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Pressure equalization in rainscreen wall systems
Rousseau, M. Z.; Poirier, G. F.; Brown, W. C.
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NRC Publications Record / Notice d'Archives des publications de CNRC:
https://nrc-publications.canada.ca/eng/view/object/?id=dfd381a2-df13-4f88-8ee8-7d0773756a36 https://publications-cnrc.canada.ca/fra/voir/objet/?id=dfd381a2-df13-4f88-8ee8-7d0773756a36b y M.Z . Rousse a u, G.F. Poirie r a nd W.C. Brow n
A pressure-equalized rainscreen (PER) wall is a multiple-line-of-defence approach
to rain penetration control. This Update defines pressure equalization and
discusses the various elements that must be incorporated in a PER w all to
minimize rain penetration due to air pressure differentials.
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 . 1 7
Rain can en ter on ly if a com bin ation of th e follow in g th ree con d ition s are p resen t: rain w ater d ep osited on th e w all, h oles an d cracks offerin g w ater leakage p ath s in w ard s an d a force or a com bin ation of forces to d rive w ater in w ard s. Strategies for rain p en etration con trol m u st en tail th e con trol of on e or m ore of th ese con tribu tin g factors.
Variou s ap p roach es to rain p en etration con trol are cu rren tly u sed in th e bu ild in g in d u stry. Th ese ran ge from sin gle-lin e-of-d efen ce assem blies (com m on ly callee-of-d face-seal w alls) to m u ltip le-d efen ce assem blies su ch as th ose th at in corp orate a rain screen , a d rain ed air sp ace an d a w ater-resistan t
m em bran e.1 Th e p ressu reequ alized rain -screen (PER) w all d esign is on e of th ese m u lti-d efen ce ap p roach es. It is based on th e op en rain screen p rin cip le,2 w h ich aim s to con trol all forces th at can d rive w ater in to the wall assembly, i.e., air pressure difference, gravity, su rface ten sion , cap illary action , an d rain d rop m om en tu m . Of th ese forces, air p ressu re d ifferen ce is often a d om in an t on e w ith th e p oten tial to d rive a con sid erable am ou n t of rain w ater in to th e w all assem bly. Th is Up d ate th erefore focu ses on th e con trol of air pressure difference across the rainscreen, and the particular elements of wall assemblies in stru m en tal in obtain in g su ch con trol.
De fining Pre ssure Equa liza t ion
Th e p ressu re equ alization con cep t is sim p le: w h en th e ou tsid e air p ressu re is tran sferred to an air sp ace beh in d th e exterior clad d in g, th e clad d in g is exp osed to a n ear-zero p ressu re d ifferen tial. In practice, the wall assembly m u st com p rise th ree com p on en ts (Figu re 1): a rain screen (i.e., ven ted clad d in g), a com p art-m en ted air ch aart-m ber an d an air barrier system . Th e air ch am ber com -p artm en ts m u st be sm all en ou gh , th e air barrier system m u st be airtigh t
Pressure Equalization in
Rainscreen Wall Systems
2
en ou gh , an d th e area of th e ven tin g th rou gh th e rain screen m u st be large en ou gh to allow su fficien t air to m ove in an d ou t of th e com p artm en ts u n d er th e ap p lied air p ressu re. In a n u tsh ell, th e strategy lies in th e con trol of airflow w ith in an d th rou gh th e w all assem bly.
In th eory, p ressu re equ alization m ean s a zero air pressure differential at all times across th e rain screen , i.e., a com p lete elim in ation of th e d rivin g force for p ressu rin d u ced w ater p en e-tration . In p ractice, h ow ever, p erfect p ressu re equ alization across th e rain -screen at all tim es is n eith er ach ievable n or n ecessary for ad equ ate rain p en e-tration con trol (th e w all assem bly m u st be d esign ed to tolerate th e en try of a sm all am ou n t of w ater w ith ou t d am age).
Prelim in ary stu d ies in d icate th at for p racti-cal p u rp oses, “ad equ ate p ressu re equ aliza-tion ” for rain p en etraaliza-tion con trol m ay be d efin ed as n ot m ore th an 25 Pa p ressu re d ifferen tial across th e rain screen .
