Industry Guid to Slcting th Bst Rsidntial Window Options for th Florida Climat Authors Ross McCluny and Paul Jindra Original Publication McCluny, R., Jindra, P., Industry Guid to Slcting th Bst Rsidntial Window Options for th Florida Climat. Publication Numbr FSEC-PF-358-00 Copyright Copyright Florida Solar Enrgy Cntr/Univrsity of Cntral Florida 1679 Clarlak Road, Cocoa, Florida 32922, USA (321) 638-1000 All rights rsrvd. Disclaimr Th Florida Solar Enrgy Cntr/Univrsity of Cntral Florida nor any agncy throf, nor any of thir mploys, maks any warranty, xprss or implid, or assums any lgal liability or rsponsibility for th accuracy, compltnss, or usfulnss of any information, apparatus, product, or procss disclosd, or rprsnts that its us would not infring privatly ownd rights. Rfrnc hrin to any spcific commrcial product, procss, or srvic by trad nam, tradmark, manufacturr, or othrwis dos not ncssarily constitut or imply its ndorsmnt, rcommndation, or favoring by th Florida Solar Enrgy Cntr/Univrsity of Cntral Florida or any agncy throf. Th viws and opinions of authors xprssd hrin do not ncssarily stat or rflct thos of th Florida Solar Enrgy Cntr/Univrsity of Cntral Florida or any agncy throf.
Industry Guid to Slcting th Bst Rsidntial Window Options for th lorida Climat Ross McCluny and Paul Jindra This documnt has bn prpard to hlp rtailrs, buildrs, distributors, and othr window industry profssionals in Florida assist homownrs and othr rsidntial window purchasrs in choosing th bst window options for Florida homs. Consumrs may find it usful in obtaining dtaild information concrning propr window placmnt and slction for Florida s gnrally hot, humid climat. A companion publication, Homownrs guid to slcting th bst rsidntial window options for th Florida climat is also availabl from th Florida Solar Enrgy Cntr.
1 Introduction It usd to b said that windows ar littl mor than hols in th insulation. It is tru to som xtnt that poorly insulatd and drafty windows in crtain climats dfat th purpos of wall insulation. Howvr, modrn high prformanc windows ar almost as good as opaqu insulatd wall sctions, at last in trms of total nrgy savings ovr long priods of tim. In som cass, thy can b shown to actually out-prform insulatd walls. Of cours, thy hav th priclss additional bnfits of providing viws to th outdoors and natural daylight illumination indoors important issus of quality and comfort. Th main purpos of a building and its windows is to provid comfort to th occupants as th sun movs through th sky, as th outdoor air tmpratur and humidity vary, and as th wind and rain com and go. If comfort can b achivd whil rducing th building s nrgy us and lowring monthly utility bills, so much th bttr. This Enrgy Guid focuss on choosing window options for rsidntial buildings in hot climats. Many of th principls offrd hr apply as wll to non-rsidntial buildings. Howvr, thr ar major diffrncs in th typs of windows availabl for ths two building classs, and thr ar normally major diffrncs in thir building occupancy schduls. This is important bcaus unoccupid buildings don t nd illumination. Nonrsidntial buildings ar most gnrally occupid during daylight hours. This provids a gratr opportunity for th us of daylighting in ths buildings to displac lctric lighting, saving nrgy in th procss. Rsidntial buildings ar usually lss occupid during daylight hours, so thr is lss chanc to sav nrgy by using daylight to displac daytim lctric lighting. With its rlativly highr fraction of rtirs, howvr, Florida offrs numrous xcptions to this gnral rul. Propr us of windows for daytim lighting of rsidntial building intriors can displac th lctric lighting that might othrwis b ndd.
2 How Do Windows Affct Enrgy Us in th Hom? Windows ar not dirctly nrgy-consuming dvics. Somtims thy caus th building to us mor nrgy than would b th cas without thm, and at othr tims thy actually rduc th building s nd for nrgy. Whn windows lt hat scap on cold wintr nights (causing th hating systm to us mor nrgy) and whn thy admit solar radiant hat on hot summr aftrnoons (causing th air cooling systm to us mor nrgy) thy incras th building s nrgy costs. How can thy rduc nrgy us? On way is to admit solar radiant hat into th building on a cold wintr day. Th solar radiation dirctly hats th building s intrior, and this hat is usd to displac nrgy that would othrwis b purchasd for hating th hous. Whn daylight illumination ntrs through a window, it dirctly illuminats th intrior of th building. If this daylight displacs lctric lighting which would othrwis b on, thr ar nrgy savings from not having to turn on th lights. Windows ar sldom slctd by consumrs on th basis of thir nrgy prformanc alon. Instad, apparanc, th oprating mchanism, color, and pric dominat slction faturs. Enrgy impacts hav bn lss important, du primarily to a lack of objctiv information on th window itslf, or availabl at th sal sit, about nrgy prformanc. Furthrmor, it is not as asy to compar windows on th basis of nrgy fficincy, as with rfrigrators, microwav ovns, and automobils. To partially allviat this difficulty, th National Fnstration Rating Council (NFRC) has dvlopd a systm for th crtification, rating, and labling of windows for nrgy prformanc. This systm is slowly ntring us by cod bodis in th U.S. Th NFRC labl is intndd to hlp th purchasr and cod officials dtrmin th nrgy prformanc of windows for which labls ar issud. Evn whn using th NFRC labling program, rducing th nrgy costs associatd with windows whil incrasing thir human comfort without xcssiv pric incrass, is a ral challng to th building dsignr. Achiving an accptabl dsign dpnds upon many factors. Which way th window facs rlativ to th dirction of