What’s Newin Building

EnergyEfficiency

U.S. Department of Energy

INSIDE

Solar Control 6

Window EnergyRating and Labeling 10

Window Checklist 13

Window Energy Glossary 15

Selecting Windowsfor Energy EfficiencyNew window technologies have increased energybenefits and comfort, and have provided more practicaloptions for consumers. This selection guide will helphomeowners, architects, and builders take advantage ofthe expanding window market. The guide contains threesections: an explanation of energy-related windowcharacteristics, a discussion of window energyperformance ratings, and a convenient checklist forwindow selection.

Selecting the right window for aspecific home invariablyrequires tradeoffs between dif-

ferent energy performance features,and with other non-energy issues. Anunderstanding of some basic energyconcepts is therefore essential tochoosing appropriate windows andskylights. As illustrated on the fol-lowing page, three major types ofenergy flow occur through windows:(1) non-solar heat losses and gains inthe form of conduction, convection,and radiation; (2) solar heat gains inthe form of radiation; and (3) airflow,both intentional (ventilation) and

unintentional (infiltration). (See theWindow Energy Glossary for expla-nations of these terms.)

Insulating ValueThe non-solar heat flow through awindow is a result of the temperaturedifference between the indoors andoutdoors. Windows lose heat to theoutside during the heating season andgain heat from the outside during thecooling season, adding to the energyneeds in a home. The effects of non-solar heat flow are generally greateron heating needs than on coolingneeds because indoor-outdoor tem-perature differences are greater dur-ing the heating season than duringthe cooling season in most regions ofthe United States. For any windowproduct, the greater the temperaturedifference from inside to out, thegreater the rate of heat flow.

A U-factor is a measure of the rateof non-solar heat flow through a win-dow or skylight. (An R-value is ameasure of the resistance of a win-dow or skylight to heat flow and isthe reciprocal of a U-factor.) LowerU-factors (or higher R values), thusindicate reduced heat flow. U-factorsallow consumers to compare theinsulating properties of different win-dows and skylights.

The insulating value of a single-pane window is due mainly to the

The three major types ofenergy flow that occurthrough windows: (1) non-solar heat losses and gains inthe form of conduction, con -vection, and radiation; (2)solar heat gains in the form ofradiation; and (3) airflow, bothintentional (ventilation) andunintentional (infiltration).

thin films of still air on the interior andmoving air on the exterior glazing sur-faces. The glazing itself doesn’t offermuch resistance to heat flow. Additionalpanes markedly reduce the U-factor bycreating still air spaces, which increaseinsulating value.

In addition to conventional double-panewindows, many manufacturers offer win-dows that incorporate relatively new tech-

nologies aimed at decreasing U-factors.These technologies include low-emittance(low-E) coatings and gas fills.

A low-E coating is a microscopicallythin, virtually invisible, metal or metallicoxide coating deposited on a glazing sur-face. The coating may be applied to oneor more of the glazing surfaces facing anair space in a multiple-pane window, or toa thin plastic film inserted between panes.The coating limits radiative heat flowbetween panes by reflecting heat backinto the home during cold weather andback to the outdoors during warm weath-er. This effect increases the insulatingvalue of the window. Most window man-ufacturers now offer windows and sky-lights with low-E coatings.

The spaces between windowpanes canbe filled with gases that insulate betterthan air. Argon, krypton, sulfur hexafluo-ride, and carbon dioxide are among the

SolarRadiation

Convectionand Conduction

ThermalRadiation

Infiltration

High-performance windows make energy-efficient homes possible with greaterfreedom of design than in the past.

gases used for this purpose. Gas fills addonly a few dollars to the prices of mostwindows and skylights. They are mosteffective when used in conjunction withlow-E coatings. For these reasons, somemanufacturers have made gas fills stan-dard in their low-E windows and sky-lights.

The insulating value of an entire win-dow can be very different from that of theglazing alone. The whole-window U-fac-tor includes the effects of the glazing, theframe, and, if present, the insulating glassspacer. (The spacer is the component in awindow that separates glazing panes. Itoften reduces the insulating value at theglazing edges.)

Since a single-pane window with ametal frame has about the same overall U-factor as a single glass pane alone, frameand glazing edge effects were not of greatconcern before multiple-pane, low-E, andgas-filled windows and skylights werewidely used. With the recent expansion ofthermally improved glazing optionsoffered by manufacturers, frame and spac-er properties now can have a more pro-

nounced influence on the U-factors ofwindows and skylights. As a result, frameand spacer options have also multiplied asmanufacturers offer improved designs.

