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Performance issues with muntin bars in sealed insulating glass units
Elmahdy, A. H.
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b y A .H. Elm a hd y
Muntin bars have become popular as a decorative element in insulating
glass units. This Update discusses some performance issues encountered
w ith the use of muntin bars, w hich can actually undermine the quest for
aesthetics.
C o n s t r u c t i o n T e c h n o l o g y U p d a t e N o . 2 8
In n ovation s in th e d esign an d m an u factu re of in su latin g glass u n its (IGUs) over th e p ast d ecad e h ave im p roved th eir th erm al p erform an ce an d d u rability. Th e in corp ora-tion of m u n tin bars betw een th e sh eets of glass in an IGU, h ow ever, is a d evelop m en t th at is m erely d ecorative, in th at th ey sim u -late th e look of old colon ial w in d ow s. Th e grille created by th e h orizon tal an d vertical m u n tin bars (as sh ow n in Figu re 1) gives th e ap p earan ce of tru e d ivid ed ligh ts w ith ou t actu ally p en etratin g th e glass or d ivid in g it in to sep arate ligh ts. Recen t IRC research h as d eterm in ed th at th e p resen ce of m u n tin bars can red u ce th e R-valu e an d con d en sation resistan ce of an IGU.1 It w as
fou n d as w ell th at certain m u n tin bars release volatile organ ic m aterials u p on exp osu re to u ltraviolet rad iation , cau sin g foggin g of th e glass.
Te st s for R-Va lue a nd Conde nsa t ion Re sist a nc e
Six IGUs w ere tested for R-valu e, in terior glass su rface tem p eratu re an d tem -p eratu re in d ex (con d en sation resis-tan ce). Test u n its w ere 1 m x 1 m , d ou ble-glazed , eith er air- or argon -filled , with either aluminum or vin yl m u n tin bars,
an d a low -em issivity (low -e) coatin g on on e su rface of th e glass. Tw o con trol u n its (n o bars) w ere also tested . (See Table 1.)
Results
Table 2 p rovid es a su m m ary of th e R-valu e test resu lts. Th e R-valu es of th e u n its w ith m u n tin bars w ere fou n d to be 2.3% to 7.3% low er th an th ose of th e u n its w ith ou t m u n tin bars. Th e red u ction in R-valu e is d u e to th erm al brid gin g in th e area of th e glazin g u n it ad jacen t to th e m u n tin bar. Th is can be exp ected becau se th e m u n tin bars, m ad e of m aterials su ch as alu m in u m (anodized or painted), vinyl, or silicone foam, u su ally h ave a h igh er th erm al con d u ctan ce th an th e air in th e cavity. (Alth ou gh n ot a p rim e con sid eration h ere, th e gas-filled u n its h ad abou t a 7% h igh er R-valu e th an th e air-filled u n its, con sisten t w ith p reviou s research .)
Figu res 1 an d 2 sh ow th e glass su rface tem p eratu res an d th e tem p eratu re in d exes for tw o of th e six IG u n its.
Performance Issues w ith
Muntin Bars in Sealed
Insulating Glass Units
Th e tem p eratu re in d ex, w h ich relates th e in terior su rface tem p eratu re of a w in d ow to th e exterior tem p eratu re an d th e th er-m al ch aracteristics of th e w in d ow, is a m easu re of con d en sation resistan ce. (See “Prin cip les of Con d en sation Con trol” in Con stru ction Tech n ology Up d ate No. 5.)2
Table 1.Test conditions
Muntin bar type IGU
filling None Aluminum Vinyl (control)
Air
•
•
•
2
Wh ere large tem p eratu re grad ien ts across th e air cavity of IGUs occu r, u n its w ith m u n tin bars m ay exh ibit low er glass su rface tem p eratu res th an u n its w ith ou t m u n tin bars, p articu larly in th e areas ad jacen t to th e m u n tin bar. Wh en air circu lates in sid e th e cavity of an IGU u n d er th e in flu en ce of n atu ral con vection , a localized h igh air velocity exists betw een th e glass an d th e
su rface of th e m u n tin bar. Th e resu lt is a h igh er rate of h eat tran sfer th rou gh th e glass, an d h en ce, a low er glass su rface tem p eratu re. Th is occu rs m ain ly in th e vicin -ity of th e m u n tin bars. A low er glass tem p eratu re m ay low er th e w in d ow ’s resis-tan ce to con d en sation so th at con d en sation is m ore likely to form , esp ecially in th e vicin ity of th e m u n tin bars.
