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Condensation problems in flat wood-frame roofs
Tamura, G. T.; Kuester, G. H.; Handegord, G. O.
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CONDENSATION PROBLEMS IN F L A T WOOD-FRAME ROOFS
G e o r g e T. T a m u r a , G e o r g e H. K u e s t e r & Gustav 0. Handegord Division of Building Re s e a r c h
National R e s e a r c h Council of Canada Ottawa, Canada K1A OR6
ABSTRACT
F l a t w o o d - f r a m e house roofs with insulation a p p l i e d between j o i s t s a r e s u s c e p t i b l e to c o n - d e n s a t i o n p r o b l e m s i n cold c l i m a t e s . Inve s t i - gation of difficulties e x p e r i e n c e d in a wood- f r a m e r o w housing p r o j e c t i n E a s t e r n Canada
showed that m a n y i n t e r r e l a t e d f a c t o r s contribute t o t h e o c c u r e n c e of p r o b l e m s and d e m o n s t r a t e d t h a t c o n t r o l of a i r leakage t h r o u g h t h e ceiling i s t h e one p r i m a r y r e q u i r e m e n t f o r s u c c e s s f u l p e r f o r m a n c e . L e s t o i t s p l a t s 'a o s s a t u r e de bois a v e c isolant e n t r e l e s s o l i v e s p o s e n t d e s p r o b l P m e s de c o n - d e n s a t i o n d a n s l e s c l i m a t s f r o i d s . L'Btude d e s d i f f i c u l t k s s u s c i t e e s p a r un e n s e m b l e de m a i s o n s e n rangke 'a o s s a t u r e de bois d a n s 1'Est du C a n a d a a indiquk l e s n o m b r e w f a c t e u r s i n t e r
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d e p e n d a n t s qui sont m i s en jeu e t a p e r m i s d ' i d e n t i f i e r l a r k g u l a t i o n d e s f u i t e s d ' a i rB
l e plafond c o m m e l a p r i n c i p a l e condition dtune a c t i o n efficace.2
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T a m u r a , K u e s t e r and Handegord IntroductionThe condensation of w a t e r vapour i n roof s p a c e s of h o u s e s d u r - ing cold w e a t h e r has long been recognized a s a potential prob- l e m in Canada. Building p r a c t i c e s developed to a l l e v i a t e these s e r i o u s p r o b l e m s have involved the u s e of vapour b a r r i e r s in the f o r m of a m e m b r a n e ( u s u a l l y polyethylene 0. 05 to 0. 10 rnm thick) i n s t a l l e d s o a s t o inhibit the flow of vapour f r o m inside the house into the c o l d e r p o r t i o n s of the e x t e r i o r e n c l o s u r e in w i n t e r , coupled with m e a s u r e s to p r o m o t e the r e m o v a l of any vapour t h a t d o e s p e n e t r a t e t o the outside. These m e a s u r e s w e r e introduced in recognition of t h e h i g h e r vapour p r e s s u r e s that e x i s t e d i n houses r e l a t i v e to outside and were i n i t i a l l y based on a s i m p l e , s t e a d y - s t a t e diffusion concept.
The m o v e m e n t of m o i s t a i r through unintentional openings a s a p a r a l l e l m e c h a n i s m i n the t r a n s p o r t of w a t e r vapour h a s come t o be r e g a r d e d a s m o r e significant t h a n vapour diffusion i n Canadian buildings. T h e building f e a t u r e s and c o n s t r u c t i o n p r a c t i c e s i n c o r p o r a t e d into t r a d i t i o n a l d e s i g n s have g e n e r a l l y acted t o m i n i m i z e the p r o b l e m s r e s u l t i n g f r o m condensation, except i n t h o s e c a s e s w h e r e some s p e c i a l feature o r e n v i r o n - m e n t a l condition d i s r u p t t h e p r o c e s s .
In r e c e n t y e a r s , e l e c t r i c heating of h o u s e s has become m o r e p r e v a l e n t , eliminating the need f o r chimneys and the combus- tion a i r r e q u i r e m e n t s a s s o c i a t e d with f u e l - f i r e d heating s y s t e m s . T h i s h a s g e n e r a l l y tended t o d e c r e a s e the natural e x h a u s t of a i r f r o m s u c h h o u s e s , giving r i s e to h i g h e r humidities i n t h e living space. In addition, an i n c r e a s i n g n u m b e r of e l e c t r i c a l l y heated row housing p r o j e c t s have utilized f l a t wood-frame r o o f s for r e a s o n s of economy o r a p p e a r a n c e . T h e s e roof s y s t e m s do not
offer the l a r g e a t t i c s p a c e which e x i s t s between the insulated ceiling and roof s t r u c t u r e t h a t i s p r e s e n t in the m o r e traditional sloping r o o f s of h o u s e s i n Canada.