Wa ll Com pone nt s for Pre ssure Equa liza t ion
Th e follow in g th ree basic com p on en ts m u st be p resen t in a w all assem bly to m in im ize rain p en etration d u e to air p ressu re d iffer-en tials (Figu re 1):
• an effective air barrier system , • an air ch am ber an d
• a rain screen .
Th ese com p on en ts m u st h ave p rop erly d esign ed featu res su ch as ven t h oles in th e
rain screen an d d elim iters to d ivid e th e air ch am ber in to a series of com p artm en ts.
Recen tly, IRC p erform ed som e exp eri-m en tal laboratory stu d ies to ch aracterize th ese featu res for variou s gen eric w all system s. In a p roject join tly sp on sored by Can ad a Mortgage an d Hou sin g Corp oration (CMHC) an d several w all system m an u fac-tu rers, IRC u sed its u n iqu e d yn am ic w all-testin g facility to stu d y th e relation sh ip betw een th e th ree com p on en ts (listed above) as a fu n ction of th e p h ysical ch arac-teristics of d ifferen t w all assem blies su b-jected to static an d d yn am ic air p ressu res. Sp ecim en s of p recast con crete p an els, brick ven eer/ stu d w all assem blies an d exterior in su lation an d fin ish system s (EIFS) w ere exam in ed for th eir p ressu re-equ alization p erform an ce. Som e w ere also su bjected to rain p en etration tests.
An air barrier system
Th e p erform an ce of th e air barrier system affects th e ability of th e w all assem bly to ach ieve p ressu re equ alization across th e rain screen . Th e air barrier system in p lace w ith in th e w all sign ifican tly red u ces th e flow of air th rou gh th e w all assem bly, an d th erefore greatly con tribu tes tow ard red u cin g th e air p ressu re d ifferen tial across th e rain -screen . Un d er d yn am ic-p ressu re con d ition s, recen t IRC stu d ies in d icate th at excessive flexibility of th e air barrier system w ill resu lt in flu ctu ation s in th e volu m e of th e air ch am ber com p artm en t. Th ese flu ctu ation s ad versely affect th e p oten tial for rap id p res-su re equ alization across th e rain screen . Un d er static-p ressu re con d ition s, th e leakage of th e air barrier system is th e d eterm in in g factor for sizin g th e ven tin g requ irem en ts. Th ese fin d in gs em p h asize th e n eed for an air Dyna m ic a nd st a t ic a ir pre ssure s
on t he ra insc re e n
Static (stead y over tim e) air p ressu res on w all assem blies are gen erated by m ech an ical system s an d stack effect w h ile d yn am ic p ressu res (flu ctu atin g qu ickly w ith tim e an d loca-tion ) are cau sed by w in d (Figu re 2). Recen t IRC research in d icates th at d yn am ic as w ell as static air p ressu res m u st be con sid ered w h en ad d ressin g w in d -d riven rain an d th e p ressu re-equ alization p erform an ce of a w all assem bly for rain p en etration con trol.3 Un d er d yn am ic p ressu res, th e
ability of th e assem bly to resp on d qu ick ly to th e ou tsid e flu ctu atin g p ressu re load is critical for ad equ ate p ressu re equ alization ; th is tim e con strain t is n ot an issu e u n d er sta-tic load in g. Th e IRC research sh ow ed th at w all assem blies resp on d d ifferen tly to d yn am ic an d static load in g, resu ltin g in d ifferen t d esign requ irem en ts for th e w all com p on en ts, p articu larly for th e ven tin g of th e rain screen an d th e com -p artm en talization of th e air ch am ber.
Figure 2.Example of wind-induced pressure differential measured across an exterior wall system (Brown et al. Field Testing of pressure-equalized rainscreen walls. ASTM. STP 1034, 1991.)
rigid ity of th eir bou n d aries (i.e., air barrier system , rain screen an d lateral d elim iters) are th e d eterm in in g factors for estim atin g th e ven tin g requ irem en t of th e rain screen . How ever, in w all system s w ith ou t a clear air sp ace, p relim in ary exp erim en tal w ork at IRC su ggests th at th eir p ressu re-equ alization p erform an ce is very sp ecific to th eir p articu lar d esign , an d th at th e d ata available can -n ot be extrap olated to ge-n erate gu id eli-n es.