th sun How much outsid shading is plannd or is prsnt for th window How bright th xtrior scn is (th brightr th scn, th gratr th potntial for glar) How dark th intrior is (th brightr th intrior surfacs, th lss th window glar) Th homownr s willingnss to oprat shading dvics to achiv thir bst prformanc Th homownr s dsir (or cod-rquirmnt) for impact rsistanc (th impact can com from storm-blown objcts or intrudrs) How critical it is to maintain an unobstructd viw to th outsid (gorgous vistas shouldn t b prmanntly blockd by window shads or othr add-ons, although tmporary blocking for privacy or solar hat rjction may b dsird at tims) Th homownr s dsir for acoustic isolation in noisy nvironmnts Whthr th window can mt asthtic dsirs for apparanc and quality Slcting Windows Thr ar som basic principls to kp in mind whn slcting windows for Florida rsidncs. Th wintr spac-hating sason is short and not svr. Thus, thr is not a strong nd to insulat a window, at last in comparison with th much gratr nd to protct it from dirct solar radiant hat gain. It is tru that insulating th window with multipl pans, insulating gass, and a spcial coating can rduc th siz of th air conditionr ndd to mt pak loads and that this smallr siz will rduc both hating and cooling costs as wll. Rducd hating and cooling quipmnt siz could sav nough construction dollars to pay for th xtra cost of th window insulation, but this should b provn by
3 calculations bfor it is accptd as truth. (Computr tools ar availabl, such as th program RESFEN, for making ths calculations. S th wb sits mntiond at th nd of this publication for mor information about such programs.) For rtrofit applications rplacing windows in an xisting building thr is sldom th chanc to sav dollars on th air conditionr, unlss it happns that th homownr nds to rplac th air conditionr at th sam tim that th windows ar rplacd. If th homownr dcids to install insulatd windows for whatvr dominant rason a fw xtra comfort bnfits can b xpctd too. Thr will b lss transfr of sound through th window, an advantag in urban sttings with frqunt road or aircraft nois, but a possibl disadvantag for rural sits whr th occupant might njoy haring bttr th sound of th wind or th chirping of birds. Insulatd windows hav lss tndncy toward condnsation and th rsultant growth of mold and mildw. On th infrqunt cold Florida wintr nights and xcssivly hot summr aftrnoons, an insulatd window will b mor comfortabl to sit nar. In trying to Solar noon prvnt unwantd solar radiant hat W gain during S summr months, 86 Sunst it is important to N raliz that th sun riss north of E Sunris Sunpath on summr solstic at southrn latitud ast and sts Figur 1. Summr solstic sunpath. north of wst during ths months (as shown in Fig. 1). It riss du ast and sts du wst only on th quinoxs, nar th 21 st of March and Sptmbr. Thus, whnvr possibl, it is bst to minimiz window xposurs toward th ast and northast and toward th wst and northwst. This can b accomplishd by th dsign and orintation of th building and by shading th window, as discussd blow. Shading Stratgis It is far bttr, for hat gain prvntion, to block th sunlight bfor it rachs th window, thrby dissipating th absorbd hat outsid whr it can b carrid away by air currnts. This mans that simpl shad trs and othr xtrior shading mthods can b vry ffctiv, both in saving air cooling nrgy and in blocking th strong glar which dirct sunlight can produc if allowd insid. If th ownr and building dsignr cannot avoid windows facing ast or wst, thn th us of xtrior oprabl shading dvics should b considrd to protct th windows from th sun. In this cas, shads can b pulld down to rduc solar radiant hat gain along with its glar and highr nrgy costs. Thy can b opnd whn th sun is not shining dirctly on th window, to provid both good xtrior viws and intrior daylight illumination. A varity of xtrior shading dvics is illustratd in Fig. 3 (pag 4). Not that in wintr, th sun riss south of du ast and sts south of du wst (Fig. 2). Solar noon Altitud Advantag can angl Sunst W b takn of this S 38 fact by putting xtrior vrtical N shads on astand wst-facing Sunris E Sunpath on wintr solstic at a southrn latitud windows (Fig. 4, Figur 2. Wintr solstic sunpath. pag 4). Proprly dsignd, vrtical protruding shads on on sid of th window can allow sunlight to ntr from th southast to ast through ast-facing windows in wintr, and from th southwst to wst through wst-facing ons, whil blocking th sun at othr sasons of th yar. With this stratgy of putting a wing wall on th window (along with a proprly sizd roof ovrhang), th worst of th solar gain conditions could b avoidd, whil still providing a viw and daylight illumination without th nd to oprat xtrior shads. With Florida s warm wintr climat, admission of any dirct solar gain, vn in wintr, may not b dsirabl for vryon, so othr shading stratgis should also b considrd.
4 If xtrior shads ar not wantd, considr th us of whit or othrwis highly rflcting intrior shads. For bst prformanc, thy should b oprabl. Whn thy ar closd, thy can rflct solar radiation back through th window to th outsid. Thy can b drawn closd whn th sun is strong and not wantd, and thn opnd whn th sun is not shining on th glass or whn accss to a good viw and daylight is dsird. Wst-facing East-facing Figur 4. Extrior vrtical shads in th form of wing walls. Extrior window shading stratgis Bahama shuttrs Extrior roll blind Sarasota shuttrs Sun scrn Slattd aluminum Vntian awning (ast or wst xposur) Porch Trllis & vins Trs Hood awning Gambrl awning (for casmnt windows) Solid aluminum awning Rollr awning (slf-storing) Figur 3. A varity of xtrior shading dvics is availabl to block th sun bfor it rachs th glass.