A p r o j e c t n e a r Ottawa incorporating t h e s e f e a t u r e s , completed i n 1963, c o n s i s t e d of 122 u n i t s , 10 with high pitched r o o f s , 55 with low pitched and 57 with flat roofs. P r o b l e m s of excessive window condensation i n cold w e a t h e r and dripping of w a t e r f r o m c e i l i n g s d u r i n g w i n t e r t h a w s had been r e p o r t e d p e r i o d i c a l l y by s o m e occupants. In m a n y i n s t a n c e s the p r o b l e m s w e r e a t t r i - buted t o a b n o r m a l l y high humidities in t h e houses a s a r e s u l t of r e d u c e d ventilation o r high m o i s t u r e g a i n s f r o m c l o t h e s drying i n d o o r s .
The p e r s i s t e n c e of p r o b l e m s even a f t e r a t t e m p t s to r e d u c e h u m - i d i t i e s , prompted a questionnaire s u r v e y by DBR/NRC i n 1968. The s u r v e y r e s u l t s i n d i c a t e d that ceiling condensation p r o b l e m s had o c c u r r e d in 28 of the 122 units; 21 c a s e s in u n i t s w i t h flat r o o f s , 6 i n t h o s e with low sloped r o o f s and one c a s e i n the units with s t e e p l y sloped r o o f s . , T h i s indication that f l a t r o o f s were m o r e s u s c e p t i b l e t o condensation p r o b l e m s was i n keeping with a g e n e r a l indication f r o m t h e housing i n d u s t r y and l e d t o a d e - t a i l e d study of p e r f o r m a n c e and c o n s t r u c t i o n f e a t u r e s in a s i x - unit r o w house within the development ( F i g u r e 1 ) .
C o n s t r u c t i o n F e a t u r e s
E a c h housing unit had a plan dimension of 7.30 x 7.60 m with two s t o r e y s of finished s p a c e over a full and s e p a r a t e basement. The f r o n t and back w a l l s w e r e insulated wood f r a m e w i t h gypsum wallboard on t h e i n t e r i o r . P a r t y walls w e r e concrete block with gypsum w a l l b o a r d on s t r a p p i n g . End w a l l s w e r e of c o n c r e t e block with wood f u r r i n g , i n s u l a t i o n and g y p s u m
4
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Tarnura, K u e s t e r and HandegordFIG. 1 T e s t Units
wallboard on strapping. End walls w e r e of concrete block with wood f u r r i n g , insulation and gypsum wallboard i n t e r i o r . De- t a i l s of the roof construction a r e shown i n Figure 2.
B U I L T - U P J O I S T R O O F I N G l N S U L A T l O N f 1 5 0 rnm T H I C K ) ( s o x 2 5 0 r n m N o M . 1
I
S H E A T H I N GI
S O F F I T V E N T ( 5 0 r n r n W I D E 1-.
E L E C T R I C A L E X T E R I O R W A L L P A R T I T I O N E X T E R I O R W A L L FIG. 2F l a t Roof Construction Detail
space w a s intended t o be continuous and open t o the s o f f i t vents a t the f r o n t and r e a r of the unit. In a number of joist spaces however, placement of insulation o v e r nailing s t r i p s o r c r o s s bridging had r e s u l t e d in blocking of t h i s a i r space. The poly- ethylene vapour b a r r i e r , although intended to provide a continu- ous m e m b r a n e under the insulation, w a s i n t e r r u p t e d by the p a r - t i t i o n w a l l s since t h e s e w e r e used t o support the roof joists i n t h e i n i t i a l e r e c t i o n p r o c e s s . Shrinkage of t h e top p l a t e s a f t e r appli- cation of the vapour b a r r i e r and w a l l b o a r d , o r o t h e r difficulties in s e c u r i n g t h e wallboard a t t h i s location, allowed an opening in the f o r m of a c r a c k t h a t connected the i n t e r i o r of the partition t o the roof space. E l e c t r i c a l wiring and plumbing s e r v i c e s w e r e a l s o c a r r i e d t h r o u g h these w a l l s , allowing openings to the roof s p a c e . At p a r t y w a l l s , the s p a c e behind f u r r i n g s t r i p s and the m a s o n r y itself afforded s i m i l a r p a s s a g e s of a i r and vapour m o v e m e n t .