Th e glass su rface tem p eratu re an d tem -p eratu re in d ex at th e location of th e m u n tin bar are su bstan tially low er th an th ose at a sim ilar location in an IGU w ith ou t a m u n tin bar. Th e exten t of th e red u ction in glass su rface tem p eratu re an d tem p eratu re in d ex d ep en d s on th e m aterial th at th e m u n tin bar is m ad e of. Un its w ith vin yl m u n tin bars sh ow ed a low er glass su rface tem p eratu re in th e area ad jacen t to th e bars th an u n its w ith alu m in u m bars. Th is d oes n ot m ean th at vin yl h as a h igh er th erm al con d u ctivity th an alu m in u m ; it d oes, h ow -ever, in d icate th at th e en viron m en t arou n d vin yl m u n tin bars is m ore con d u cive to con d en sation th an th at arou n d alu m in u m bars. It is su sp ected th at th e gap betw een th e m u n tin bar an d th e glass su rface is n ot th e sam e in each case, or th at in som e cases th e bar m ay actu ally h ave tou ch ed th e glass su rface. (Un d er large tem p eratu re grad ien ts across th e IGU cavity, th e p ressu re in sid e th e cavity d ecreases, cau sin g th e glass to d eflect in w ard s. Th is can resu lt in a d ecrease in th e gap betw een th e glass su rface an d th e m u n tin bar.)
Fogging Te st s
As p art of n orm al qu ality con trol, IGUs u n d ergo an exten sive testin g p rogram to exam in e th e seal in tegrity an d th eir overall d u rability. Th e Can ad ian (CGSB 1990) an d American (ASTM E773, E774, 1994) standards u sed d o n ot in clu d e sp ecific tests for u n its equ ip p ed w ith m u n tin bars an d h en ce IGUs are bein g in stalled in bu ild in gs w ith ou t testin g in accord an ce w ith an y recogn ized stan d ard s. (Note: th e Can ad ian stan d ard is n ow bein g revised to cover m u n tin bars.)
Ultraviolet (“foggin g”) tests w ere con -d u cte-d on 54 IGUs, rep resen tin g m an y vari-ation s in d esign an d con figu rvari-ation : sp acer bars m ad e of m etal, corru gated m etal strip an d silicon e foam , an d m u n tin bars m ad e of alu m in u m , vin yl an d silicon e foam .
The volatile (fogging) test serves to identify th e p oten tial for failu re d u e to d egrad ation of th e organ ic m aterials an d su bsequ en t release of volatile com p ou n d s, w h ich form ch em ical d ep osits in th e IGUs. Th is test
12.0 (77%) 11.8 (76%) 7.8 (66%) 8.0 (67%) 8.0 (67%) 11.3 (75%) 11.1 (75%) 9.9 (72%) 8.0 (67%) 7.6 (66%) 11.4 (75%) 11.5 (76%) 11.1 (75%) 8.1 (70%) 9.2 (70%) 8.8 (69%) 8.4 (68%) 9.5 (71%) 8.5 (68%) 8.7 (68%) 12.1 (77%) N/A 11.5 (76%) 11.3 (75%) 11.6 (76%) 11.6 (76%) 11.2 (75%) 11.3 (75%) 11.6 (76%) 11.4 (75%) 11.7 (76%) 11.4 (75%) 11.2 (75%) 11.8 (76%) 12.1 (77%) 11.4 (75%) 11.4 (75%) 11.4 (75%) 11.3 (75%) 12.1 (77%)
Figure 1.Glass surface temperatures and temperature indexes for air-filled, vinyl grille unit
Figure 2.Glass surface temperatures and temperature indexes for air-filled, no grille control unit
3
Table 2.R-value test results
w as p erform ed accord in g to th e p roced u re d escribed in th e CAN/ CGSB 12.8 stan d ard (CGSB, 1990).3
For each of th e 18 IGU con -figu ration s, th ree u n its w ere tested . Tw o u n its of each set con tain ed a low -e coatin g on on e glass p an e, w h ile th e th ird u n it — th e con trol u n it — w as m ad e u p of clear glass on both p an es. For th e test, th e u n its were exposed for seven days to u ltraviolet rad iation em itted from a stan d ard su n lam p w ith a m in im u m ou tp u t of 0.4 m W/ cm2 w h en m easu red
w ith a su n lam p tester at a
d istan ce of 300 m m (see Figu re 3). Th e u n its w ere m ou n ted in a box equ ip p ed w ith th e su n lam p an d a cold p late. Wh en th e u ltraviolet rad iation is ap p lied , volatile m aterials evap orate. Th ese m aterials th en con d en se u n d er th e cold p late. (Du rin g th e test, th e cold p late w as p laced on th e su r-face of th e low -e glass p an e (for on e u n it) an d on th e clear glass p an e of th e secon d coated glass u n it. Th is w as d on e to exam -in e th e -in flu en ce of th e low -e coat-in g on th e d etection of th e volatile d ep osits.)
