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Exterior wall construction
OF BUILDING RESEARCH NATIONAL RESEARCH COUNCIL OTTAWA, CANADA
EXTERIOR WALL CONSTRUCTION
--
by W.H. B a l l
The v a r i o u s f u n c t i o n s t h a t a w a l l performs i n s e r v i c e can be simply s t a t e d . ?lest? f u n c t i o n s may t h e n be recognized as c o n s i - d e r a t i o n s i n t h e d e s i g n of a w a l l construction o r i n t h e a n a l y s i s of t h e adequacy of a s p e c i f i c c o n s t r u c ' t i o n .
Corlsiderat i o n s i n Val1 Construc.cion
The f o l l o w i n g c o n s i d e r a t i o n s a r e those which may apply t o a w a l l c o n s t r u c t i o n . They may be d i v i d e d i n t o two c l a s s e s : g e n e r a l c o n s i d e r a t i o n s and s p e c i a l c o n s i d e r a ~ i o n s . General Considerat i o n s : 1. s t r e n g t h ; 2 . c o n t r o l of h e a t flow;
3.
c o n t r o l of w a t e r vapour flow; 4 , i n s i d e f i n i s h ; j. o u t s i d e f i l L i s h . S2ec i a l Consiberat i o n s :6 .
r e s i s t a n c e co f i r e ;7. r e s i s t a n c e .co sound transmission;
8.
aesthetic v a l u e .'llhere i s , i n a d d i t i o n , t h e o v e r - a l l c o n s i u e r a t i o n of c o s t which must k e lnclud-ed i n a complete a n a l y s i s of a ivall c o n s t r u c t i o n .
The terin c o s c slere i s i n ~ e n d e d t o m c l u d e n o t only che c o n s t r u c t i o n c o s t but a l s o t h e cosL of maintenance. Thus f o r comparative p u r -
p o s e s t h e a n n u z l c o s t of a c o n s ' c r u c t i o ~ ~ i s t h e most r e a s o n a b l e b a s i s f o r comparison.
All cf t h e s e c o n s i d e r a t i o n s can a l s o be a p p l i e d t o roof c ~ n s c r u c t i o n . ~ i n c l u d i n g c e i l i n g s . I n t h i s n o t e , however, only w a l l s
w i ll be d i s c u s s e d .
De.~a i i e d Considerat i o n s i n Wall Perf or~mance
---
1. -. S t r e n g t h
The f i r s t c o n s i d e r a t i o n i s s t r e n g t h . The requirement f o r s t r e n ~ t h can be broken down i n t o two $ a r t s :
a ) S t r e n g t h t o s u p p o r t v e r t i c a l loads. --
- --
--
-
V e r t i c a l l o a d s a r e t h o s e wh1c.h r e s u l t from t h e weight of t h e roof and t h o s e whish may r e s u l t from snow, wind, w a t e r , and occupancy l o a d s w i t h i n b u i l d i n g s ;
b ) S t r e n g t h t o support h o r i z o n t a l l o a d s . -
Hosizonxal l o a d s
are
t h o s e commonly a s s o c i a t e d w i t h wind and, depending on t h e d i r e c t i o n of t h e wind r e l a t i v e t o t h e s t r u c t u r e , may produce t r a n s v e r s e l o a d s i n some walls and r a c k i n g loads i n o t h e r s .S t r e n g t h i s perhaps t h e most important c o n s i d e r a t i o n because i n t h e development of b u i l d i n g p r a c t i c e t h e e s s e n t i a l s of s h e l t e r e r e comrnmly provided by a sheathed wood-frame c o n s t r u c t i o n
o r a l o a d - b e a r i n g masonry w a l l w i t h a s h z a t n e d roof s b r u c t u r e ,
T h s sheathed wood-frame c o n s t r u c t i o r i and 8 Inches of
masonry have evolved a s t h e elements which
are
t h e p r i n c i p a l c o n t r i - b u t o r s t o s t m n g t h i n house walls. For wood-frame c o n s t r u c t i o n 2 byLl s t u d s a t
16
i n c h c e n t r e s a r e s t i l l used. This o i z e of member and spaciCg was not developed on a c a l c u l a t e d o r s c i e n t i f i c b a s i s but has e v r ~ l v e d as a r e s u l t of e x p e r i e n c e . More r e c e n t l y 2 by4
s t u d s a t 24 i n c h c e n t r e s a r e being used i n s i n g l e - s t o r e y houses and a l s ol a i g e r rnemSers a r e used a t even g r e a t e r spacings, a.s i n post-and-beam c o n s t ~ u c t i o n . These developments have been t h e r e s u l t of t h e