Th e air ch am ber n eed s to be d ivid ed in to sm aller, sep arate com p artm en ts.2 Rem em ber th at sin ce w in d p ressu re is d yn am ic, th e p ressu re in d u ced on th e bu ild in g façad e varies n ot on ly w ith tim e, bu t also w ith its location on th e façad e. For exam p le, th e air p ressu re in d u ced by w in d can be fairly u n iform n ear th e cen tre of th e w alls, bu t steep grad ien ts (variation s) can d evelop tow ard s th e bu ild in g ed ges an d th e roof lin e.7 Th is sp atial variation in p ressu re can in d u ce lateral airflow w ith in th e ch am ber u n less it is d ivid ed at su itable in tervals. Th e com p artm en tation of th e air ch am ber in to sm aller air com p artm en ts red u ces th e ran ge of w in d -in d u ced p ressu res su stain ed by each of th ese com p artm en ts, resu ltin g in a better p oten tial for p ressu re equ alization across th e rain screen .
In th e 1960s, Gard en2su ggested th at com p artm en ts be sm aller at location s of large p ressu re variation s (su ch as bu ild in g ed ges an d p arap ets) an d larger in location s of sm aller p ressu re grad ien ts, su ch as in th e cen tral p ortion of th e façad e. Based on th ese p rem ises, Gard en su ggested th at com p artm en t h eigh t sh ou ld n ot exceed 6 m (abou t tw o stories) w h ile com p artm en t w id th cou ld be u p to 6 m in th e cen tral p ortion of th e façad e an d abou t 1.2 m at bu ild in g ed ges an d p arap ets. Recen tly, Can ad ian research in volvin g w in d tu n n el stu d ies8 h as been in itiated for th e d evelop m en t of m ore d efin itive gu id elin es on com p artm en t sizes over th e bu ild in g façad e for PER w all system s. Th ese stu d ies con firm th at Gard en ’s ru le-of-th u m b abou t th e location s on a façad e th at are m ost in n eed of of com p art-m en tation is valid ; th e n eed for sart-m all com p artm en ts at p arap et level is also stressed (Figu re 3).9,10
Th e com p artm en t d elim iters close in th e top , bottom an d sid es of th e com p artm en t. Th ese d elim iters n eed to be som ew h at im p erviou s to air an d p rop erly con n ected to th e rain screen an d to th e p lan e of air-tigh tn ess of th e air barrier system in ord er barrier system w ith in th e bu ild in g en velop e,
as called for in th e 1995 ed ition of th e Nation al Bu ild in g Cod e of Can ad a.4
An effective air barrier system h as low air p erm ean ce, stru ctu ral stren gth , an d con -tin u ity over gap s, join ts an d in terfaces.5 Th e air barrier system m u st be d esign ed to resist air p ressu re in d u ced by m ech an ical ven tilation , stack effect an d w in d w h ile still lim itin g air leakage. Th e system m u st also be rigid en ou gh to su stain th ese air p ressu res w ith a resu ltin g d eflection th at can be accom m od ated w ith in th e w all assem bly. Th ese air p ressu res m u st be tran sferred to th e stru ctu re of th e bu ild in g.
An air chamber
Th e air ch am ber is located betw een th e rain screen an d th e air barrier system . Win d p ressu res in d u ced on th e rain screen are tran sferred to th is sp ace as air is d isp laced betw een th e ou tsid e an d th e ch am ber th rou gh th e ven t h oles located in th e rain -screen . Th e air ch am ber can con sist of a clear air sp ace or oth er ad equ ate op tion s. As an exam p le, in a recen t IRC exp erim en -tal stu d y, on e w all assem bly w ith an air ch am ber m ad e w ith a geosyn th etic d im p led p lastic sh eet w ith a geotextile bon d ed to th e d im p les p rovid ed sim ilar p ressu re equ alization p erform an ce to th at of th e assem bly w ith a clear air sp ace. In som e w all system s, sp ecially d esign ed in su lation m aterials are in ten d ed to p rovid e th e requ ired air ch am ber for p ressu re equ aliza-tion p u rp oses: th e air ch am ber m ay be form ed as a grid of n arrow ch an n els w ith in th e in su lation m aterial, or th e air ch am ber can be h ou sed w ith in th e in su lation m aterial itself w h en its p h ysical p rop erties p rovid e th e requ ired “air p erm eability.” In gen eral, for d yn am ic p ressu re equ alization , th e volu m e of th e air com p artm en ts an d th e
IRC’s Can ad ian Con stru ction Materials Cen tre (CCMC) d evelop ed an evalu ation p rotocol to assess th e “effectiven ess” of air barrier system s.6 It covers requ irem en ts for m axim u m
allow able air leakage rates for air barrier system s for w alls of low -rise bu ild in gs, stru ctu ral air p ressu res (static an d d yn am ic) an d m aterial d u rability. CCMC’s allow able air leakage requ irem en ts for air barrier system s are a fu n ction of th e w ater vap ou r p erm ean ce of th e ou term ost n on -ven ted layer of th e w all assem bly. In an y case, th e m axim u m air leakage rate allow able by CCMC for th e air barrier system in exterior w alls of low -rise bu ild in gs is 0.20 L/ (s·m2) at 75 Pa
to create th e requ ired lateral bou n d ary. In form ation on th e p erform an ce of an y m aterial an d assem bly for th at p u rp ose is scarce at th e m om en t. In p rin cip le, w all com p on en ts in p lace for oth er p u rp oses, su ch as m etal sh elf an gles, can act as d elim iters. Rigid sh eet m etal an d foam p lastic in su lation strip s cou ld likely be u sed as d elim iters, as lon g as th ey can be m ad e relatively airtigh t an d can be in stalled to su stain th e lateral air p ressu re load s.