Window frames can be made of alu-minum, steel, wood, vinyl, fiberglass, orcomposites of these materials. Wood,fiberglass, and vinyl frames are betterinsulators than metal. Some aluminumframes are designed with internal thermalbreaks, non-metal components that reduceheat flow through the frame. These ther-mally broken aluminum frames can resistheat flow considerably better than alu-minum frames without thermal breaks.Composite frames may use two or morematerials (e.g. aluminum-clad wood,vinyl-clad wood) to optimize their designand performance, and typically have insu-lating values intermediate between thoseof the materials comprising them. Framegeometry, as well as material type, alsostrongly influences thermal performanceproperties.

Spacers can be made of aluminum,steel, fiberglass, foam, or combinations ofthese materials. Spacer thermal perfor-

Representative Window U-Factors*e is the emittance of thelow-E coated surface.Values are for 3-foot-by-5-foot windows. U-factors varysomewhat with window size.

Source: ASHRAE Handbook—Fundamentals, American Society ofHeating, Refrigerating, and Air-Conditioning Engineers, Inc., Atlanta,GA, 1993.

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*,1/2-inch air space 0.37 0.49 0.67

Double glass, e = 0.10*,1/2-inch argon space 0.34 0.46 0.64

Triple glass, e = 0.10 on twopanes*, 1/2-inch argon spaces 0.23 0.36 0.53

Quadruple glass,e = 0.10 on two panes*, 1/4-inch krypton spaces 0.22 ----- -----

Outdoor air temperature and indoor air rela-tive humidity combinations at which conden-sation will occur on the center of the glass forsingle, double, and triple glazings, some withlow-E coatings and gas fills.On or above each curve, the conditions are rightfor condensation. Below each curve, condensa -tion will not occur on that glazing type as longas the glazing is exposed to room air circula -tion. Values are based on winter conditions:70˚F indoor air temperature, 15 mph outdoorair velocity, and no incident solar radiation.

Source: WINDOW 4.1 (a computer program for calculatingthe thermal and optical properties of windows), LawrenceBerkeley National Laboratory, Berkeley, CA, 1994.

mance is as much a function of geometryas of composition. For example, somewell-designed metal spacers insulatealmost as well as foam.

The table on page 3 shows representa-tive U-factors for window glazing, frame,and spacer combinations under winterdesign conditions. Due to their orientationand their greater projected surface areas,domed and other shaped tilted and hori-zontal skylights have significantly higherU-factors than do vertical windows ofsimilar materials and opening sizes.

Preventing Condensation

Air can hold varying amounts of watervapor or moisture. The warmer the air is,the more moisture it can hold. Theamount of moisture in the air, expressedas a percentage of the maximum amountthe air could hold at a given temperature,is called its relative humidity. For healthand comfort, indoor air should containsome moisture. The relative humidityshould generally be between 30% and40% at normal room temperature.

The relative humidity of air can beincreased by adding more moisture or byreducing the temperature. When the rela-tive humidity reaches 100%, the air canhold no more moisture, and water beginsto condense from it. The temperature atwhich this condensation occurs is calledthe dew point temperature of the air.When moist air comes in contact with acold surface in a home, it may be cooledto its dew point temperature, resulting incondensation on the surface.

Single-glazed windows characteristical -ly suffer from water condensation prob -lems and the formation of frost on theinside surface of the glass in winter.

Windows don’t cause condensation, buthistorically they have been the first andmost obvious place it occurs. This isbecause windows generally have lowerthermal resistances than insulated walls,ceilings, and floors. As a result, theirinside temperatures are usually lower thanthose of other surfaces in a home duringcold weather. If the air in a home is humidenough, water will condense from it whenit is cooled at a window surface.Condensation is most often thought of asa cold climate winter problem. However,in hot, humid weather, moisture can con-dense on the outside surface of a poorlyinsulated window in an air-conditionedbuilding.

Left unchecked, condensation can dam-age window frames, sills, and interiorshades. Water can deteriorate the sur-rounding paint, wallpaper, plasterboard,and furnishings. In severe cases, it canseep into adjoining walls, causing damageto the insulation and framing.

The indoor air coming in contact withenergy-efficient windows is less likely tobe cooled to its dew point temperaturebecause the inside surface temperaturesremain higher during cold weather than dothose of windows with single glazing, tra-ditional metal spacers, and metal frames.

The figure on page 4 illustrates condi-tions under which condensation will formon the center of the glass of five glazingtypes with widely varied U-factors. Thegraph shows clearly that the risk of con-densation at the center of the glass isreduced as the insulating value of theglass increases. Even at an outdoor airtemperature of -30˚F, the indoor air rela-tive humidity must be nearly 70% beforecondensation will form on the triple glaz-ing with two low-E coatings and a gas fill.On the other hand, at an outdoor tempera-ture of 10˚F, condensation will form onthe single glazing at an indoor relativehumidity of only 18%.