availability of s p e c i a l sheathing n l a t e r i a l s and a l s o , i n p a r t , from a b e t t e r understanding of the s t r e n g t h r e q u i r e m e n ~ s f o r walls through e x g e r i e n c e and l a b o r a t o r y t e s t s . I n the c a s e of , o l i d masonry walls t h e r e h a s been l i t t l e change. The use of hollow-coacrete block and c l a y t i l e as a back-up f o r b r i c k h a s been i n t ~ o d u c e d . There h a s been t h e i n t e r e s t i n g development of t h e SCR B r i c k f o r $-inch t h i c k load- b e a r i n g v , a l l s , l h e hccestance OF 'chis ch5nnt.r s ' c r ~ l c t u r a l wall has
been r e l a t i v e l y simple keczuse stendarc! t e s G nethods ~ ~ & L T E b e e ~ developed alcrig it-ith a basis fcr . e v a l u a t i o n ; a l t h c u g n :it w.us+u be
admitzed th25 t h e basis f o r e v a l u a t i o n i s l a r g e l y
tl-,ax
of ccjniparisun w i t h xsll s $k.a"c;~ave beer, ~ s e d w i t h apparent s u c c e s s .The b i g guestl.on t h a t has t o be a:?s: -red from the p o i n t
of v i 9 w of s t r e ~ g t k ~ i s : "What i s t h e m b i n u n s t r e ~ g - 6 5 - t h a t a w a l l must have t o do t h e j o b ? ' ' There is no ready
answer
t o t h i s guesLion but it i s worth a s k i n g because i t has ecoliomic i m g l i c a t i o r , ~ , I nz e n e r a l t h e l e s s m a t e r i a l t h e r e i.s i n a w a l l -chs l e s s i t w i l l c o s t . T h e r e f o r e , i n d e s i g n i n g w a l l s f o r s t r e n g t h and economy t h e m a t e r i a l
t h z t goes i n t o a w a l l must be put t o work i n t h e b e s t p o s s i b l e way. F o r ex,mple, t h e s u g g e s t i o n i s so;iaetj.rnes made t h a t i t i s wise t o have two l a y e r s of i n s u l a t i n g s h e a t h i n g on a wal-1, one on t h e
i n s i d e and cne
cn
che out@ic:c, Jeca-Lise nok o n l y wlll I t be s t r o n g e r buk i t w i l l p r o v i 3 e insulation and a p l a s t e r base t o o n But t h i s,/
3
Lo be overdone and t h e i n s u l a t i o n requirement underdone. It i s b e s t t o c o n s i d e r one requirement a t a time and provide f o r t h a t r e q u i r e - ment i n t h e most economical way.
I
C o c t r o l of H e a . ~ PlowG-
The v~ood-frame wall and t h e masonry w a l l do n o t compare f a v o u r a b l y when t h e m a t t e r of h e a t flow i s c o n s i d e r e d . The s t a n d a r d wood-frame wall h a s
a
space f o r a t l e a s t34
i n c h e s of i n s u l a t i o n and i t has been hown t h a t t h i s t h i c k n e s s of i n s u l a t i o n i s economical f o r most r e g i o n s af Canada. I n o r d e r t o provide t h i s same t h i c k n e s s ofi n s u l a t i o n i n a masonry w a l l it would be n e c e s s a r y t o v i r t u a l l y b u i l d a wood-frame w a l l on t h e i n s i d e of a masonry w a l l .
It
i s customary t o use a l e s s e r t h i c k n e s s of i n s u l a t i o n on masonry walls p l a c e d b e t - ween 2- by 2-inch f u r r i n g s t r i p s . Thus aboutI$
i n c h e s of i n s u l a t i o ni s r e a l i z e d and I n a.ny s t r i c t comparison one should charge t h e masonry w a l l w i t h t h e c o s t of p r o v i d i n g t h e space f o r t h e i n s u l a t i o n . T h i s does not xean, however, t h a t a mascniy w a l l with l$- i n c h e s of good i n s u l a t i n g on i z i s
-
p o t a good w a l l but by comparison w i t h t h e poss- i b i l f t i e s for p r o v i d i n g maximuv (and economical) r e s i s t a n c e t o h e a t flow i n a wo~d-frame w a l l i t does P a l l somewhat s h o r t .I f a s u b s t a n t i a l t h i c k n e s s of i n s u l a t i o n i s placed i n a
wood-frame w a l l t h e i n s u l a t i n g value of any o t h e r m a t e r i a l s t h a t may go i n t o t h e w a l l can be ignored and t h e y can be s e l e c t e d without con- c e r n f o r t h i s , ?or exainple it would not make an a p p r e c i a b l e d i f f e r - ence whether t h e s h e a t h i n g m a t e r i a l had any i n s u l a t i o n value a t a l l .