Spcializd Window Options If th us of xtrior and intrior shads is unaccptabl, thr ar othr options that can b slctd in th window itslf to rduc Glass prism (though not ncssarily liminat) solar gain ffcts. Ths options can b ffctiv vn if intrior shads ar installd too. On way is to choos a spctrally slctiv glazing systm for th window, using spcially tintd glass or coatd glass that blocks much of th solar gain without advrsly affcting th viw through th window. To undrstand this option rquirs som knowldg of th solar spctrum. An arly xprimnt in optics (Sir Isaac Nwton, about 1723), diagramd in Fig. 5 rvald that whit light is composd of a rainbow of colors, spanning what is calld th visibl portion of th solar spctrum. Th complt solar spctrum is plottd in Fig. 6, showing that th visibl portion contains lss than half th nrgy of th total solar spctrum. All of th solar spctrum producs hat whn absorbd by intrior surfacs, but only th fairly narrow visibl portion producs th snsation of vision. Th rst is invisibl Blu Grn Yllow Orang Rd Figur 5. Colors of th visibl spctrum. Spctral irradianc in W/(m 2 nm) 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 Ultraviolt (UV) radiation. This includs what ar calld infrard and ultraviolt radiation. Nithr of ths wavlngth bands contribut to vision and cannot b calld light. Th visibl transmittanc of glass is simply th fraction of incidnt light transmittd by th glass to th intrior. It has th symbol VT (or T v ) in window product litratur and is a numbr ranging from 0% to 100% (or 0 to 1.0 in fractional trms). Spctral slctivity dnots th ability of glass (or its coating) to transmit radiation in a tailord way ovr th spctrum. In this documnt, th principal applications of spctral slctivity ar two: th transmittanc might b high ovr th whol solar spctrum but low outsid this rgion, or it might b high ovr just th visibl spctrum and low vrywhr ls. UV VIS NIR Solar spctrum Human y snsitivity (visibl portion of th spctrum) 0.0 0 500 1000 1500 2000 2500 Wavlngth in nm Blu GrnYllow Rd Orang Nar infrard (NIR) Figur 6. Comparison of th solar spctrum to th human y color snsitivity. 5
6 Th total nrgy spctrum xtnds wll byond th solar spctrum limits, and all warm objcts radiat som nrgy outsid th solar spctrum. A spcial kind of glass coating calld low-mittanc or low- was dvlopd for cold climats, to xploit th diffrnc btwn th wavlngths of incidnt solar radiation and thos much longr wavlngths of radiation mittd by warm intrior objcts, as shown by th blu boundary in Fig. 7. Cold-Climat Window Coatings Th transmittanc of low- coatd glass is high ovr th solar spctrum to captur th maximum solar hat possibl, but low (with high rflctanc) byond th uppr limit of th solar spctrum, shown in Fig. 6 (pag 5). In a cold climat, radiation from th warm innr pan of a two-pan window to th coldr outr 2 Spctral irradianc in W/ (m nm) 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 Hot climat transmittanc Human y rspons Solar spctrum Cold climat transmittanc 23.9 C blackbody 0.0 0.5 1 5 10 50 Wavlngth in micromtrs Figur 7. Solar spctrum at a window, blackbody radiation spctrum from th intrior, and two idalizd window transmittanc spctra.
7 pan has wavlngths much gratr than thos of th solar spctrum. Th laws of physics tll us that this longwavlngth infrard radiation propagats btwn th glazings in both dirctions, but that th nt nrgy flow is from th warm innr glazing across th gas spac to th cold outr pan, whr it is absorbd by th bar glass and thn r-mittd to th outdoors, causing a loss of th building s intrior hat. Th low- coating was originally dvlopd to stop this wintrtim hat loss. Th coating s physical proprtis allow this in ithr of two diffrnt configurations, missiv or rflctiv (s Fig. 8, a and b. A warm innr pan with a low- coating (8a) is a poor mittr of longwav radiation to th cold outr pan, and thnc to th cold outsid air, thus trapping this radiant hat indoors. Low-missiv configuration Cold-climat low- coatd windows a. b. Or High-rflctiv configuration c. Cold Warm Absorptiv longwav convrsion Or Hot-climat coatd windows d. Cold Warm Solar dirct rflction Or * * Hot Cool Hot Cool Warm Cool Warm Cool Long-wavlngth IR Solar nar IR Solar nar IR absorbr (longwav convrtor) * Cold-climat low- coating Hot-climat solar nar IR rflctiv coating Scond pan optional in principl Figur 8. Low mittanc coatings for radiativ nrgy trapping or rjction.