Condensation is even more likely tooccur at window spacers and frames,which are usually less insulating than thecorresponding glazings. With so manyinsulating glazing types available, effortsto prevent condensation have shiftedtoward the development of better insulat-ing spacers and frames.

Recommendations for SelectingWindow U-Factors

When shopping for windows and sky-lights, pay close attention to whether theU-factor listed by the manufacturerapplies to the glazing only or to the entireunit. If it is for the glazing only, the over-all U-factor may be considerably higherbecause of the frame and spacer effects.These effects increase with decreasingtotal window area. Compare differentwindow types or makes by their total U-factors, which are best obtained fromNFRC labels. (See Window EnergyRating and Labeling, page 10.) New win-dow energy ratings and the RESFENcomputer program can be used to estimatethe relative energy usage associated with aparticular window type and U-factor.

Avoid aluminum-frame windows with-out thermal breaks if possible. Even inmilder climates, these windows tend tohave low inside surface temperatures dur-ing the heating season, giving rise to con-densation problems. Aluminum-framewindows with properly designed thermalbreaks can be used in moderate climates.Wood, vinyl, and fiberglass are the bestframe materials for maximum insulatingvalue.

Single-pane windows are impractical inheating-dominated climates. In theseregions, multiple-pane, low-E, and gas-filled window configurations are advis-able. In most climates, glazings with low-E coatings and gas fills will be a choicethat provides significant energy savings ina cost-effective product. Low-E and gasfills have now become a common optionfor many manufacturers, which reducestheir added cost. The resultant total win-dow U-factor should be 0.5 or lower andpreferably below 0.4 for maximum energysavings. Consumers should select win-dows with long warranty periods, whichindicate sound window design and con-struction, and a reduced probability ofinsulating glass seal failure or gas leak-age, which would reduce performance.Remember that lower window and sky-light U-factors mean less energy con-sumption, lower utility bills, and greatercomfort in the living space.

Daily total solar heat gainthrough 1/8-inch clear sin -gle glass for various win -dow orientations on veryclear days at 40˚N lati -tude (for example,Columbus, Ohio, andBoulder, Colorado).

Source: ASHRAE Handbook—Fundamentals, American Societyof Heating, Refrigerating, and Air-Conditioning Engineers, Inc.,Atlanta, GA, 1993.

Solar Control

Solar transmission through windows andskylights can provide free heating duringthe heating season, but it can cause ahome to overheat during the cooling sea-son. Depending upon orientation, shadingand climate, solar-induced cooling costscan be greater than heating benefits inmany regions of the United States. In fact,solar transmission through windows andskylights may account for 30% or more ofthe cooling requirements in a residence insome climates.

Because the sun’s position in the skychanges throughout the day and from oneseason to another, window orientation hasa strong bearing on solar heat gain. Thefigure above shows the solar heat gainthrough 1/8-inch clear single glass for var-ious window orientations on very cleardays in the heating and cooling seasons at

40˚N latitude. South-facing windowsallow the greatest and potentially mostbeneficial solar heat gain during the heat-ing season, while admitting relatively lit-tle of the solar heat that contributes tocooling requirements during the coolingseason. The reverse is true for skylightsand east- and west-facing windows. Northexposures transmit only minimal solarheat at any time. The ultimate importanceof these climatic and orientation effectswill depend on the type of glazing underconsideration.

The Solar Heat Gain Coefficient(SHGC) is a measure of the rate of solarheat flowing through a window or sky-light. (AShading Coefficient (SC) is theprevious standard indicator of a window’sshading ability and for simple glazings isapproximately equal to the solar heat gaincoefficient multiplied by 1.15.) Solar heatgain coefficients allow consumers to com-pare the solar heat gain properties of dif-ferent windows and skylights. The solarheat gain coefficient accounts for both thetransmissive glazing element, as well asthe opaque frame and sash.