Also f o r i n s u l a t e d masmry walls t h e r e need be l i t t l e concern f o r t h e i n s u l a t i n g value of t h e p l a s t e r
o r
p l a s t e r base.I n s u l a t i o n i s perhaps i n t h e b e s t p o s i t i o n of any m a t e r i a l s i n w a l l c o n s t r v c t i o ~ from t h e economic viewpoint sLnce s u b s t a n t i a l t h i c k n e s s can be purchased and i n s t a l l e d a t low-cost, and up to a
p o i r t t h o annual c o s t of t h e wall i s reduced mer!c,dly over an unin-
I
s u l a t e d w a l l .There i s one a e p e c t cf h e a t flow o r heat l o s s from a w a l l tha: has n o t Seen taken i n t o account i n t h e p r e c e d i n g comments. That a s p e c t 1s t h e p a s s i b i l i c y o r o u t s i d e a i r being blown i n t o w a l l s and s o d i s p l a c i n g warmed a i r . It h a s been suggested t h a t s h e e t m a t e r i a l s a p o l i e d t o The ou5sitie of a w a l l w i l l reduce h e a t LOSS by d e c r e a s i n g t h e movements of air i n and out of w a l l s p a c e s , and t k a t narrow boards
and a lappet l a y a r of' s h e a t h i n g paper w i l l a l l o w r e l a t i v e l y f r e e movement of a l r i n and out of w a l l s . There i s not m ~ c h evidence t o sup?cn-t e i t h e r ~ u t g e s t i o n . Experimentz c o n d u c t ~ d in Norwzy i n d i c a t e t h a t f;uildir;g pepers ;vith J o i n t s lapped t i g h t l y w i l l provide a v e r y l a r g e percentage of t h e t o t a l r e s i s t a n c e t o a i r flow i n a w a l l con- s z r u c t i o n .
I
The Control of Water-Vapour Flow-
--
--- -.The p r i n c i p a l means of c o n t r o l l i n g water-vapour flow i n t o w a l l s i s by t h e a p p l i c a t i o r , of vapour b a r r i e r s on t h e warm s i d e of
Y
w a l l s . This reduces t h e amount of w a t e r vapour t h a t e n t e r s t h e w a l l s .A s an added p r e c a u t i o n it i s wise t o c o n s i d e r t h e e a s e w i t h which w a t e r vapour can l e a v e t h e wall t o t h e outei.de s i n c e vapour b a r r i e r s g r e not p e r f e c z b a r r i e r s .
'The s e l e c t i o n of a vapour b a r r i e r may n o t be important from an. economic gboint of view s i n c e t h e a j p l . i e d c o ~ t of t h e b e s t vapour b a r r i e r s nay be as low a s
5
o r4
c e n t s a quare f o o t . Tke d i f f e r e n c e i n c o s t ~f membranes i s small, s o one might j u s t as w e l l have a good one t o s z a r t w i t h .The use of f o i l - b a c k e d wallboard i s one means of combining
tllis f u n c t i o n w i t h t h e i n t e r i o r f i n i s h , Foil-backed l a t h i s q u e s t i o n - a b l e as a vapour b a r r i e r because of zhe g r e a t l e n g t h of crac14 t h a t r e s u l t s .
We a r e o f t e n a s k e ~ about t h e s u i t a b i l i t y of i n t e r i o r p a i n t s
as :lapour b E r r i e r s . Here it can o n l y be sa.id t h a t g e n e r a l 1 7 :~wo o r t h r z e c o a t s of good o i l , rub:jer base, o r alkyd p a l n t s w ~ l l ! ~ r o v i d e s u b s t a n t l a l r e s i s t a n c e t o water vapour f l o w . I s u g g e s t thzii t h e y should o n l y be used as vapour b a r r i e r s where it i s not p r a c t i c a l o r economical t o use a membrane b a r r i e r .