After exp osu re, th e u n its w ere exam in ed in d ivid u ally in a view in g box for th e p res-en ce of ch em ical d ep osits. In p ractice, an IGU fails th e stan d ard foggin g test w h en th e op erator, view in g at a righ t an gle, observes an y oily d ep osits or traces of foggin g on th e su rface of th e glass. Wh en view ed at a righ t an gle, m ost IGUs p ass th e test. In th e IRC tests, view in g w as d on e at both a righ t an gle an d an off-an gle. Wh en view ed at a righ t an gle, m ost of th e u n its tested by IRC sh ow ed n o sign of d ep osits. How ever, w h en view ed at an off-an gle, m an y u n its sh ow ed traces of d ep osits becau se th e reflection / refraction of ligh t th rou gh th e th in film d ep osits m akes th em m ore visible.
In the case where the cold plate was placed on th e low -e glass p an e, volatile m aterials w ere observed u n d er th e p late (in d icatin g failu re). Th e con trol u n its (n o coatin g) sh ow ed n o foggin g. Th e low -e coatin g evd en tly en h an ces th e visibility of th e ch em i-cal d ep osits. Th is can be ascribed to th e coatin g’s relatively rou gh su rface (at th e m icroscop ic level) com p ared to th e su rface of float glass. If th e d ep osits are sm aller th an th e valleys in th e low -e coatin g, th e d ep osits ten d to settle in th ese valleys an d becom e m ore visible.
Unit description Argon R-value Percentage concentration m2• K/W variation from the
(%) control unit (%)
Air filled , n o grille (con trol u n it for air-filled u n its) N/ A 0.41 N/ A Air filled , w h ite alu m in u m grille (3 x 3)* N/ A 0.38 –7.3 Argon filled , w h ite alu m in u m grille (3 x 3) 86.0 0.43 –2.3 Air filled , w h ite vin yl grille (3 x 3) N/ A 0.38 –7.3 Argon filled , w h ite vin yl grille (3 x 3) 92.1 0.42 –4.5
Argon filled , n o grille (con trol u n it for argon -filled u n its) 83.6 0.44 7.3 relative to air filled
Figure 3.Volatile (fogging) exposure box
Fan Auto transform to fan Auto transform to lamp Air flow 150-mm diameter cooling plates – 2 required Thermocouple (Tc)
Insulating glass unit 350 x 500 mm G.E. Sunlamp RS 275 W Power socket 250 mm 550 mm Cooling water temperature determined at this point Tc Tc Tc Water in Out
Plywood box lined with aluminum foil
70-mm hole
* 3 x 3 grille means that the IG unit is divided into three sections (vertical) and three sections (horizontal) by means of two vertical muntin bars and two horizontal muntin bars (see Figure 1).
Both th e Can ad ian an d Am erican stan d ard s on th e d u rability of IG u n its in clu d e a test to assess the degradation of organic m aterials in th e IGU on exp osu re to UV rad ia-tion . How ever, th ere are d ifferen ces in th e standards: The American stan d ard calls for off-an gle view in g (w h ereas th e Can ad ian stan d ard calls for straight-on view-in g) bu t requ ires less tim e for UV exp osu re th an th e Can ad ian on e.
Material Degradation With UV Exposure Th e d egrad ation of organ ic com p ou n d s u n d er u ltraviolet (UV) exp osu re h as been a kn ow n p h en om en on for years. IGUs m ay fail th e test for foggin g d ep osits w h eth er or n ot th ere are m u n tin bars in th e cavity. Th is is becau se organ ic m aterials are u sed in m ost com m on sealan ts or oth er com p o-n eo-n ts io-n th e IGU. Th e io-n corp oratioo-n of m u n tin bars in trod u ces an ad d ition al sou rce of volatile m aterial th at m ay cau se failu re. It is u n likely th at th ere is an y in -p lan t p roblem w ith th e p ain ts th at are baked on or an od ized on th e alu m in u m bars, bu t th ere cou ld be if tou ch -u p p ain t is u sed on flaw s or ch ip s in th e an od ized coat.
Mu n tin bars m ad e of silicon e foam are an excep tion : n o p roblem s w ere d etected w ith th em . With m etal bars, p rep aration in clu d es cu ttin g, u su ally in volvin g th e u se of lu bri-can ts an d coolan ts. Th e resu lt bri-can be th e in trod u ction of oil in to th e IGU. Th is h ap -p en s often , -p articu larly w h en th e m u n tin bars are n ot p rop erly w ash ed after cu ttin g. Oil in th e IGU also cou ld be th e resu lt of h an d lin g th e glass sh eets an d oth er com p o-n eo-n ts w ith coo-n tam io-n ated gloves or fio-n gers.
