Energy Efficient Windows and Doors

8/3/2019 Energy Efficient Windows and Doors

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Chapter 7Energy-Efficientwindows and Doors

Cost Codes: R = reducedN = negligibleS = small ($0.25 and $1.00/ft2 of floor area)

Recommendations

1. Minimize glass area.2. Shade glass areas us ing porches, treesawnings, sunscreens, shutters, sun control

films, inte rior shades or blinds.3. Choose reduced transmittance glassproducts with low shading coefficients.4. Base glass and shading choices in largepart on compass orientation.5. Choose windows with low infil trationratings and durable hardware. Choose well-weather-stripped doors.6. Choose windows and doors with maximumopenable area for improved ventilation(casements; awning type; pocket or swing ingglass doors to porches).7. Choose casement windows for rooms withonly one outside wall to create a wingwalleffect for improved ventilation.8. Use screen doors for ventilation.9. Consider using insulated glass.

Maximum Combined Total

FirstCost

RS/M/H

MN

N S

S/M/H

M/HSM

H

OO PotentialCooling

5-1 05-15

5-150-10

5

5

0-5

0-50-5

30

SavingsHeating

5-1 0-

-

-

5-10

-

-

-15-20

30


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Marketing Energy-EfficientWindows and Doors

View, daylight, ventilation. These have been thefunctions of win dow s for centuries. But since theenergy crisis, possibly no other house component hascom e under as muc h scrutiny. Th at is because thewindow is the weak link in the building envelope.It admits solar heat, tends to leak, and is a poorinsulator relative to the rest of the building. M orethan one unhappy hom e owne r call their windows"holes in the insulation."But manufacturers are responding almost daily withnew window products claiming greater energy effi-ciency. As a result, kno wing wh at windo w featuresto look for can be confusing for the builder. This isparticularly true in Florida because man y of theenergy-efficient features promote d by nation al win-dow manufacturers save heating energy, but notnecessarily cooling energy.This chapter discusses window s an d d oors forFlorida homes. After reading it you will kn ow notonly what to look for in currently available windowproducts b ut also which features are desirable so thatyou can evaluate future products. You r knowledg eof windows, as demonstrated in your homes, will beon e of your best selling tools.Keep in mind that there are four key items that cansignificantly affect the home owner's utility bills:solar heat passing through the glassair leakage through an d arou nd the windowheat flow outw ard through the w indow in winterventilation of the ho use.Start by selling a potential buyer on your choice ofhome plans that have reduced glass area, emphasiz-ing these advantages:less expensive to build and operatelower initial cost for air conditionerless of the sun's heat entering the house, thereforeless air conditioning neededless heat escaping in wintergreater thermal comfort for anyone sitting orstanding near (sm aller) window s, because of lessradiant heat exchange between the glass and theperson in winter or summer.You can also point out other benefits of less windowarea. The hom e buyer will not have to spend asmuch o n wind ow coverings. Th e house will gener-ally be quieter. It will be mo re secure with fewer orsmaller points of entry.

Windows allow you to look out, and let in lightand breezes. But windows are the weak linkin the building envelope.

Regardless of all these benefits, many ho me buyerswill want more glass for the sake of light andappearance. As an alternative to small windows andwindow areas, you can provide larger window areasthat are well shaded. A big advan tage to their use isthat glare is much less of a potential problem thanwith small unshaded windows.There are many methods of shading windows.W he n you select trees, awnings, sunscreens, reflec-tive glass or interior blinds, you have som ethingtangible a nd visible to sell. Sh ow clients the shadingdevice and explain that it will cut out unwanted heatfrom the sun. Men tion that the sun's heat enteringthe house can account for about 20% o 30%of theirair conditioning bill.Explain that the so uth side receives less sun insummer and more in winter, so seasonal shading isideal for south-facing windows. Indicate ho w youhave taken special care to minimize eastwardly and


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Marketina Enerav-Efficient Windows and Doors

westwardly facing windows, which are difficult toshade, but have assured adequa te shading for win-dows that d o appear on those sides of the house.(Refer to Chapters 3 an d 4 for more marketingsuggestions.)

Daily Solar Heat Gain for 118" Single Glass

North South East Root North South East Roo for orWest West

North- and south-facing glass is preferred toeast, west, and roof (horizontal) facing glass.