The damage i n cerrns of p a i n t f a i l u r e , d a n ~ g e d i n s u l a t i o r l , and r o t t i n g of m a t e r i a l s that czn r e s u l t from t h e omission of vapour bar-
r l e r s , o r a poor i n s t a l l a t i o n , could c e r t a i n l y r e s u l t in c ~ s t s amount- i n g t o
many
t i m e s t h e c o s t ofa
membrane b a r r i e r .I n 5 e r i o r F i n i s h
-
---
l n e f u n c t i o n of t h e i n t e r i o r f i n i s h can be ~ E a e e d f a i r l y s i n p l y a s " t n e p r o v i s i o n 01'
a
p a i n c a b l e , cneanable, and b a r a b l e s u r -f a c e " . ' l h e problems a r e all familfar
-
plaster crzclcin~, and popping91% m i l s and J o l n t -f i l l i n g groulems i n d r y - w a l l ~ o n s t r 1 ~ ~ c t ; o n . 'me s e l e c t i o n o f an i n c e r i o r f i n l s h i s ol1,en a m a ~ t e r of p r s c n a i . choice
a n 3 t h e r e aygears t o be Fe:r ~ e c n i ? l c a l c c m s ~ ? . : ! . e r a - ~ ~ o n s thzf: sugges-G any g e n e r a l ;i ,zri;;ags amon2 i ~ a t e r i a l s used 517 average : ~ ~ - : t e .
f ' j n c t i : ~ , .,;' Yh9z ;.zberj.oi-, i_-?Lch a r e 5 : vc:ri _!- It ~k~32;Cj- shed r a i n >?ill p,naw, an.2 !nay i n sore vzsllc p r r ~ v i d e resist-.r;ze t o t h e
e n t r y cf wind. It s h a i l l d be d u r a b l e uniler t!?e co:ldi.ti;rj .=f exposLre and s h ~ u l J lidire d i m e n s i o n a l s t z ' u i l i t y 3 - ~ e r a s i m i l a r r s 7 ? ; e of condi-
i 2 o n s . It shauld n ~ t provide
a
b a r r i e r L a t h e rnt2ver.1eA?L o - ~ t w a r d of w a t e r v a p ~ r t h a t nlay mcve through o r by-pass t h e va2o.iu. b a r r i e r . F i n a l l y t h z e x t e r i o r f i n i s h should p ~ c v i a e 2 p l f a s i n g 2>s?arancz.I n ;iood-frame w a l l s , i n c l u d i n g b r i c k - t e ~ e e ~ c o ~ s t r u z t l o ! ~ , khe oheathifig paper i s tlze element of t h e e x t e r i o r f i n i u l - 1 which i s
l i k e l y t o p o v f d e -The g r e a t e s t r e s i s t a n c e t o water-.vapx,r flow. The principzl purpose cf t h e s h e a t h i n g paper i s t o keep l i q u l d w a t e r 2nd
sj.r ogt of thc w a l l . 'Th'ils, a me]-nbrane shoulcl bz s e l e c t e d t h a t w i l l . I l f i ( i t.:ater and a t t h e same time p a s s w a t e r v a p o u r . S h e a t h i n g papers
BN 26
5
a m , i n f r c t , r e p ~ ~ 8 n t e d by t h e comrron s a t u r a t e d b u i l d i n g papers t h a t a r e i'amiliar t o a l l . U n f a r t u n a t e l y sGme m a n u f s c t u r e r s t r y t o make t h e i r p r o d u c t s t o o w a t e r r e s i s t a n t and, by s o doing, m k e them t o 3 r e s i s t a n t t o w a t e r vapour f l o w .
S t u c c o and b r i c k v e n e e - s i d i n g s a r e a l l f a i r l y permeable t o w a t e r vapour flow. Painted w9od s3din;s do n o t meet t h i s f u n c t i o n v e r y w e l l because, a s (ras p r e v i o u s l y n e n t l ~ n e d , a p a i n t f i l m t ~ i l l provide a s u b s t a n t i a l resistance t o k ' a t e r - v a j o u r f l o w , Nany have d i s c o v e r e d t h i s t o t h e i r l o s s , a s p?.int blistering i s no s t r a n g e r i n t h e b u i l d i n g s c e n e . Kowever, i n most h ~ u e e s t h e s l c u a t i o n i s n o t s e r i o u s s i n c e t h e s i d i n g i s lapped s o t h a t ';he k r & l l can b r e a t h e outward throdgh t h e
c r a c k s between boards. The ouvward maverxent of w a t e r vapour i s prob- a b l y r e s p o n s i b l e f o r much of t h e painc f a i l u r e t h a t i s r e p o r t e d .