Additional glazing layers provide morebarriers to solar radiation, thus reducingthe solar heat gain coefficient of a win-dow. Tinted glazings, such as bronze andgreen, provide lower solar heat gain coef-

The NFRC label provides useful informa -tion—such as the window’s U-Factor,Solar Heat Gain Coefficient, VisibleLight Transmittance, and Air Leakagerating—to help you choose wisely.

ficients than does clear glass. Low-E coat-ings can be engineered to reduce windowsolar heat gain coefficients by rejectingmore of the incident solar radiation.Spectrally selective glazings, includingsome low-E coated glazings with lowsolar heat gain coefficients and new lightblue and light blue-green tinted glazings,block out much of the sun’s heat whilemaintaining higher visible transmittancesand more neutral colors than more heavilytinted bronze and grey glazings. High-transmittance, low-E coatings, used inconjunction with a tinted outer glass layer,also reduce solar heat gain by preventingthe absorbed heat from reaching the interi-

or space. Mirror-like reflective glazingsare commonly used in office buildings,but only occasionally chosen for resi-dences. While they may have very lowsolar heat gain coefficients, they block somuch of the light and view that they arenot normally desirable in homes.

The table below shows representativesolar heat gain coefficients and visibletransmittances for glazings with typicalwood or vinyl frames and aluminumspacers. Aluminum-frame windows ofcomparable size and glazing type general-ly have slightly higher solar heat gaincoefficients because of their thinnerframes and greater glazing areas.

Solar Heat Gain Coefficient

Visible Transmittance

glass total glass totalGLAZING 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 0.33retrofit 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 0.49outer pane, 1/2-inch air space

Double glass, green tint 0.75 0.54 0.60 0.47outer pane, 1/2-inch air space

Double glass, clear, 0.76 0.55 0.65 0.51e = 0.20*, 1/2-inch air space

Double glass, 0.45 0.33 0.35 0.29“Southern” low-E, e=0.08*,on tint, 1/2 inch air space

Double glass, spectrally selective, 0.72 0.52 0.40 0.33e = 0.04*, 1/2-inch argon space

Triple glass, clear, 0.68 0.50 0.49 0.39e = 0.08 on two panes*,3/8 to 1/2-inch air or argon spaces

Representative WindowSolar Heat GainCoefficients and VisibleTransmittancesValues are for a 3-foot-by-5-foot horizontal sliderwindow with wood or vinylframes and aluminumspacers. Solar heat gaincoefficients vary somewhatwith window size.

*e is the emittance of the low-E coated surface; emittancewill vary slightly with specificproducts.

Source: WINDOW 4.1 (a computerprogram for calculating the ther -mal and optical properties of win -dows), Lawrence Berkeley NationalLaboratory, Berkeley, CA, 1994.

Ultraviolet Protection

Ultraviolet radiation is the main compo-nent of sunlight that can fade and damagedrapes, carpets, furniture, and paintingswhen transmitted through windows andskylights. Efforts to produce windowglazings that transmit less ultravioletenergy have met with some success. Ingeneral, windows and skylights with plas-tic glazing layers or low-E coatingsreduce ultraviolet transmission. Evenwithout any ultraviolet radiation, sunlightcan still cause fading of fabrics and otherfurnishings.

Recommendations for Solar Control

Consumers should consider two aspectsof window selection to control solargain—the selection of the window itselfand the choice of interior or exteriorshading devices. Traditional windowswith clear glass required the use of shad-ing devices to obtain adequate perfor-mance, especially when the orientationadmitted substantial sunlight in summer.However, modern high-performance win-dows can do such a good job of control-ling sunlight that the importance of theseshading systems is reduced.

Window Solar Heat Gain Coefficientsshould ideally be selected according toorientation, but it may not always bepractical to do so. If south exposures areto admit beneficial solar heat during theheating season, their Solar Heat GainCoefficients should be high. These highcoefficients will not usually result inoverheating problems during the coolingseason because of the lower solar radia-

tion levels at that time on south-facingwindows, especially those with adequateroof overhangs.

Skylights and east- and west-orientedwindows may warrant lower solar heatgain coefficients since they transmit themost solar heat during cooling periods. Inmost climates, there is not much point inspending more money to obtain lowersolar heat gain coefficients for north-fac-ing windows.

In hot, sunny climates, select windowswith spectrally selective glazings to pro-vide low solar heat gain coefficients with-out loss of light. Darker tinted glazingsalso provide lower solar heat gain coeffi-cients, but they will yield somewhatdecreased outdoor visibility, particularlyat night. Where glare is a concern, thiseffect may be desired, but under otherconditions it may not. In climates wherecooling loads are large, look for windowswith a SHGC of 0.4 or less. To maintaingood light transmittance and visibility,select windows whose glazings have visi-ble transmittance of 0.6 or higher.

In some hot climates, where wintersare mild, it might seem reasonable toselect a single-glazed window with a lowSolar Heat Gain Coefficient rather than amore typical double glazing. However,single glazings have a more limited rangeof solar control (even if laminated glassand glue-on plastic films are considered),so a double-glazed window with appro-priate glazing choice as noted above, maybe the best overall solution, even in hotclimates.