8 This sam nrgy trapping function is rtaind if th manufacturr placs th low- coating on th outr pan, facing inward (8b), vn though that glass is coldr than th innr on. Th xplanation hr dpnds on th high longwav rflctanc associatd with a typical low- coating. Th longwav radiation rflcts back toward th warmr glass. A common nam for this longwav radiation is far infrard (FIR) radiation, bcaus its wavlngths ar far from th visibl spctral rgion, farthr than th nar infrard (NIR) radiation that xtnds from th dg of th visibl portion of th spctrum to th nd of th solar spctrum. Windows with convntional, cold-climat low- coatings ffctivly trap radiant hat insid th building, making it warmr than it othrwis would b without th coating, and rducing th nd for supplmntal hating. This works grat for cold northrn climats, but not in sunny Florida! W might call th convntional low- coating a northrn low- on, or mor accuratly a cold-climat low- coating (to includ our southrn hmisphr nighbors in this discussion, whr southrn latituds ar th coldr ons). Anothr nam for this coating is high solar gain low-. In summary, typical cold-climat low- coatings mit poorly and rflct wll in th far infrard rgion of th spctrum; thir rflctanc is mad to chang from high to low btwn th solar spctrum and th FIR, admitting solar radiation to warm th intrior whil trapping radiant hat insid. Hot Climat Glazing Systms Th low-mittanc principl can also b applid to a hot-climat glazing systm. (Th systm in this cas combins a spcific glass, plus coatings.) Th nwr glazing systm has bn dvlopd to mt th nds of Floridians and othrs living in hot climats (who, on an annualizd basis, want to xclud solar nrgy, not trap it insid). Idal hot - and cold - climat spctral transmittancs ar contrastd in Fig. 7 (pag 6). Crucial to th diffrnc btwn thm is th choic of th xact wavlngth at which th transmittanc dparts from its low valu at longr wavlngths (to high at shortr ons). This illustrats an application of th gnral spctral slctivity principl. Th nd ffct is to rduc th transmittanc of nar infrard radiation, lowring solar gain, but without a loss of visibl transmittanc. Th hot-climat transmittanc is high only ovr th visibl portion of th solar spctrum. Th rmaining infrard radiation, which maks up ovr half th total solar radiation, is blockd from ntring th building. Th trminology in this cas is hot climat glazing systm with low- coating(s). Anothr gnric trm usd for this class of glazing systm is low solar gain low-, but this trminology can b mislading bcaus th low- quality xtnds ovr diffrnt spctral rgions in th two diffrnt low- coating typs, as dscribd in th following two sctions. Spctral Slction by Absorption On way of achiving hot-climat solar NIR rjction is by spctrally slctiv absorption, as illustratd in Fig. 8c. An uncoatd sht of glass is slctd that has high absorptanc ovr th nar infrard portion of th solar spctrum but good transmittanc ovr th visibl portion. As a rsult, such glass will absorb much of th solar NIR radiation and hat up. But bing spctrally slctiv, much mor solar nrgy is absorbd outsid th visibl spctrum than insid it. In ordr to protct th intrior of th building from th hat conductd and convctd away from this hot glass, a scond pan must b addd on th intrior sid, with an insulating gap in btwn. Th rsult is a doubl- pan insulatd glazing unit (IGU) with a spctrally slctiv absorbr for th outr pan. It hlps if a cold-climat low- coating is addd to th outr pan s innr surfac. Th rason for this apparnt paradox of
9 mixing cold- and hot-climat tchniqus is to rduc th infrard r-radiation from th hot outr glass to th coolr innr on. Th rsult of this combination (Fig. 8c) is just th opposit of that for th cold-climat low- coating; it is not to trap hat insid th building but to protct th innr pan of glass from th hat of th outr on, thus kping solar hat out of th building. If an insulating gas is also usd btwn th pans, th systm works vn bttr. This arrangmnt offrs a good way of achiving a high ovrall spctral slctivity for hot climats. In two-pan glazing systms, th four glass surfacs ar convntionally numbrd from th outsid in. Thus, th coating can b placd on ithr surfac 2 or surfac 3 as shown in Fig. 8c. (Not that if this absorbing typ of hot-climat glazing systm is flippd ovr, with th insid now facing th outsid, th rsult is a good window for a cold climat. For mor information about this clvr window dsign, s th sidbar on flip windows (pag 12). Spctral Slction by Dirct Solar Rflction Thr is a scond basic way of rjcting th solar NIR radiation by working dirctly on this radiation without convrting it to hat first. In this approach, th rjction is achivd by a nw typ of coating rathr than by absorption in th glass. This coating is mad to hav a high rflctanc ovr th solar Window for hot climats Rflctd infrard radiation from th sun, kping it outdoors Transmittd visibl radiation from th sun, sky and surrounding objcts, providing viw and daylight illumination Figur 9. A window with spctrally slctiv optical proprtis for hot climats rflcts th NIR componnt of incidnt solar radiation whil admitting visibl radiation. nar IR. Th transmittanc of this coating is high ovr th visibl portion of th spctrum for good viw and daylight admission, but low ovr th NIR to rduc solar hating of th intrior. S Fig. 9. Soft Coatings Th bst-prforming of th hot-climat coatings is mad with a procss that rsults in th coatings bing rlativly soft. In consqunc, thy cannot b placd on th outsid of th window glass xposd to rain and wathr nor on th insid of th uncovrd glass, sinc washing th windows could damag thm. As a rsult, ths highly spctrally slctiv soft coatings ar put insid a two-pan sald glazing unit, to protct thm from damag. Th addd insulation of th prmanntly trappd gas in th gap has othr bnfits as wll. Ths bnfits ar dscribd subsquntly, as ar th liabilitis. Hard Coatings Som manufacturrs sll toughr coatings (now typically manufacturd by a lowr-cost pyrolytic procss), but thir spctral prformanc is gnrally not quit as good as that of th softr coatings. With just a littl sacrific on th nrgy prformanc, howvr, choosing a singl-pan window with a hard coating is a good option in th Florida climat. Singl pan windows ar not as costly as doubl pan ons. As th tchnology of making ths coatings advancs, so too will thir nrgy prformanc, causing thm in many ways to approach th idal. If on of ths hard coatings is usd, thn th scond pan bcoms functionally optional it is not rquird for this principl to work. But bcaus prsnt-day hard coatings hav far from idal NIR rflction charactristics, th glass undr thm gts somwhat hot and a scond clar pan is still addd to isolat this hat from th intrior and its occupants. Coating Trminology Th trm low- clarly nds qualification, spcially whn its hot-climat applications ar addd. For th coldclimat vrsion, it is only rquird that th coating s mittanc b low (Fig. 8a), or quivalntly, its rflctanc high (Fig. 8b), ovr th far infrard portion of th Trminology - continud on pag 15