A rainscreen
A rain screen is th e first lin e of d efen ce again st w in d -d riven rain , an d as su ch is su bjected to all forces lead in g to rain p en e-tration th rou gh its op en in gs an d im p erfec-tion s. Pressu re equ alizaerfec-tion across th e rain screen m in im izes w ater en try in to th e w all d u e to on e force, air p ressu re d ifferen -tial. For p ressu re equ alization to take p lace, th e rain screen m u st be ven ted ; th at is, h oles m u st be p resen t in th e rain screen so th at en ou gh air can be exch an ged
betw een th e ou tsid e an d each com p artm en t of th e air ch am ber. Tw o m ajor issu es arise con cern in g th e ven tin g requ irem en ts: th e am ou n t of ven tin g n eed ed for ad equ ate p ressu re equ alization u n d er static an d d yn am ic p ressu res, an d th e p lacem en t of th e ven tin g for each com p artm en t.
How m uch v e nting
Un d er d yn am ic-p ressu re con d ition s, th e rain screen ven tin g requ irem en ts are m ain ly d riven by th e volu m e of air in th e ch am ber com p artm en t, th e resistan ce to airflow at th e ven t h oles, w ith in th e ch am ber as w ell as betw een th e com p artm en ts, an d th e rigid ity of th e w all assem bly. In d eed , th e larger th e volu m e of air in th e com p artm en t,
th e larger th e volu m e of air th at h as to be d isp laced in an d ou t of th e com p artm en t to obtain ad equ ate p ressu re equ alization across th e rain screen ; h en ce, th e larger th e total area of venting. The more rigid the assembly, th e sm aller th e ven tin g area requ ired . Th e
volu m e/ ven tin g ratio of th e w all assem bly,
i.e., th e volu m e of th e com p artm en t d ivid ed by the effective cross-sectional area of the vent h oles, is a critical ch aracteristic of th e assem -bly for ach ievin g d yn am ic-p ressu re equ al-ization . Again , IRC’s lim ited exp erim en tal w ork su ggests th at ch am ber com p artm en ts of sm all volu m e an d h igh rigid ity (su ch as th e sp ecim en illu strated in Figu re 4) sh ou ld h ave a volu m e/ ven tin g ratio of 50 m or less (i.e., ventingRS≥volumeCOMPARTMENT / 50 m ). Chamber compartments larger in volume and less rigid (see sp ecim en in Figu re 5) sh ou ld h ave a volu m e/ ven tin g ratio of 25 m or less (i.e., ventingRS≥volumeCOMPARTMENT / 25 m ). In oth er w ord s, th e sm aller th e volu m e of th e com p artm en t an d th e m ore rigid it is, th e less ven tin g requ ired .