Explain to clients that cracks aroun d windo ws anddoors can cause a loss of up to 10% of their heatedor cooled air. Sho w them the door weatherstrippingand the caulking around the windows and glassdoors. If you have chosen window products that are

two, three o r four times tighter than industry stand-ards, with hardware guaranteed to be d urable (10-year or lifetime guarantees), make sure to point thatout. Potential home buyers who have lived in anapartment or house with poorly sealed windows willappreciate superior products.Promote the ventilation aspects of awning an dcasement wind ows an d pocketing or swinging glassdoors by demonstrating how the entire windowedarea opens to provide the greatest ventilation withthe least amount of glass (see illustration below).Consider selling screen doors as a ventilation-enhancing option, both for front doors and doorsopening to the garage or other outside area. Manyhome owne rs add screen doors later, so why notshow an appropriate one on your model?Most ho me buyers recognize that insulated (doublepane) glass is an energy saver, so merely pointingout that you have used it may be sufficient. Explainthat in winter when it is 40F outside and 70Finside, a single layer of non-insulated glass isn'teffective enough in deterring the escape of heat.Furthermore, anyone sitting or standing nea r glasswill be far mo re comfortable if it is insulated. Also,insulated glass reduces moisture condensation andusually will reduce sound transmission as well.If you have chosen insulated steel doors, promotethem in a similar fashion.

Horizontalliding@ Casement(45%) (90%)Hopper (45%)

Jalousie

Effective open area of various types of windows (percentages).


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Selecting and InstallingEnergy-Efficient Windows and Doors1. Reduced glass area Although interior shading devices are subject to the

control of the home owner and can be ineffective ifYour approach to reducing the amount of glass in not operated properly, most of them can be effectivethe home is best begun at the time you select the if designed and controlled correctly. Windowhouse plan. Check the plans. Will smaller windows coverings which have white (highly reflective) backssignificantly affect the marketability of the house? and are opaque (no light passing through) are mostCan an 8-foot-wide section of windows be replaced effective.by a 6-foot section? Consult the architect ordesigner to explore the possibilities. In some situa- I \tions, front doors with glass may be less desirablethan solid doors. In other cases, larger glass areascan be tolerated for marketing and aesthetic reasons,without significant energy penalties, if they areoriented properly and shaded well.2. Shaded glassSun entering through windows in a home canaccount for 30%of the air conditioning load. Typi-cally, residents use interior drapes or blinds to helpreject the sun's heat, reducing the solar load to about20%of the air conditioning load. You can takesteps to reduce that even more. Choose Bahamashutters, awnings, trees, trellises, window films orsunscreens for exterior shading. Each of these iseffective. Exterior shading is generally more effec-tive than interior shading because the heat absorbed Vertical or horizontal blinds, white-backedby the shade is dissipated outdoors rather than inside draperies and shades are effective interiorthe house. shading options.

enetian awning

Gambrel awning id aluminumor casement windows)Exterior shading options.


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8/01 /90 Selecting and Installing Energy-Efficient Windows and Doors

3. Reduced-transmittance glass tics. Inform the windo w supplier of your intentions.Some films have not worked well on certain win-Not a ll alternatives to clear glass are c reated equal. dow s, especially on insulated glass units.Som e glass absorbs a lot of sun, and part of th atabsorbed heat reradiates into the house. Othe r kindsof glass are highly reflective and reject most radiantheat to the outside; others are only slightly reflective.Some appear almost clear; others look tinted.There is a way to compare the available choices.Ask the dealer or manufacturer about two keymeasurable qualities: the "shading coefficient" andthe "visible transmittance." A low shading coeffi-cient is desired to block sola r heat. A high visibletransm ittance is desirable for a clear app earance an dletting light in (see box). How ever, if the exteriorscene is com posed of unshaded expanses of brightly

Total solar heat gain throughShading - window and shading deviceCoeff ic ient -

(SC) Total solar heat gain through asingle sheet of 1/8" glass

Amount of lightthat comes

% Visib le Light = through window X 100Tran smi tta nce Amount of l ightincident o nwindow surface

reflecting surfaces (buildings, parking lots, or pavedareas, etc.), the visible transm ittance should belowered some wha t to minimize glare. Values forcertain glasses appe ar in the ad jacent table.If you c anno t find a commercially available windowwith the com bination of visible transm ittance andshading coefficient you desire, a relatively low-costalternative is to use clear glass with a n ap plied plas-tic sun-co ntrol film having th e desired characteris-