10 Cost Incntivs On can xpct to pay mor for an nrgy fficint window or shading systm. Th xtra initial cost will b offst by savings on th monthly nrgy bill. Aftr-thfact window upgrads always ntail a pric prmium. Th payback tim is th tim it taks for ths savings to qual or xcd th xtra cost of th bttr windows. (Dividing th initial cost by th annual dollar savings yilds th simpl, as opposd to discountd, payback tim in yars.) Payback tims of only a fw yars can b vry complling to a homownr, spcially whn it is pointd out that th window lasts, and continus to sav mony, for many mor yars into th futur. On can vn gt spcial mortgags for nrgy-fficint homs and spcial hom improvmnt loans for nrgy fficint rplacmnt windows. Othr incntivs ar availabl from tim to tim. Somtims lctric utility companis offr financial or othr incntivs for slcting nrgy-fficint windows. Whatvr discounts, subsidis, or rbats ar availabl, th total nt cost to th homownr, including installation costs, should b dividd by th anticipatd monthly nrgy savings whn computing payback tim. Mor sophisticatd masurs of lif-cycl costs ar availabl, but a good simpl payback tim is likly to b rflctd as also favorabl whn othr, mor accurat, lif-cycl costs ar calculatd. As mntiond, bttr windows and shads can mak it possibl to install a smallr air conditionr (A/C). In this cas, som or all of th xtra cost of th bttr window can b offst by th rducd air conditionr cost. Thr ar cass whr th rducd A/C cost mor than pays for th xtra window costs, making th payback tim zro. Th window has paid for itslf th instant it is installd in such cass, and all futur nrgy savings ar compltly fr of cost. Th homownr pays nrgy bills monthly. During tims of pak lctrical dmand, such as during a nighttim wintr cold front or on a vry hot summr aftrnoon, th homownr pays th sam rat pr kilowatt hour of lctricity as at othr tims. It costs th lctric utility mor, howvr, to produc pak load powr. Som utilitis provid incntivs to homownrs for installing dvics or using stratgis that rduc thir pak lctrical dmand. For windows, this usually mans insulatd windows that also limit solar gain. Th nrgy conomics of windows, thrfor, is not asily dtrmind by th homownr alon. Som assistanc in dtrmining window costs and savings is providd on th Efficint Windows Collaborativ wb sit listd at th nd of this publication. Aftr-th-fact window upgrads always ntail a pric prmium. Pak load is a rlativ trm. It dos not occur uniformly for all mmbrs within a powr sharing community. Your domstic pak load may not coincid with your community s pak load, spcially in mixd rsidntial/commrcial rgions. Air conditionr sizing is rlativ too. It nd not amount to th sam hat pumping rquirmnt from idntically built houss. Air conditionr sizing dpnds on your lifstyl. Only th utility itslf xprincs th aggrgat pak dmand of th local ara. (Minor paks may appar in addition to th major on.) During Florida s rlativly fw wintr cold snaps, fficint hot climat windows slightly worsn th daytim pak load to th utility by admitting lss radiant solar hat to warm th intrior. Othr Slction actors Enrgy costs ar not th whol story, sinc a window provids far mor than just nrgy control. Th valuabl assts of bttr visual and thrmal comfort, asthtic dsign, natural daylight, and acoustic isolation ar oftn mor important to th homownr than just a window s nrgy costs. Ths faturs, not asily givn dollar valus, ar difficult to combin with a window s cost accounting in th dcision-making procss. Th homownr knows thir valu, howvr, and may vn b abl to assign a prsonal montary valu to thm. Though most Florida homownrs ar vry conscious of initial pric, thy can oftn b prsuadd to pay th xtra cost of a bttr
11 window whn it combins lowrd nrgy bills along with th othr lss tangibl assts. Fortunatly, most comfort and asthtic faturs of a window ar compatibl with good nrgy consrvation as wll. It is important to raliz that a window can b xpctd to last many yars. Futur nrgy prics cannot b prdictd with clarity, though fw bliv thy will drop. An invstmnt today in nrgy fficint windows can b a valuabl hdg against nrgy pric inflation, shltring th homownr against futur pric incrass. Installing bttr windows can b thought of as a form of insuranc. Thy protct us from futur conomic fluctuations that might mak a hom much mor xpnsiv to oprat. Providing th homownr with an nrgy-fficint window that also lowrs th lctric load of th hom during pak hours bnfits vryon. Th window sllr bnfits through th highr profits gnratd by slling mor xpnsiv windows. Th homownr bnfits through lowrd lctric bills. Th utility bnfits through lowrd pak loads. Socity bnfits through rducd air pollution missions at th powr plant and from th rsulting clanr air for all of us. Window Prformanc Indics Th solar hat transmittanc of a window is masurd by th solar hat gain cofficint, or SHGC. This is th fraction of incidnt solar radiation that ntrs th building as hat gain by all mchanisms. Also, th fraction of sunlight, skylight, and rflctd daylight incidnt on a window that ntrs as light is masurd by th visibl transmittanc. Hot-climat glazing systms ar normally ar intndd to mak th window pan high in VT whil low in SHGC. Th ratio of ths two prformanc indicators is calld th light-to-solar-gain ratio, or LSG LSG = VT. SHGC In gnral, th highr th LSG, th bttr th hotclimat prformanc, with th mphasis bing on lowring th SHGC valu. LSG numbrs gratr than 1.0 should b chosn, and thos xcding 1.5 offr th bst protction from th hating rays of th sun whil still providing good viws of th outdoors and ltting in plnty of daylight. Th LSG valu is sldom publishd by manufacturrs, nor dos it appar dirctly on window nrgy labls. To calculat it, divid th visibl transmittanc of th glazing plus fram by its solar hat gain cofficint two charactristics spcifid by th NFRC that ar publishd by rputabl window manufacturrs. If a window manufacturr dos not publish SHGC valus, but instad th oldr shading cofficint, SC, th rsults usually will b littl diffrnt if SHGC is rplacd by SC in th abov formula. Thr is a caution in this, howvr. Th NFRC valus for both th VT and th SHGC apply to th whol window, including opaqu framing lmnts. It is an ara-wightd sum of th transmittancs of all parts of th window, including glass, opaqu fram, and smi-opaqu parts. Thus, th visibl transmittanc valu usd in calculating th LSG abov should b th NFRC standard on (for th whol window). Whn using SC in th dnominator of th LSG quation, howvr, th numrator should contain th visibl transmittanc of th glass only. High-LSG glass provids th bst nrgy and comfort prformanc in Florida buildings. Th xtra cost for such glass can oftn b offst by smallr air conditionrs (rducd pak load) and lowr monthly lctric bills (rducd avrag nrgy us). Sinc high-lsg glass is most oftn offrd only in doubl pan modls, th xtra bnfits of doubl pan windows ar a bonus. Th solar gain could always b rducd still furthr by lowring VT, whil kping LSG th sam. This should not b ovrdon. A VT blow about 0.3 to 0.35 would look somwhat xtrm.