Exterior or interior shading devices—such as awnings, louvered screens, sun-screens, venetian blinds, roller shades,and drapes—are essential for shadingclear glass, and can complement andenhance the performance of windowswith low Solar Heat Gain Coefficients.One advantage of many shading devicesis that they can be adjusted to vary solarheat transmission with the time of dayand season. But windows with “built-in”lower Solar Heat Gain Coefficients pro-

Commercially available windows mustmeet numerous standards and build -ing regulations addressing condensa -tion resistance, sound control, main -tenance requirements, and overalldurability of the unit.

vide better visibility and require less man-agement and maintenance in today’s busyhouseholds.

Exterior shading devices are moreeffective than interior devices in reducingsolar heat gain because they block radia-tion before it passes through a window.Light-colored shades are preferable todark ones because they reflect more, andabsorb less, radiation. Horizontally orient-ed adjustable shading devices are appro-priate for south-facing windows, whilevertically oriented adjustable devices aremore effective for shading windows oneast and west orientations.

Sash Type EffectiveOpen Area*

Casement 90%Awning 75%Hopper 45%Horizontal sliding 45%Single-hung 45%Double-hung 45%

Representative Window Ventilation Areas*Effects of window screens are notincluded.

Source: R.K. Vieira and K.G. Sheinkopf, Energy-Efficient Florida Home Building, FSEC-GP-33-88, Florida Solar Energy Center, CapeCanaveral, FL, 1988.

Ventilation and Airtightness

Airflow through and around windowsoccurs by design as ventilation and inad-vertently as infiltration. The use of win-dows for natural ventilation is as old asarchitecture itself. Opening windows, par-ticularly on opposite sides of a livingspace, can cool a home for free. The sashtype of a window influences the ventila-tion airflow rate through the window rela-tive to its size. Some common sash typesand their effective open areas for ventila-tion purposes are shown in the tableabove. Casement windows are especiallyeffective for ventilation because they tendto direct the greatest airflow into the liv-ing space when fully open.

Infiltration is the uncontrolled leakageof air into a building from the exteriorthrough joints and cracks around window

and skylight frames, sash, and glazings.This leakage can account for up to 10%of the energy usage in a home. The air-tightness of a window depends on boththe characteristics of the window—suchas sash type and overall quality of win-dow construction—and the quality of theinstallation. Operable windows with com-pressing seals are generally more airtightthan purely sliding seals, because of theway the sash element seals against theframing.

An air leakage rating is a standardizedmeasure of the rate of infiltration througha window or skylight under specific envi-ronmental conditions. Air leakage ratingsallow consumers to compare the airtight-ness of different windows and skylightsas manufactured products; they do notaccount for leakage between the installedproduct and the wall or roof. Lower airleakage ratings indicate greater airtight-ness.

Airflow Recommendations

In milder climates, or in spring and fall inmore severe climates, operable windowscan provide ventilation, improve comfort,and reduce the need for air conditioning.Operable windows are often specified tomeet building code requirements foremergency egress. Although operablewindows are sometimes useful in house-hold areas with high moisture production,such as bathrooms, kitchens, and laundryrooms, exhaust fans provide more reliablecontrol throughout the year.

Select windows with air leakage rat-ings that meet or exceed standard industryrequirements of 0.37 cfm/ft2 to minimizediscomfort from uncontrolled infiltration.Select windows with even lower valuesfor particularly windy sites or harsh cli-mates. Check the seals between windowcomponents for airtightness. To minimizeinfiltration around installed windows, fol-low the manufacturer’s installation proce-dures carefully and seal and caulk jointsand cracks.

Window Energy Rating and Labeling

Many windows, skylights, andglazed doors now bear energy rat-ings or labels, similar to those

being placed on household appliances, toassist consumers in selecting energy-effi-cient products. The labels have beendeveloped by a non-profit group, theNational Fenestration Rating Council(NFRC). The following interview withNFRC staff, provides homeowners, archi-tects, and builders with some importantinformation on these new window energyratings.

Q Why are energy ratings or labelsimportant for windows and sky -lights?

A Fenestration—windows, skylights,and glazed doors—can account forover 25% of the heating and coolingenergy bills in a typical home.Designers, builders, and homeown-ers have never had a tool for deter-mining or comparing the energy per-formances of fenestration products toassist them in their purchase deci-sions. Many manufacturers offer avariety of energy-efficient products,but have not been able to demon-strate their superiority through com-parable performance ratings.

Q How will designers and homeown -ers use these energy labels?