More research is needed to determine why IGUs w ith ou t m u n tin bars can also fail th e foggin g test. It is p ossible th at th e sealan ts, ed ge seals, or d essican ts, or som e com bin a-tion th ereof, are resp on sible for th e failu re.
I m plic a t ions for t he I ndust ry
Th e p resen ce of m u n tin bars in sid e th e IGUs h as con sid erable effects on th e in terior glass su rface tem p eratu re an d th e tem p eratu re in d ex of th e IGUs. Th e red u ction in glass su rface tem p eratu res on th e w arm sid e ad jacen t to th e m u n tin bar red u ces th e con -d en sation resistan ce in th at area. (Th e p resen ce of th e m u n tin bar lead s to a sm aller gap w ith in th e IGU, w h ich in creases th e air velocity in th e cavity, in creasin g th e th erm al coefficien t an d d ecreasin g th e su rface tem p eratu re of th e glass.) Th ese red u ction s are n ot large, h ow ever, an d in all likelih ood m ost h om eow n ers w ou ld n ot n otice an y d ifferen ce in h eatin g bills as a resu lt.
Th e issu e of foggin g m ay be of greater con cern . Most h om eow n ers w ou ld n otice foggin g of th e u n its becau se th ey w ou ld n orm ally look ou t th eir w in d ow s at m an y d ifferen t an gles. Th ey m ay believe th at th e foggin g p roblem is on e of con d en sation
betw een th e p an es cau sed by a failed seal. Un til fu rth er research is d on e, arch itects, bu ild ers an d ren ovators sh ou ld th in k care-fu lly abou t w h eth er or n ot to u se IGUs w ith in tern al m u n tin bars, as th ey in trod u ce an ad d ition al sou rce of volatile m aterial th at can cau se aesth etic failu re.
As an altern ative, th e aesth etic of m u n tin bars can be ach ieved by in stallin g w ood grilles on th e room sid e of th e w in d ow s. Th e grilles can h ave a solid fram e th at is p in n ed tigh tly to th e sash . Th e w in d ow w ou ld , of cou rse, be m ore d ifficu lt to clean on th e in sid e.
Sum m a ry
Tests con d u cted by IRC on in su latin g glass u n its w ith m u n tin bars betw een th e p an es revealed p erform an ce issu es th at are a con -cern , su ch as low er R-valu es, low er in terior glass su rface tem p eratu res an d h en ce low er con d en sation resistan ce in th e vicin ity of th e m u n tin bars. An ad d ition al p roblem id en tified w as foggin g betw een th e p an es th at occu rred becau se of th e release of volatile organ ic m aterials from th e m u n tin bars on exp osu re to u ltraviolet rad iation .
Re fe re nc e s
1. Elm ah d y, A.H. Th erm al Ch aracteristics an d Du rability of Sealed In su latin g Glass Un its In corp oratin g Mu n tin Bars Un d er Ultraviolet Exp osu re. ASHRAE
Tran saction s 1998, V. 104, Part 1, Am erican Society of Heatin g, Refrigeratin g an d Air-Con d ition in g En gin eers, Atlan ta, GA, 1998, 8 p . 2. Brow n , W.C. Win d ow Con d en sation in
Historic Bu ild in gs th at Have Been Ad ap ted for New Uses. Con stru ction Tech n ology Up d ate No. 5, In stitu te for Research in Con stru ction , Nation al Research Cou n cil of Can ad a, 1997, 4 p . 3. CAN/ CGSB 12.8 (CGSB 1990), In su latin g
Glass Un its, Can ad ian Gen eral Stan d ard s Board , Ottaw a, 1990.
Dr. A .H. Elm a hd yis a sen ior research officer in th e Bu ild in g En velop e an d S tru ctu re Program of th e N ation al Research Cou n cil’s In stitu te for Research in Con stru ction .
“Construction Te chnology Up d a te s” is a se rie s of te chnica l a rticle s conta ining p ra ctica l inform a tion d istille d from re ce nt construction re se a rch.
For more information, contact Institute for Research in Construction, National Research Council of Canada, Ottaw a K1A 0R6
Telephone: (613) 993-2607; Facsimile: (613) 952-7673; Internet: http://irc.nrc-cnrc.gc.ca
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Nation al Research Cou n cil of Can ad a Decem ber 1999