Speci f icat ions of Var ious Wind ow Glasses% VisibleGlass Type (Daylight) Shading WinterTransmittance Coefficient R-Value

Single clear 90 1OO .86Single green 84-86 .85-.87 .86Insulated clear 8 1 .89 1.72Low-e clear 76 .72 2-3insulatedInsulated green 76-78 .71-.76 .76Single bronze 69 .85 .86Insulated bronze 62-67 .71-.78 1.72Single plus (10-90) (.2-.9) .86control filmAnderson Sun 44 .38 3.13Glass, bronzeReflective 27 .51 .86bronze singleReflective 25 .42 1.72bronze insulatedHeat Mirror*66lclear 55 .50 4.0066/bronze 42 .41 4.0044/clear 38 .35 4.2044lbronze 22 .25 4.20

" See illustration below.

Heat mi r ro r f i lm i sp laced be tween tw opanes of glass.


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Selecting and Installing Energy-Efficient Windows and Doors

4. OrientationHouse design and site planning should place most orall glass areas on the no rth and south sides. Avoideasterly and westerly facing windows, or provideporches or other shading. If left unsha ded, makecertain to choose a glass or film with a lo w shadingcoefficient.In N orth an d C entral Flo rida, select clear glass forsouth windows and shade them using seasonalmethods such as decidu ous trees (Chap ter 3), over-hangs (Chapter 4), retractable awnings, and interiorwhite-backed blinds or shades. These permit wintersun to enter the room. W hite mini-blinds canreduce solar heat an d still permit ventilation w hilemaintaining privacy, making them a good choice foran energy-conserving window treatment. See Chap-ters 3 and 4 for m ore information on orientation.

Bahama shutters can be an effective shadingstrategy for east- or west-facing windows.

5. Tight-fitting windows, doors

0.375 cfm/ft for windows tested at 25 miles-per-hour wind speed. The standa rd for glass doors is0.50 cfm/ft2 of total glass area. Mo st windows anddoors easily pass this test. Som e window s have rat-ings as low as 0.02 cfm /ft. Use test data for com-parisons. Mak e sure the tests were done at 25 mphor 1.56 pounds per square foot (psf), not 15 mph.Be aware, however, that the w indow you obtainmay o r may not perform exac tly like the tested unit,since the ma nufactu rer usually selects the unit to betested. Also, infiltration tests do no t measure howwell the windo w seal will hold up over the lifetimeof the product.W indow seals vary not only between m anufacturers,but certain types of windows have in herently bettersealing capabilities. Casement and awning window stypically have low infiltration ratings (abo ut 0.05cfm/ft), while single-hung, doub le-hung and slidingwind ows typically have h igher infiltration ratings

You know that windows and doors should fit tightlyto save energy. But how d o you know if the pro-ducts you a re considering purchasing will betight-fitting?Windows are tested for air infiltration in accordancewith the American Society for Testing andManufacturing (ASTM ) Test E-283. Product litera-ture will sometimes list the results of the test.Request the results from your dealer or the manufac-turer if you don't see them. The results will beexpressed in cubic feet per m inute per foot (cfm /ft)of sash for windows, and in cubic feet per minuteper squ are foot (cfm /ft2) of overall area for glassdoor units. The industry standard is a maxim um of


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8/01/90 Selecting and Installing Energy-Efficient Windows and Doors

(0.15 to 0.30 cfm/ft). However, most of the crankoperators on aluminum awning windows are notvery durable- ventually leading to leaks. Mostwood-frame casement and awning windows havecompression seals and a separate hook-type lock inaddition to the operating crank. They seal tightlywhen closed. Jalousie windows have severe prob-lems with operator reliability and air leakage. Theyare not recommended for heated or air-conditionedbuildings.Installation. Proper installation of windows isimpo rtant in preventing air leakage. Use spray insu-lation (foam caulk) around frame edges and caulkinside and o ut with a long-lasting material. Poorinstallation can defeat the quality built into a win-dow unit. Wind ow m ovement after installation canopen up cracks. Air gaps arou nd windows canshort-circuit the tight seals provided in goodwindows.