For static-p ressu re equ aliz ation across th e rain screen , th e effective ven tin g n eed ed d ep en d s on th e leakage ch aracteristics of th e air barrier system an d , to a lesser exten t, of th e com p artm en t d elim iters: th e larger th e total leakage op en in gs of th e air barrier system , th e larger th e ven tin g area h as to be. Th is p ressu re equ alization d esign ch aracteristic is gen erally referred to as th e
ven tin g/ leak age ratio of th e w all assem bly,
i.e., th e effective total cross-section al area of th e ven t h oles d ivid ed by th e equ ivalen t leakage area (ELA) of th e air barrier system . Lim ited laboratory exp erim en tation at IRC su ggests th at, for static load in g, th e effective total cross-section al area of op en in gs in th e rain screen sh ou ld be at least 20 tim es th at of th e air leakage area of th e air barrier system (i.e., ven tin g RS≥20 x ELAABS).
Th e ven t h oles m u st be d esign ed to let in air, n ot w ater; i.e., th e op en in gs m u st be sh ield ed from d irect rain en try. Th is sh ield in g red u ces th e free area of th e ven t-in g op en t-in gs an d n eed s to be accou n ted for in th e estim ation of th e effectiven ess of th e ven tin g p rovid ed .
In gen eral, for a w all w ith a p rop erly fu n ction in g air barrier system , th e ven tin g requ ired for d yn am ic p ressu re equ alization across th e rain screen w ill likely be larger th an th at for static p ressu re load in g. In an y even t, after estim atin g both ven tin g requ irem en ts, th e larger of th em sh ou ld be selected for th e d esign of th e ven t h oles
4 Construction Technology Update No. 17
Figure 3.General pattern for façade compart-mentation Area of small compartments at building edges and parapet Area of larger compartments in the centre
An y effective rain p en etration con trol strategy sh ou ld assu m e th at som e rain w ill en ter th e w all at som e tim e d u r-in g th e service life of th e w all assem bly; th at w ater m u st be d isp osed of qu ickly. Th e in n er su rface of th e com p artm en ts m u st be w ater-resistan t, an d th e com p artm en t m u st be d rain ed at th e bottom w ith th e u se of a flash in g.
(see box “Estim atin g requ ired ven tin g”). Th ese gu id elin es d o n ot p rovid e an
absolu te figu re for th e ven tin g requ irem en ts for all situ ation s bu t rath er an ord er of m ag-n itu d e to aim for d u riag-n g th e d esigag-n of p ro-totyp e w all assem blies.
W he re to v e nt
With in a com p artm en t, all th e ven t h oles sh ou ld be at th e sam e h eigh t. Th e com m on ap p roach h as been to d istribu te th e op en -in gs u n iform ly across th e bottom of th e com p artm en t in ord er to obtain a m ean p ressu re in th e com p artm en t close en ou gh to th e ou tsid e p ressu re an yw h ere on th e ou tsid e face of th e corresp on d in g rain screen . Placin g th e ven t h oles at th e bottom of th e com p artm en t p rovid es th e ad d ed ben efit th at th e ven t h oles can also p rovid e u n i-form d rain age of th e com p artm en t.
Wind tunnel studies at the Boundary Layer Win d Tu n n el Laboratory at th e Un iversity of Western On tario h ave sh ow n th at gath erin g all th e op en in gs h orizon tally on th e corn er of th e com p artm en t fu rth est from th e ed ge of th e bu ild in g ten d s to p ressu rize th e com p art-m en t. Th is ap p roach art-m ay offer an extra art-m ar-gin of safety w ith resp ect to rain p en etration con trol, p articu larly at bu ild in g ed ges an d p arap ets exp osed to steep p ressu re grad ien ts. Further investigation of this new development an d its p ractical ap p lication s is w arran ted .11
Sum m a ry
Control the airflow. An effective PER w all
m in im izes th e am ou n t of w ater th at can en ter th e w all assem bly, lim its h ow far w ater can get in to th e assem bly an d p rovid es a d rain age rou te back ou tsid e to red u ce th e len gth of tim e th e w ater rem ain s in bu ild in g m aterials. A PER w all d esign aim s to con trol all forces actin g on a w etted clad d in g su rface. Air p ressu re d ifferen ce across th e exterior clad d in g is con sid ered a sign ifican t force in d rivin g rain in to th e w all. To con trol th is Est im a t ing re quire d ve nt ing
Here is a sim p lified exam p le to illu strate th e step s for estim atin g th e effective ven tin g requ ired for static an d d yn am ic p ressu re equ alization across th e rain screen :
Let u s assu m e a rigid assem bly (su ch as th e w all assem bly of Figu re 4), w ith an air leakage rate of 0.1 L/ s/ m2at 75 Pa corresp on d in g to an Equ ivalen t Leakage Area (ELA) of 28 m m2. Th e com p artm en t volu m e is estim ated to be 0.04 m3.