12 lip Windows for Improvd Wintr Prformanc Th doubl pan absorptiv glazing systm for hot climats has a usful fatur for rgions with pronouncd sasonal swings in climat: if th positions of th two glass pans ar flippd ovr from thir summrtim positions during th cold wintr, th systm convrts to a solar radiant hatr. In th cold-day position, solar radiation passs through th clar outr pan and is absorbd by th innr on, which hats up and dumps most of its hat to th intrior, warming th building. Th low- coating on th innr pan now rducs th radiation of hat from this hot innr pan to th cold outr on, trapping th hat insid. Flipping it back ovr maks it a hot-climat glazing systm sinc th solar hat is now absorbd in th outr pan of glass which is insulatd from th intrior of th building. (Th glazing srvs as ithr a hot- or a cold-climat systm dpnding on its orintation.) A masur of prformanc for a flip window is th asymmtry indx, th ratio of th wintr and summr SHGC s J = SHGC w. SHGC s Flip windows ar manufacturd and sold in svral aras, including Europ, mainly to aid in claning th outsid of a window from th insid, spcially usful in high-ris apartmnts. Such a flippabl window, quippd with a hot-climat glazing systm basd on spctrally slctiv absorption in on of th two pans, can sav nrgy in both hot or cold sasons. Th subtropical Florida pninsula dos not xprinc tru xtrms in summr/wintr climats, but th worldwid application for flip windows is substantial. J-valus of 2.3 hav bn achivd. Wathr Damag Protction Damag from hurricans and othr svr wathr has ld som Florida countis to rquir impact-rsistant windows. Th nw Florida unifid statwid building cod rquirs impact rsistant fnstrations in countis having highst suscptibility to damag. On way of achiving this goal is to us laminatd glass pans. Such pans consist of two shts of glass stuck togthr with a layr of polyvinyl butyral. At last on manufacturr offrs such a glazing systm with a spctrally slctiv (not a convntional low-) hot-climat coating sandwichd in th middl of th laminatd glass. Othr manufacturrs ar xpctd to follow with similar or altrnat mans of rducing th SHGC (without ovrly rducing th VT). In addition to th damag protction providd, this is an xcllnt way to achiv som of th bnfits of a spctrally slctiv coating without having to rsort to full doubl-pan windows just for solar gain control. Th othr accptd way to mt th damag protction rquirmnts of th building cod is with xtrior shads and shuttrs that can b drawn or rolld tightly ovr th window, protcting it in tims of svr wathr. This option has th bnfits for nrgy consrvation dtaild prviously for shading dvics. Sinc som form of damag protction is rquird in much of Florida, alrt homownrs will compar th rlativ costs of damag-rsistant glazing vrsus attachd xtrior shads or shuttrs. This comparison, to b complt, should includ th pric of rducd air conditionr installation and th annual savd nrgy costs for th two as wll.
13 Mor on Shads No mattr how good th window, if it facs ast or wst and is not adquatly shadd, it can produc srious glar, pak A/C loads, and localizd ovr-hating problms insid th building. Th localizd ovr-hating can mak th aras affctd vry uncomfortabl for rsidnts. Ths ar th rasons that th us of shading dvics was discussd so xtnsivly at th bginning of this Enrgy Guid. As dscribd prviously, xtrior shads ar mor ffctiv than intrior ons. Shads locatd on th room sid of th window hat up whn th sun shins on thm, vn if thy rflct som of th incidnt sunshin back out through th window. In consqunc, thy can still admit a lot of hat into th building, making th room lss comfortabl and causing th air conditionr to work hardr. Shads locatd outsid, howvr, rlas narly all of th solar hat absorbd by thm to th surrounding air and outsid objcts. For this rason, it is much mor ffctiv to block incidnt solar hat bfor it rachs th window. Intrior shads ar most ffctiv if thy hav high rflctivity ovr th whol solar spctrum on th outward-facing sid. This mans that th outsid apparanc of such shads will b ithr bright whit or shiny. Th inward sid can b just about any color, txtur, and pattrn wantd. If th outsid apparanc of th shad is mirror-lik or xcssivly shiny, this can b an asthtic problm to som popl, so it is probably bst to hav a dullr, diffusly rflcting whit shad. Som shads offr a partially or fully trappd air spac btwn th outward- and inward-facing sids, and should bttr kp th hat that is absorbd by th shad s outr surfac away from th intrior air. This rducs th load on th air conditionr and maks th shad mor comfortabl to sit nar. (For bst rsults with such shads, th dgs should fathr sal to th dgs of th window, trapping th warm air btwn th shad and th window. In this cas, th hat so trappd has a bttr chanc of bing conductd through th window to th outsid.) Window Orintation and Shading Th windows chosn for a hous should rflct th individual outdoor conditions in th dirctions thy fac. An xtrior that is gnrally dark, for xampl, with rducd viws of th sky and th sun, probably rquirs high visibl transmittanc glass to provid good viw and to admit rflctd daylight. Although a low SHGC is gnrally dsirabl, it is lss important in this cas du to th lssr solar radiation incidnt on th window. If, on th othr hand, th window looks out onto clar opn sky and brightly rflcting surrounding objcts, it is bst to hav a lowr visibl transmittanc, and as low a SHGC as possibl. (High-LSG glass is not th primary goal hr.) Again, adding shads to th window can modrat this sky brightnss, rducing th nd for th low transmittanc and low SHGC glass. Wll-shadd windows probably do not bnfit gratly from high-lsg glass, so th xtra cost of this glazing fatur can b avoidd with such windows. Whn choosing oprabl shads, howvr, it is bst to slct high-lsg glass for protction during th tims whn occupants forgt to, or choos not to, clos th shads. Grn Windows Many consumrs sm willing to pay mor for a product labld grn. Th achivmnt of a claim to nvironmntal frindlinss, as judgd by an indpndnt agncy, can do wondrs in th markting of nrgyfficint windows. Th grn buildings movmnt is basd on a prsumption that nrgy fficint buildings ar good for th nvironmnt. Of cours if th numbr of buildings incrass mor than thir fficincy, th nt impact on th nvironmnt is ngativ. On an hypothtical basis at last, it is clar that by saving nrgy, a building would b rsponsibl for lss air and watr pollution at th powr plant, lss dpltion of fossil fuls and its attndant consquncs (or rducd advrs consquncs of nuclar powr production), and rducd missions of crtain gass.