A Energy labels will show a variety ofproduct performance attributes,enabling designers to compare andselect products directly, based oneach project’s specific energy per-formance needs. Until now, design-ers have had to spend too much timetrying to understand a mixed bag ofrating techniques, test methods, andperformance claims. A nationwidesystem for rating whole-productenergy performance will not onlygive designers the energy informa-tion they seek, but will also permitdirect product comparisons.

Homeowners have faced a simi-lar dilemma. When selecting fenes-tration products for a remodelingproject or new construction, home-owners have had no way to comparethe energy performances of twoproducts directly. This difficulty hasbeen compounded by the differentenergy rating techniques employedby the various industry segments.Window energy labels will enableconsumers to compare productsdirectly, regardless of glazing andframe type.

Q How will the energy ratings bedetermined?

A The energy ratings are determinedusing advanced computer toolsdeveloped in the United States andCanada, combined with standard-ized product performance testing.The WINDOW 4.1 program, devel-oped at Lawrence Berkeley National

In evaluating the cost of new orreplacement window units, the aesthet -ic, functional, and energy performancecharacteristics come together. Tomake a better decision, it is useful tothink of the life-cycle cost of the unit.

The When shopping for windows, look for theNational 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 andSolar Heat Gain Coefficient, which rate the efficiency of the entire window (glass and frame), arethe 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 theyour assurance that Independent Certification andthis window has been Inspection Agency (IA) selectedindependently rated. by the window manufacturer.

The U-factor is ameasure of heattransmission due toa temperature dif-

Name of the window ference. The smallermanufacturer.the U-factor, the

less heat is trans-mitted. Description of the

particular productThe Solar Heat Gain to which this label isCoefficient is a attached.measure of the rateof solar heat flowthrough the window. Since ratings can

vary depending onThe Visible Light the typical size ofTransmittance value windows in differentis a measure of the building types, valuesfraction of visible are given for bothlight 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.

NFRC Label

SM

National Fenestration Rating Council

1300 Spring Street, Suite 120Silver Spring, MD 20910

Telephone: (301) 589-6372Fax: (301) 589-0854

e-mail: NFRCUSA@aol.comWeb: http://www.nfrc.org

Laboratory, is one of the fundamen- a result of this effort, consumerstal building blocks of the rating sys- across the country now have energytem. This program and the FRAME rating labels on windows, skylights,program are used to calculate the U- and glazed doors analogous to thosefactors and Solar Heat Gain on automobiles, appliances, andCoefficients of windows. Air leakage insulation.and other energy performance attrib-utes are also being rated. Soon,homeowners will see two new rat- to the public?ings, the Fenestration Heating Rating

Q Are these computer tools available

(FHR) and Fenestration Cooling Yes. These computer tools are avail-Rating (FCR), which provide a com-parative index of heating and cooling

A able through the NationalFenestration 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 designcosts associated with a particular professionals in the proper use ofwindow type and orientation in a these window-related computerspecific geographic location, based tools. For more information on theseon local utility costs. computer programs, contact:

Q Who is responsible for implementing National Fenestration Rating Councilthe window energy performance rat - 1300 Spring Street, Suite 120

ing and labeling program? Silver Spring, MD 20910Telephone: (301) 589-6372

AFax: (301) 589-0854The National Fenestration Ratinge-mail: NFRCUSA@aol.com

Council has developed and is imple- Web: http://www.nfrc.orgmenting this rating and labeling sys-tem. NFRC is a non-profit coalition -of manufacturers, builders, state and erenced or used?federal energy officials, private and

Q Where might I see NFRC labels ref

government laboratories, utilities,consumers, and others workingtogether to develop a nationwide

A Several state building codes andother organizations with an interestin promoting energy efficiency, such

energy performance rating systemas utilities, are already referencing

that is fair, accurate, and credible. AsNFRC 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 manywindow manufacturers, architects, orbuilders.

Choosing a better-performing windowto save on fuel costs will also improvecomfort and performance in otherareas. When shopping for windows, lookfor the NFRC Label as your guide.

Window ChecklistFor Design, Specification, and Installation

This checklist guides homeowners, architects, and builders in selecting residentialwindows and skylights. Selecting the right window can be difficult because of themany 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 designprocess. Note that each entry below does not apply to all circumstances and that somegeneral guidance may appear to be contradictory because all of the detailed conditionscannot 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 localbuilding 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 -

dows, consider the following points:❏ Look for NFRC U-factor ratings andlabels to guide window selection. ❏ Select light-colored shading devices to

minimize solar heat gains.❏ Select double-pane windows in all cli-mates where heating is needed. Select ❏ Select exterior shading devices to mini-double- or triple-pane windows with mize the inward flow of absorbed solarlow-E coatings and gas fills in cold cli- heat.mates to reduce heat losses and con- ❏ Select interior shading devices todensation. reduce solar heat gains while providing

