of the important elements in determining subsequently released inward. The Solar the exterior appearance of a building. Heat Gain Coefficient has replaced the Shading Coefficient as the standard indi- Gas Fill: A gas other than air placed cator of a window’s shading ability. It is between window or skylight glazing panes expressed as a number without units to reduce the U-factor by suppressing con- between 0 and 1. A window with a lower duction and convection. Solar Heat Gain Coefficient transmits less solar heat, and provides better shading. Glazing: The glass or plastic panes in a window or skylight. Spectrally Selective Glazing: A specially engineered low-E coated or tinted glazing Infiltration: The inadvertent flow of air that blocks out much of the sun’s heat into a building through breaks in the exte- while transmitting substantial daylight. rior surfaces of the building. It can occur through joints and cracks around window U-Factor (U-Value): A measure of the and skylight frames, sash, and glazings. rate of heat flow through a material or assembly. It is expressed in units of Low-Emittance (Low-E) Coating: Btu/hr-ft2-˚F or W/m2-˚C. Window manu- Microscopically thin, virtually invisible, facturers and engineers commonly use the metal or metallic oxide layers deposited on U-factor to describe the rate of non-solar a window or skylight glazing surface pri- heat loss or gain through a window or marily to reduce the U-factor by suppress- skylight. Lower window U-factors have ing radiative heat flow through the win- greater resistance to heat flow and better dow or skylight. insulating value. Radiation: The transfer of heat in the form Visible Transmittance: The percentage or of electromagnetic waves from one sepa- fraction of visible light transmitted by a rate surface to another. Energy from the window or skylight. sun reaches the earth by radiation, and a Selecting Windows for Energy Efficiency What’s New New window technologies have increased in Building energy benefits and comfort, and have provided more practical options for consumers. This Energy selection guide will help homeowners, architects, and builders take advantage of the expanding window market. The guide contains Efficiency three sections: an explanation of energy-related window characteristics, a discussion of window energy performance ratings, and a convenient We would like to acknowledge the many win- dow and glazing industry reviewers of this doc- checklist for window selection. ument. We appreciate the time they took to assure the usefulness of this document. Special thanks to V&WPatio Door & Window Co., Inc. of Berkeley, California, for making one of their Resources window retrofit installations available for photo illustration of this brochure. This work was supported by the U.S. Department of Energy, In seeking information concerning windows and energy efficiency in general, Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Building there are several local resources worth investigating: Technology, State and Community Programs, Office of Building Systems. • Local utilities • State or municipal energy agencies This document was prepared as an account of work sponsored by the United States Government. While this • Regional universities with architecture, construction, or extension programs document is believed to contain correct information, neither the United States Government nor any agency • Bookstores thereof, nor The Regents of the University of • Product literature at home improvement centers California, nor any of their employees, makes any war- ranty, express or implied, or assumes any legal respon - • Local chapters of the American Institute of Architects sibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process dis - • Local builder’s associations closed, or represents that its use would not infringe pri- vately owned rights. Reference herein to any specific commercial product, process, or service by its trade Recommended Web Sites name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recom - Search for specific window and glazing manufacturers and trade organiza - mendation, or favoring by the United States Government or any agency thereof, or The Regents of tions by name on the World Wide Web. the University of California. The views and opinions of authors expressed herein do not necessarily state or American Architectural Manufacturers Association: http://www.AAMAnet.org reflect those of the United States Government or any agency thereof, or The Regents of the University of American Institute of Architects: http://www.aia.org California. American Solar Energy Society: http://www.ases.org/solar Produced for DOE’s Office of Energy Efficiency and Renewable Energy by the Lawrence Berkeley National American Society of Heating, Refrigerating & Air Conditioning Engineers: Laboratory, a DOE national laboratory. http://www.ashrae.org Home Energy Magazine: http://www.homeenergy.org DOE/GO-DE-AC03-76SF00098 National Association of Home Builders: http://www.nahb.com PUB-788 January 1997 - 5000 National Fenestration Rating Council: http://www.nfrc.org National Wood Window and Door Association: http://www.nwwda.org For More Information National Research Council of Canada: http://www.cisti.nrc.ca:80/irc/irccontents.html Energy Efficiency and Natural Resources Canada: http://www.NRCan.gc.ca Renewable Energy Clearinghouse P.O. Box 3048 Passive Solar Industries Council: http://www.psic.org Merrifield, VA 22116 U.S. Department of Energy: http://www.eren.doe.gov (800) DOE-EREC (800) 363-3732 Recommended Reading (703) 893-0400 fax http://erecbbs.nciinc.com/ Residential Windows: A Guide to New Technologies and Energy Performance, by John Carmody, Stephen Selkowitz, and Lisa Heschong, W.W. Norton & Company, 1996; http://www.wwnorton.com. Printed on recycled paper using soy-based inks.
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