The r o l e of t h e e x c e r i o r f i n i s h i n keeping w a t e r and wind out or w a l l s i s perhaps t h e ~nos'c irnaortant c o n s i d e r a t i o n i n t h e over- a l l p i c t u r e of wall performance. Fdater w i l l cause m a t e r i a l s t o l o s e strerzgth, change dimension, anti 1 1 1 2 ~ l e a d t o a shortened. l i f e f o r t h e
m a t e r i a l because of r o t o r f r e e z i n , a c t i o n , The e x c l u s i o n of wind h a s a l r e a d y been mentioned a s a f a c t o r i n connection w i t h h e a t l o s s . Wind
i s c e r t a i n l y a f a c t o r which can c o n t r i b u t e t o wall l e a k a g e . h a l l s a r e much more s u s c e p t i b l e t o l ~ a k a g e d u r i n g wind-driven r a i n t h a n t h e y a r e
t o r a i n without wind.
In o u r e f f o m s t o greven': wall leakage t h e r e h a s been a tendency toward t h e s e a l i n g of t h e c x t e r i o r wall s u r f a c e s t o cry t o n r J c u c e a moisture-impermeable s u r f a c e . The p e r f e c t s e a l i n g of a wall. s u r f a c e i s almost impossible s i n c e f i n e c r a c k s and j o i n t s t h a ~ cannot be s e e n w i l l a l l o w 'che passage of v a t e r wheLher
it
i s wind d r i q e n o r n o t . It t h e n becomes necessary t o consLder t h e b e s t p o s s i b l e means of p r o t e c t i ~ n f o r t h e v a r i o u s elements af t h ewall.
For exam>le, by c o n s t r u c t i n g t h e wall i n two cornpo, e n t s , i n n e r and o u t e r , t h e r e w i l lbe a nuch be c t ~ r chz2ce of improving o v e r - a l l
wall
performance, Theinner cornpineilk can c o n t a i n t h e s c r u 2 t u r a l
,
i n s u l a t i o n and i n c e r i o rf i i l l s h elements, and tr?e o u t e r corn~onent can s e r v e simply as a weather s c r e e n i n r e s p z c t t o r a i n ;-3enotra; .on, T h i s p r c p o s a l w i l l be recog-
n i ~ c d a s t y p i c a l of a c a v i t y - w a l l c o n s t r u c t i o n o r t h e s t a n d a r d b r i c k - veneer consbr&>bion. To provide She maximum protection a g a i n s t t h e ec.Gry or wetep i n t o the w a l l t h e c a v i e y behind t h e e x t e r i o r f l n l s h s h c u l d be f r e e l y d r a i n e d anC v e n t l l a c e d s o t h a t wind a c t i o n rr:&y have
a minlnim e f f e c t 32 w a l l l e a k a g e .
IL
may t h u s be p o s s i b l e t o reduce t h e y ~ s s i b i 1 i ' c ; ; r of damage t o w a l l s m e t o w a t e r p e n e t r a t i o n t ~ , t h a t which may occur as a r e s u l t of t h e a b s o r p t i v e p r o p e r t i e s of t h e ex- te1.i.o~ c l a d d i n g when a f i l m of w a t e r e x i s t s on i t s , s u r f a c e . Dimen- s i o n a l s t a b i l i t y w i t h v a r i a t i o n i n n o i s t u r e c o n t e n t and temperature and ~ e ~ i s t c ? n e s t o d e t e r i o r a t i o n due t o f r e e z e thaw a c t i o n w ~ u ~ . c s t i l . 1 be r e q u i r e d of claddin;; n ~ a t e r i a l s a s now, but t h e f u ~ c t i o n 3 2 cbe c l a d d i n 2 would not be s o d i r e c t l y a s s o c i a t e d w i t h t h e o v e r - a l l p e r - i'or;~lznce s f t h e whole w a l l .Wood o r s t u c c o c l a d w a l l s can be b u i l t on a s i m i l a r basis
a ~ p i y i n g t h e s e m a t e r i a l s on f u r r i i l g s t r i p s p l a c e d over t h e s h e a - ~ h i n g e r . S t ~ : c c c f i n i s h e s would r e q u i r e a backing which n e e d only f u n c t i o n ij?g t h e appl i c a t i o n p r o c e s s . No g r e a t d i f P i c u l C y would o c c u r w i t h
04 o r o t h e r s h e e t s i d i n c s . V e r t i c a l f u r r i n g s t r i p s provide n a t u r a l chaanels an2 h o r i z o n t a l f u r r i n g s t r i p s can be gapped o r l l o w d r a i n a g e and ventilation.