6. Maximum ventilation openingsSwinging glass doors that open ont o porches canprovide greater ven tilation than sliding glass doors.Consider a single 3-foot-wide swinging door (withor without French-style lights) instead of 5- or 6-foot sliding glass doors. O r consider a double-opening swinging door. Alternatively, select a slidingglass doo r that will pocket inside or just outside of awall. Telescoping doors are also available and donot inhibit the placement of wall insulation.Single-hung, double-hung a nd sliding windows pro-vide an openable area that is less than half of thewindow area as sho wn in the adjacent illustration.Awning and casement windows provide 60% o 90%openable area.A su mmary of the relative performance of w indowtypes (with single-hung the basis of comparison) isgiven below.

Make sure you select doors that ar e well weather-stripped, an d install them w ith a snug fit. W henchoosing glass doors, consider tight-fitting swingingdoors instead of sliding doors, which frequently leak.

French doors are an efficient way toincrease ventilation in a room.

O O ( > O . *o-+oQ *oQ , O~eG,yC8'"$" " Q ~ > O * $+Windowtype 4e Q +deea\J'CCasementAwningAluminumawningSingle hungDouble hungSliding/rollingJalousieSliding doorFrench doorFixed glass

0 0 0 00 0 0 N.A. 0 0

1 Window (not door) rating, based on minimizingglass area while providing egress in common sizes.Based on weatherstripped, locking models.

3 Optional egress hardware may be required onsmaller units.Typical aluminum awning window hardware.

Relative performance of window types.


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7. Casements as wingwallsRooms with windows on only one wall do not typi-cally have cross-ventilation. One way to provide itis with two widely spaced windows, with short wallsextending like wings from the wall adjacent to thewindows. This creates positive and negative pres-sure zones, and breezes will be more likely to enterand leave the room. Look at the figure below andrefer to Chapter 4 for more information onwingwalls.

Range of winddirections for whichroom is effectively YventilatedWingwalls create inlets and outlets for

ventilation of rooms with one outside wall.A single casement window that is hinged so air canenter from each side can perform the same functionas a wingwall. Note that most casement windows

I 1A single casement window can actas a wingwall.are hinged in this manner to permit cleaning of bothsides of glass from inside the house. If two win-dows are used, whether together or apart, theyshould be hinged so they act as a wingwall. Thishinging is different from most standardarrangements.

A double casement window hinged as showncan act as a wingwall.8. Screen doorsA great method for increasing ventilation is throughdoorways. A solid opaque door blocks the daytimesun, but augmented with a screen door it can beopened to provide ventilation at night. For appear-ance's sake, wall pockets can be designed to concealscreen doors when they are not in use. If a doublefront door is used, use a double screen door. Use ascreen door on the garage service door, too.9. Insulated glassExcept for radiant heat from the sun, heat flowsthrough a given type of glass at a rate directlyrelated to the indoor-outdoor temperature difference.In hot climates like Florida, indoor and outdoorsummer temperatures differ by 20F at most. Theaverage summer temperature difference over 24hours is typically less than 10F. Conductionthrough window glass is thus not a meaningful partof the cooling load. However, it is a concern in theheating season. The question is, how much extracan one justify paying for insulated glass? Theanswer is in the table on the next page.

Insulated glass is most important whentemperature differences between outside andinside are high.


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Selecting and Installing Energy-E fficient Windows and Doors

Annual Savings of Using Insulated Glasslnstead of Single-pane Glass($/sq.ft)'South Central NorthFlorida Florida Florida

Heat Pump (COP=2.0)2 0.15 0.15 0.20Electric Resistance 0.15 0.20 0.25To obtain a reasonable return o n investment,how m uch more can a home owner justifypaying for insulated glassthan single pane glass ($ /~ q. ft )~

South Central NorthFlorida Florida FloridaHeat Pump(COP=2.0) 1.65 2.00 2.25Electric Resistance 1.75 2.60 3.30

Annual Savings of Using Clear Low-e Windowslnstead of Clear Insulated Windows ( $/ ~q .f t) ~South Central NorthFlorida Florida FloridaLow- e surface 3 0 0.05 0.05Low-e surface 2 0.15 0.15 0.15Heat Mirror 88 0.20 0.20 0.20

To obtain a reasonable retu rn on investment,how much more can a home owner justifypaying for low-e window thanclear insulated windows ($ /~ q. ft )~South Central NorthFlorida Florida Florida

Low-e surface 3 0 0.55 0.75Low- e surface 2 1.75 1.75 1.75Heat Mirror 88 2.25 2.50 2.75' Assume an air conditioner with SEER=8.0, electricity costs at 8.5 centslkwh. All values based on 10%duct loss. All values rounded to nearest $0.05. Heat ing set point = 6aF, Cooling set point = 78OF.