❏ To reduce frame and edge heat losses for privacy and aesthetics, or whenand condensation in all climates where exterior shading devices cannot beheating is needed, select windows with used.wood, vinyl, fiberglass, or properly ❏ Select horizontally oriented shadingdesigned, thermally broken aluminum devices for south-facing windows andframes. vertically oriented shading devices for

❏ Use heavy drapes, thermal shades, or east- and west-facing windows.thermal shutters to provide additional ❏ Specify overhangs, exterior awnings, orwindow insulation in cold climates. the planting of deciduous trees and

shrubs to shade south-facing windowsSolar Control and Ultraviolet during the summer while allowing ben-Protection eficial solar heat gains during the win-

❏ ter.Look for NFRC Solar Heat GainCoefficient ratings and labels to guidewindow selection. Daylight and View

❏ Select windows with spectrally selec- ❏ Look for NFRC Visible Lighttive glazings (special tints or modified Transmittance ratings and labels tolow-E coatings) to reduce solar heat guide window selection.gains (SHGC less than 0.4) while ❏ Select window size, location, and glassmaintaining high visible transmittance type to provide adequate daylight lev-(glass transmittance greater than 0.6). els in each space.

❏ Select tinted windows to reduce solar ❏ 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.

❏ Select special glazings (with plastic ❏ Specify window sizes and positions inlayers or low-E coatings) to reduce walls to take advantage of desirableultraviolet transmission in rooms with views.materials subject to fading. (If this is a

❏ Position windows away from brightcritical concern, consult expert assis-external surfaces that create glare.tance.)

Ventilation and Airtightness ❏ Select laminated glass or tempered

❏ glass with screens for skylights and forSelect operable windows for rooms requir-windows near doors or close to theing substantial ventilation during mildfloor.weather and to meet building code egress

requirements. ❏ Select windows with locks or latches

❏ that can be easily opened from the inte-Select casement or awning windows torior but cannot be opened from themaximize effective ventilation area.exterior.

❏ Select awning windows to better excludeprecipitation while ventilating.

Maintenance, Durability, and❏ Position operable windows in opposite Lifetime

walls of living spaces to maximize cross-❏ Check warranties for indication ofventilation.

durability and lifetime before selecting❏ Select fixed windows or windows with windows and skylights.

compression seals to minimize infiltration.❏ Check the quality of window construc-

❏ Select windows and skylights with contin- tion.uous edge seals to minimize infiltration.

❏ Use protective paints, stains, or❏ 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

❏ Follow the manufacturer’s instructionsinstructions.to maintain glazing, sash, frame, andhardware in good repair.

Sound Control

❏ Position windows away from external Installationsources of extreme noise.

❏ Check all applicable building codes❏ 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

❏ Follow the manufacturer’s installationthe exterior.instructions carefully.

Privacy, Safety, and SecurityEconomics

❏ Select interior shading devices that❏ Consider the relative effects on utilityobscure direct view for additional privacy.

bills when selecting windows and sky-❏ 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 theenergy and cost savings provided byenergy-efficient windows and sky-lights.

❏ Consider the effects on the resale valueof a home when selecting windows andskylights.

❏ Check local, state, and federal energyefficiency programs and utility energyconservation programs for economicincentives for installing energy-effi-cient windows and skylights.

New materials and improvements to allparts of the window assembly havecontributed to their high performance.These double-glazed windows with vinylframes 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 isexpressed in units of cubic feet per minute R-Value: A measure of the resistance of a

per square foot (cfm/ft2) 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-ft2-˚F/Btu. A highwindow’s air leakage rating, the better its window R-value, has a greater resistanceairtightness. to heat flow and a higher insulating value.

Conduction: The flow of heat through a Shading Coefficient (SC): A measure ofsolid material, such as glass or wood, and the ability of a window or skylight tofrom one material to another in an assem- transmit solar heat, relative to that abilitybly, such as a window, through direct con- for 1/8-inch clear, double-strength, singletact. glass. It is equal to the Solar Heat Gain

Coefficient multiplied by 1.15 and isConvection: The flow of heat through a expressed as a number without unitscirculating gas or liquid, such as the air in between 0 and 1. A window with a lowera room or the air or gas between window- Shading Coefficient transmits less solarpanes. 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 admittedsuch as shades or blinds. The placement of through a window or skylight, bothwindow openings in a building wall is one directly transmitted, and absorbed andof the important elements in determining subsequently released inward. The Solarthe 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 isbetween window or skylight glazing panes expressed as a number without unitsto reduce the U-factor by suppressing con- between 0 and 1. A window with a lower

duction and convection. Solar Heat Gain Coefficient transmits lesssolar heat, and provides better shading.