14 Th lattr mans smallr contributions to th inadvrtnt climat modification that has bn linkd to global warming. Ths bnfits, thrfor, would also accru from any nrgy fficint windows prsnt in a building. Against ths bnfits must b balancd th xtra nrgy and matrials costs rquird to manufactur high prformanc windows. On way of viwing this nvironmntal impact would b to claim that th xtra nvironmntal costs attributabl to th manufacturing of nrgy-fficint windows ar mor than balancd by thir nvironmntal lif-cycl savings. Such argumnts ar pron to rrors and misrprsntations, but ar currntly popular in today s cultur. If you dsir to markt or promot nrgyfficint windows as grn, at last b mindful of th words of Chris Hayhurst, writing in Look for th Labl in th May/Jun 2000 issu of E MAGAZINE. Still, don t just dash into th stor and fall for vry nvironmntally frindly labl you s. Not all claims of nvironmntal or social good ar lgitimat. A gnral rul: If th labl coms dirctly from th manufacturr or from a privat trad association, procd with caution. It may b a grnwashing, unjustifid nvironmntal claims. On th othr hand, if th labl has bn awardd by an indpndnt, third-party program with no vstd intrst in th product itslf, it s probably for ral. Summary Rcommndations In slcting windows for th Florida hom, th first thing to considr is thir orintation rlativ to th points of th compass. In nw construction, this lads to a fw simpl rcommndations. Minimiz ast - and wst - xposurs, which ar difficult to shad arly or lat in th day. Don t worry too much about north - facing windows (th glazing rcommndations blow should b sufficint). Us a rasonably-sizd roof ovrhang to protct south - facing windows and a modst on on th north sid. Kp natural shading. Don t cut down shad trs prior to construction. Locat th rsidnc to tak fr advantag of tr shading, spcially from thir uppr canopis. Exprinc has shown that th bst xtrior shad for ast - or wst - facing windows is a tall tr full with lavs. If it is som distanc from th building, it will provid a nic scn to viw whn th sun is on th othr sid of th hous. It will also block low sun arly in th morning or lat in th aftrnoon. Blocking th sun s rays bfor thy rach th window is th most ffctiv prvntiv stratgy that can b usd. Natur is th bst landscap architct. (Dciduous trs ar an ffctiv shading dvic for south-facing windows in climatic zons with substantial wintr/summr tmpratur diffrncs. Thy provid good shad in summr but drop thir lavs in wintr to admit solar radiation into th dwlling. Dciduous trs ar of lss pronouncd bnfit for Florida, howvr, du to our mild wintrs and high summr sun angls: for south - facing windows, dirct summr sun is asily blockd by roof ovrhangs.) For guidanc on roof ovrhangs, s FSEC publication Roof Ovrhangs and Solar Tim, R. McCluny, FSEC-RM-5-80, (Supplmnt to Sun Position in Florida, FSEC-DN-4-83, Rv. March, 1985. If ast - or wst - facing windows cannot b avoidd, ithr bcaus th building alrady xists or bcaus th bst viws ar in thos dirctions, thn follow our graduatd rcommndations for protcting thm, and th occupants, from th hat and glar ffcts of dirct sun ntry into th hous. In ths rcommndations, tall xtrior trs, sparatd from th hous, wr mphasizd first; th rmaining rcommndations follow. Th nxt bst outdoor shad for ast - or wst - facing windows is a tall xisting building, opaqu fnc, tall and dnsly foliatd hdg row, or possibly an ovrhad trllis filld with grn vins. Thn coms th larg varity of awnings and othr xtrior shads attachd to th window, as shown in
15 Fig. 2. If ffctiv shading alrady xists for th windows, uninsulatd windows with singl pan clar glass can b purchasd. Extrior shads hav th valu that hat absorbd by thm is carrid away by th winds and radiation, and dosn t hat up th hous as much as an intrior on might do. Th nxt bst shad is a brightly-rflcting oprabl intrior shad that can b closd to rflct unwantd solar hat and glar, and opnd for good viws and daylight accss. Th final choic to b mad is th kind of glazing for th window. If modrat amounts of dirct-bam sunlight can rach this glass during th hottst parts of th day, thn high- LSG glass should b usd in this window, prfrably singl pan, if such a product is availabl. If th high-lsg valu chosn is only availabl in a soft-coat IGU, th homownr will hav no choic but to us an IGU. Ths doubl-pan insulatd windows may b dsird for othr rasons, such as improvd comfort on cold nights or hot days, to rduc th siz and cost of th air conditioning systm, for acoustic isolation, or to protct against possibl futur addd costs from th utility company if clar, singl-pan glass wr usd. (If th building was built bfor th nw costs or othr pnaltis for using uninsulatd windows