For static loads:
Ven tin g RS (m2) ≥20 x ELAABS(m2)
Ven tin g RS (m m2) ≥20 x 28 m m2, th at is, 560 m m2 For dynamic loads:
Venting RS (m2)≥volu m e COMPARTMENT (m3)/ 50 m
Ven tin g RS (m2) ≥0.04 m3/ 50 m , th at is, 800 m m2
Com p are th e static an d d yn am ic requ irem en ts, an d select th e larger valu e; in th is p articu lar exam p le, th at is th e d yn am ic ven tin g requ irem en t of 800 m m2for th at com p artm en t.
Figure 4.Example of a rigid assembly with a chamber of small volume
Figure 5.Example of a flexible assembly
Air barrier system – plywood (13 mm) fastened to wood stud frame
Air chamber – air space (25 mm) Rainscreen
– brick veneer (89 mm) and vents Interior
Exterior
Section
In PER w alls, ven t h oles are in stalled in th e rain screen to allow air to m ove in an d ou t of th e air ch am ber u n d er th e ap p lied air p res-su re load so th at th e air p resres-su re d ifferen tial across th e rain screen is m in im ized . Th ese op en in gs are n ot in ten d ed to in d u ce th e “ven -tilation ” of th e ch am ber, th at is, to get a flow of air com in g in at th e bottom of th e com p art-m en t an d coart-m in g ou t at th e top , or vice versa. In fact, it is qu ite th e op p osite: in ord er to con trol rain p en etration d u e to p ressu re d ifferen tial across th e rain screen , th ere m u st be little airflow th rou gh th e ch am ber. For th is reason , all ven t h oles sh ou ld be located at th e sam e h eigh t level of th e com p artm en t.
force, th e air barrier system , th e air ch am ber an d th e rain screen m u st be d esign ed an d bu ilt to w ork togeth er to con trol airflow.
Achieve dynamic and static pressure
equalization. Dyn am ic p ressu re equ
aliza-tion across th e rain screen requ ires d ifferen t w all ch aracteristics th an static p ressu re equ alization . For d yn am ic p ressu re equ al-ization , th e rigid ity of th e com p artm en t bou n d aries, th e volu m e of th e com p artm en t an d th e area of rain screen ven tin g are th e d eterm in in g factors. For static p ressu re equ alization , th e airtigh tn ess of th e air barrier system an d th e area of rain screen ven tin g are im p ortan t.
Build an effective air
barrier system. A good
air barrier system is a key com p on en t of a d u rable, fu n ction in g w all system in m ore th an on e w ay. Th e tigh ter an d th e m ore rigid th e air barrier system , th e less ven tin g requ ired to obtain d yn am ic an d static p res-su re equ alization across th e rain screen .
Compartmentalize the
air chamber. Th e
location s on a bu ild in g façad e exp osed to w in d -d riven rain th at requ ire p ressu re equ alization to ach ieve rain p en etration con trol in clu d e bu ild in g ed ges, p arap ets an d arch itectu ral p rojection s. Con sequ en tly th e air ch am ber n eed s to be d ivid ed in to sm aller com p artm en ts at these locations while larger compartments are u su ally su fficien t in th e cen tre of th e façad e.
Introduce sufficient venting in the rain-screen at the bottom of the compartment.
Trad ition al d rain age op en in gs in cu rren t w all system s m ay n ot be su fficien t to p rovid e th e n ecessary ven tin g for p ressu re equ alization . Dynamic pressure equalization likely requires m ore ven tin g of th e rain screen th an static p ressu re equ alization . Th e ven t h oles m u st be d esign ed to let in on ly air, n ot w ater, so they must be shielded from direct water entry.
Remember: PER walls
≠
pressureequal-ization. PER w alls are n ot on ly abou t p res-sure equalization across the rainscreen. Other forces are at w ork as w ell, n ot th e least of w h ich is gravity; th eir con trol is p art of th e PER w all strategy for rain p en etration con trol in exterior w alls. On e sh ou ld assu m e th at som e rain w ill en ter at som e tim e d u rin g th e service life of an y w all assem bly; th at w ater m u st be d isp osed of qu ickly. Drain age of th e air compartment is an important feature; prop-erly detailed and sloped flashings and drainage ch an n els are n ecessary for th at reason . M.Z . Rousse a uis a research officer an d W.C. Brow nis 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 .