Seasonally adjusted value. Equivalent of 2.5 to 3.0 COP units.Based on 10% internal rate of return on investment. Also assumes fuel prices increasing at 5% peryear and a 20 year life for window products. All values rounded to nearest $0.05.Assume an air conditioner with SEER=8.0, electricity costs at 8.5 centslkwh. Choice of heat typeinsignificant. All values rounded to nearest $0.05. Heating set point = 68"F, Cooling set point = 78F.

Low-emissivity (low-e) insulated glass units haveentered the market. These units have on e surfacewith a special coating. Th e main effect of the coat-ing is an increased resistance to heat flow. This pro-duct is ideal for the no rthern U.S., since it makes aninsulated window unit that is the equivalent of atriple-pane unit. But again, ho w much m ore shouldyou pay for such a unit in Florida?

emissivity coating. Th e table lists values that depen don wh ich surface has the low-e coating. Surface 2 ispreferred for hot climates. Unfortunately, mostmanufacturers sell the un it with surface 3 as thelow-e surface. Du e to heating of th e glass itself, thisis less desirable than surface 2. Ask your supplier ifyou can order your windows with the low-e coatingon surface 2.

If you a re convinced that insulated glass is cost- The values given do not refer to low-transmittanceeffective, then ex amin e the table abov e to see how glass choices, some of which use reflective glass ormuch mo re you can justifiably pay for a clear, low- films in conj unctio n with a low-e surface. (See thesection o n low -transmittance glass in this chapter.)Outside Newcomers o n the market are single-layer low-eglass and low-e windo w films. In general, they will

The low-e coating perform nearly as well as insulated glass.in insulated units Compare the additional cost with the justifiablefaces the airspace costs in the insulated glass table.and absorbs heat.Having the coating Aside from energy savings, most insulated glass pro-on surface 2 rather ducts offer greater noise control and improved ther-than surface more mal comfort to home occupants. They also reduceeffectively block the the number of times condensation will appear on thesun's heat from the roo m side of the glass. Th at alone may ma ke thehouse. difference worthwhile for some applications.


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SummaryMinimizing unshaded glass is a principal require-ment for an energy-efficient Florida house. The siteplanning and house plan selection shou ld aim atproviding windows oriented for summer shading.Shading can also be provided by awnings, shutters,sunscreens, window films and interior windowcoverings.In selecting window s, you should g enerally chooseglass with a low shading coefficient and high visibletransm ittance. Use insulated glass when its cost isjustifiable. Use tight-fitting windows an d doors withdurable hardware.Select floor plans tha t perm it cross-ventilation, andthen provide maximum open area with minimumglass. Consider swinging glass doors or pocketedsliding glass doo rs to porches, and casement win-dow s for rooms with one outside wall. Use screeneddoors to provide excellent ventilation inlets andoutlets.

For further information"Window Treatment for Energy Conservation,"W.R. McCluney, FSEC-EN-4-80, January 1985."Light Without Heat Gain - Glazing with a Differ-ence," Bradley J. Davids, Architectural Lighting,June 1987."Windows for Hot Climates," R. Vieira, ProgressiveBuilder, November 1986."A Buyer's Guide - High Performance Windows,"Practical Homeowner, October 1987.Sola r Con trol an d Shad ing Devices, Aladar O lgyayand Victor Olgyay, Princeton University Press, Prin-ceton, NJ, 1957.Architectural Aluminum Manufacturer's Associa-tion, 2700 River Road, Des Plaines, IL 60018,(3 12)699-7310 .National Wood W indow & Door Association, 1400East Touhy Ave., Suite G54, Des Plaines, IL60018, (3 12)299-5200.Vinyl Window & Doo r Institute, 355 LexingtonAve., New York, NY 1001 7, (212)370-9341.

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Energy Efficient Windows and Doors