ar imposd, th building may b grand-fathrd in xmptd from th nw rquirmnts.) Finally, not that although plastic, applid window films ar a common rtrofit choic for modrating glar and ovrhating problms on ast - and wst - facing windows, thy ar lss ffctiv than th prvious rcommndations. In ordr for thm to block sufficint solar hat, thy gnrally ar quit dark, and hav low visibl transmittancs. If thy ar dark nough to provid ffctiv shilding from th glar and hat of dirct sunlight, thy may b so dark whn th sun is on th othr sid of th hous that thy badly dgrad th viw. Of cours, window film manufacturrs ar continuously improving th films spctrally slctiv prformanc, and window films applid to singl pan clar glass can hav LSG ratios a littl gratr than 1.0, but ths latst products may not b widly promotd by rtailrs possibly for rasons of cost. If th homownr is considring window film, it is important to raliz that som window manufacturrs void thir warrantis if window films ar applid to thir products. Th main rason is that most window films accomplish thir solar glar rduction by absorbing solar radiation insid th film, causing it to hat up. Som of th hat in th film will conduct to th glass on which it is applid, causing this glass to xpand. If it xpands nough, strsss can build up in th window and caus it to fail, most oftn through a shattring of th glass. To s comparisons of som nrgy-savings for diffrnt window typs in Miami, Tampa, and Jacksonvill, visit th wb sit of th Efficint Windows Collaborativ at http://www.fficintwindows.org. This sit also offrs a spcial fact sht, Slcting Efficint Windows for Homs in Florida, that can b downloadd and printd. Computr softwar for making such calculations is dscribd at http://www.fficintwindows.org/softwar.html. Trminology - continud from pag 9 spctrum. Th mittanc can vary outsid that particular rang. Indiscriminat us of th trm low- can lad to misundrstanding bcaus th xact lowr wavlngth at which th mittanc dparts from its low valu, as th wavlngth dcrass, is crucial to th diffrnt cold/hot applications of th low- principl. (Equivalntly, in trms of th rflctanc, for th hot-climat vrsion th rflctanc must b high ovr both th NIR and FIR rgions.)
16 or Mor Information Choosing th Bst Rsidntial Window Options for th Florida Climat Guid for Florida Homownrs, R. McCluny and P. Jindra, FSEC Enrgy Not: FSEC-PF-358-00 (to b publishd). Contact th Efficint Windows Collaborativ, Allianc to Sav Enrgy, 1200 18 th St., N.W., Suit 900, Washington, DC 20036 (Phon: 202-857-0666) and visit th fnstrations wb sit of th Florida Solar Enrgy Cntr at http://www.fsc.ucf.du/~fn/. This publication and othrs can b found thr as wll. Th following somwhat mor tchnical publications ar availabl from th Florida Solar Enrgy Cntr Publications Offic. Low-E and Othr Coatings and Glass Tints for Window Glass, R. McCluny and P. Jindra, Florida Solar Enrgy Cntr Tchnical Not: (FSEC-PF-359-00) (to b publishd). Rbuilding for Efficincy: Improving th Enrgy Us of Rconstructd Rsidncs in South Florida, coauthors D. Parkr, P. Fairy, C. Guymard, R. McCluny, J. McIlvain, and T. Stdman, Dcmbr 1992, prpard for U. S. Dpartmnt of Enrgy, Florida Enrgy Offic, and Florida Powr & Light. (FSEC-CR-562-92). Slcting Windows for South Florida Rsidncs, R. McCluny and C. Guymard, prpard for Florida Enrgy Offic, 2740 Cntrviw Dr., Tallahass, FL 32399, 15 January 1993, (FSEC-CR-577-93). Fnstration Solar Gain Analysis, Florida Solar Enrgy Cntr, 1 Novmbr 1996. (FSEC-GP-65-96). Lt Thr B Daylight, Window Rhabilitation Guid for Historic Buildings, Th Window Confrnc and Exposition for Historic Buildings II, National Park Srvic, Washington, DC, 19-21 Fbruary 1997, pp. IV-45-54. Rsidntial Windows: A Guid to Nw Tchnologis and Enrgy Prformanc, co-authors Carmody, John; Slkowitz, Stphn; Hschong, Lisa. W. W. Norton & Company, Inc., 550 Fifth Av., NY, NY 10110 (1996). 214 pp. Th bnfits of using window shads, R. McCluny and L. Mills, ASHRAE Journal, Novmbr 1993, Vol. 35, No. 11, pp. 20-26. (FSEC-PF-263-93). Choosing th Bst Windows for Hot Climats, Ross McCluny, Proc. Innovativ Housing 93 Confrnc, Vancouvr, BC, Canada, 21-25 Jun 1993, pp. 366-375. (FSEC-PF-253-93) Daylighting in Amrica. Som Practical Suggstions for Propr Usag, Ross McCluny, Lighting Dsign & Application, Vol. 15, No. 7, July 1985, pp. 36-38. (FSEC-PF-88-85). A Daylighting Chcklist, Ross McCluny, Solar Ag Magazin, April 1985, p. 84. Som of ths paprs, and additional information including this Enrgy Not, ar availabl on th FSEC Fnstration Wb sit: http://www.fsc.ucf.du/~fn/
lorida Solar Enrgy Cntr 1679 Clarlak Road Cocoa, Florida 32922-5703 lorida Solar Enrgy Cntr is a rsarch institut of th Univrsity of Cntral lorida M2-06