Glazing: The glass or plastic panes in aSpectrally Selective Glazing: A speciallywindow or skylight.engineered low-E coated or tinted glazing

Infiltration: that blocks out much of the sun’s heatThe inadvertent flow of airwhile transmitting substantial daylight.into a building through breaks in the exte-

rior surfaces of the building. It can occurU-Factor (U-Value): A measure of thethrough joints and cracks around windowrate of heat flow through a material orand skylight frames, sash, and glazings.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 themetal or metallic oxide layers deposited on U-factor to describe the rate of non-solara window or skylight glazing surface pri- heat loss or gain through a window ormarily to reduce the U-factor by suppress- skylight. Lower window U-factors haveing radiative heat flow through the win- greater resistance to heat flow and betterdow or skylight. insulating value.

Radiation: The transfer of heat in the form Visible Transmittance: The percentage orof electromagnetic waves from one sepa- fraction of visible light transmitted by arate surface to another. Energy from the window or skylight.sun reaches the earth by radiation, and a

Selecting Windows forEnergy EfficiencyNew window technologies have increasedenergy benefits and comfort, and have providedmore practical options for consumers. Thisselection guide will help homeowners,architects, and builders take advantage of theexpanding window market. The guide containsthree sections: an explanation of energy-relatedwindow characteristics, a discussion of windowenergy performance ratings, and a convenientchecklist for window selection.

Resources

In seeking information concerning windows and energy efficiency in general,there are several local resources worth investigating:

• Local utilities• State or municipal energy agencies• Regional universities with architecture, construction, or extension programs• Bookstores• Product literature at home improvement centers• Local chapters of the American Institute of Architects• Local builder’s associations

Recommended Web Sites

Search for specific window and glazing manufacturers and trade organiza -tions by name on the World Wide Web.

American Architectural Manufacturers Association: http://www.AAMAnet.org

American Institute of Architects: http://www.aia.org

American Solar Energy Society: http://www.ases.org/solar

American Society of Heating, Refrigerating & Air Conditioning Engineers:http://www.ashrae.org

Home Energy Magazine: http://www.homeenergy.org

National Association of Home Builders: http://www.nahb.com

National Fenestration Rating Council: http://www.nfrc.org

National Wood Window and Door Association: http://www.nwwda.org

National Research Council of Canada: http://www.cisti.nrc.ca:80/irc/irccontents.html

Natural Resources Canada: http://www.NRCan.gc.ca

Passive Solar Industries Council: http://www.psic.org

U.S. Department of Energy: http://www.eren.doe.gov

Recommended Reading

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.

What’s Newin BuildingEnergyEfficiency

We would like to acknowledge the many win-dow and glazing industry reviewers of this doc-ument. We appreciate the time they took toassure the usefulness of this document. Specialthanks to V&WPatio Door & Window Co., Inc.of Berkeley, California, for making one of theirwindow retrofit installations available for photoillustration of this brochure. This work wassupported by the U.S. Department of Energy,Assistant Secretary for Energy Efficiency andRenewable Energy, Office of BuildingTechnology, State and Community Programs,Office of Building Systems.

This document was prepared as an account of worksponsored by the United States Government. While thisdocument is believed to contain correct information,neither the United States Government nor any agencythereof, nor The Regents of the University ofCalifornia, nor any of their employees, makes any war-ranty, express or implied, or assumes any legal respon-sibility for the accuracy, completeness, or usefulness ofany information, apparatus, product, or process dis -closed, or represents that its use would not infringe pri-vately owned rights. Reference herein to any specificcommercial product, process, or service by its tradename, trademark, manufacturer, or otherwise, does notnecessarily constitute or imply its endorsement, recom -mendation, or favoring by the United StatesGovernment or any agency thereof, or The Regents ofthe University of California. The views and opinions ofauthors expressed herein do not necessarily state orreflect those of the United States Government or anyagency thereof, or The Regents of the University ofCalifornia.

Produced for DOE’s Office of Energy Efficiency andRenewable Energy by the Lawrence Berkeley NationalLaboratory, a DOE national laboratory.

DOE/GO-DE-AC03-76SF00098PUB-788 January 1997 - 5000

For More Information

Energy Efficiency andRenewable Energy ClearinghouseP.O. Box 3048Merrifield, VA 22116(800) DOE-EREC(800) 363-3732(703) 893-0400 faxhttp://erecbbs.nciinc.com/

Printed on recycled paper using soy-based inks.