G.F. Poirie ris an evalu ation officer with IRC’s Can ad ian Con stru ction Materials Cen tre.
Re fe re nc e s
1. Ch ow n , G.A., Poirier, G.F. an d W.C. Brow n . Evolu tion of Wall Design for Con trollin g Rain Pen etration . Con stru ction Technology Update No. 9, Institute for Research in Construction, Nation al Research Cou n cil of Can ad a, 1997, 6 p .
2. Gard en , G.K. Rain Pen etration an d its Con trol, Can ad ian Bu ild in g Digest No. 40, Division of Bu ild in g Research , Nation al Research Cou n cil of Can ad a, 1963, 4 p .
3. Poirier, G.F. an d W.C. Brow n . Pressu re Equ alization an d th e Con trol of Rain w ater Pen etration u n d er Dyn am ic Win d Load in g, Con stru ction Can ad a, March / Ap ril 1994, p . 45-47. 4 Nation al Bu ild in g Cod e of Can ad a 1995. Can ad ian
Com m ission on Bu ild in g an d Fire Cod es, Nation al Research Cou n cil of Can ad a, Ottaw a, 1995. NRCC 38726.
5. An Air Barrier for th e Bu ild in g En velop e, Proceed in gs of Bu ild in g Scien ce In sigh t ’86, In stitu te for Research in Con stru ction , Nation al Research Cou n cil of Can ad a, 1989, 24 p . NRCC 29943. http://irc.nrc-cnrc.gc.ca/bsi/86_E.html
6. Air Barrier System s for Walls of Low -rise Bu ild in gs: Perform an ce an d Assessm en t. In stitu te for Research in Con stru ction , Nation al Research Cou n cil of Can ad a, March 1997, 40 p . NRCC 40635.
7. Dalgliesh, W.A and W.R. Schriever. Wind Pressures and Suctions on Roofs. Canadian Building Digest No 68. Division of Building Research , Nation al Research Cou n cil of Can ad a, 1965, 4p . 8. In cu let, D. an d D. Su rry. Th e In flu en ce of Un stead y Pressu re
Grad ien ts on Com p artm en talization Requ irem en ts for Pressu re-Equ alized Rain screen s. Can ad a Mortgage an d Hou sin g Corp oration , Ju n e 1996.
9. Skerlj, P.F. an d D. Su rry. A Stu d y of Mean Pressu re Grad ien ts, Mean Cavity Pressu res, an d Resu ltin g Resid u al Mean Pressu res across a Rain screen for a Rep resen tative Bu ild in g. CMHC Rep ort, Sep tem ber 1994. Can ad a Mortgage an d Hou sin g Corp oration , Ottaw a.
10. A Stu d y of Mean Pressu re Grad ien ts, Mean Cavity Pressu res, an d Resu ltin g Resid u al Mean Pressu res across a Rain screen for a Rep resen tative Bu ild in g. CMHC Research & Develop m en t High ligh ts Tech n ical Series 96-207, Can ad a Mortgage an d Hou sin g Corp oration , Ottaw a.
11. In cu let, D. an d D. Su rry. Op tim u m Ven t Location s for Partially-Pressu rized Rain screen s. CMHC rep ort BLWT-SS30-1997, Sep tem ber 1997, 183 p .
“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
© 1998
Nation al Research Cou n cil of Can ad a Ju ly 1998
ISSN 1206-1220 At th is stage of kn ow led ge
ad van cem en t, th e com p reh en -sive d esign of sp ecific p res-su re-equ alized rain screen w all system s for a given bu ild in g con figu ration an d clim atic exp osu re sh ou ld be su p p orted by ad d ition al research an d test-in g. At th e d esign stage, a p ro-totyp e w all assem bly can be tested u n d er en viron m en tal con d ition s rep resen tative of th ose th at th e bu ild in g en ve-lop e is exp ected to exp erien ce. Laboratory stu d ies u n d er con -trolled static an d d yn am ic p ressu res an d w in d tu n n el stu d ies can be p erform ed to evalu ate th e p ressu re equ aliza-tion resp on se of com p artm en ts, an d assist in fin alizin g th e d esign in th at resp ect.