When t h e s e "as-built" d e t a i l s w e r e c o n s i d e r e d i n r e l a t i o n to the p a t t e r n of staining t h a t had o c c u r r e d on t h e upper f l o o r ceilings of the s i x u n i t s , i t a p p e a r e h that m a n y of the m a j o r w a t e r stains coincided with t h o s e joist s p a c e s w h e r e t h e a i r s p a c e above the insulation w a s completely o r p a r t i a l l y blocked ( F i g u r e 3 ) . In m a n y c a s e s , the l o c a t i o n s of s e v e r e staining w e r e a l s o adjacent t o p a r t i t i o n s o r p a r t y w a l l s where potential leakage openings t h r o u g h t h e ceiling could o c c u r .
R e m e d i a l M e a s u r e s
It w a s recognized t h a t s t a c k effect due t o building heating pro- d u c e s a n a i r p r e s s u r e difference t h a t p r o m o t e s t h e flow of humid inside a i r through openings and c r a c k s into the cold roof space, and that t h i s effect could be g r e a t e r in h o u s e s with n o chimney ( 1 ) . R e m e d i a l a p p r o a c h e s designed t o r e d u c e the u n d e s i r a b l e p r e s -
6
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T a r n u r a , K u e s t e r a n d Handegord B L O C K E D P A R T I A L L B L O C K E D FREE C E I L l N G E X H A U S T L I G H T F I X T U R E C E I L I N G S T A l NFIG.
3Location of Blockage i n J o i s t S p a c e a n d Ceiling S t a i n s i n Row Housing Unit
s u r e d i f f e r e n c e a c r o s s the ceiling w e r e suggested and w e r e applied in two of the s i x u n i t s ; one involving p r e s s u r i z a t i o n of the roof s p a c e , the o t h e r using continuous exhaust f r o m t h e living s p a c e . The r e m a i n i n g units w e r e left unchanged. P r e s s u r i z a t i o n of t h e roof s p a c e in one unit was a c c o m p l i s h e d
3 using a roof -mounted c e n t r i f u g a l f a n d i s c h a r g i n g 0.059 m / s into a plenum under t h e soffit a t the r e a r of the unit t o supply a i r t h r o u g h t h e existing continuous soffit vent. The soffit vent a t the f r o n t w a s c o v e r e d and sealed. T h e flow r a t e w a s adjusted
2
t o produce a positive p r e s s u r e of 2. 5 N / m a s m e a s u r e d in one of the c e n t r a l joist s p a c e s . P o s i t i v e e x h a u s t f r o m t h e living s p a c e of a second unit, r e m o t e f r o m the f i r s t , w a s provided by
3 connecting t h e existing washroom e x h a u s t ducts t o 0.038 m / s fan located on the roof. I n s t r u m e n t a t i o n was provided to r e c o r d t e m p e r a t u r e and humidity in a l l s i x u n i t s and weekly m e a s u r e
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m e n t s w e r e m a d e of p r e s s u r e d i f f e r e n c e s a c r o s s the ceiling. O b s e r v a t i o n s of ceiling conditions and window condensation w e r e a l s o c a r r i e d out.O b s e r v a t i o n s
O b s e r v a t i o n s w e r e begun i n November 1968 but no v i s i b l e evi- dence of condensation difficulties w a s a p p a r e n t in any of the s i x units d u r i n g t h e m i d w i n t e r thaw t h a t had precipitated p r o b l e m s in p r e v i o u s y e a r s . The occupants of a unit i n a n adjacent, i d e n t i c a l block did e x p e r i e n c e a r e c u r r e n c e of ceiling m o i s t u r e p r o b l e m s and t h i s unit w a s subsequently included i n the study. M e a s u r e m e n t s of a i r p r e s s u r e d i f f e r e n c e s a c r o s s the ceiling
of the c o n t r o l units indicated that when t h e outside t e m p e r a t u r e was -18°C u n d e r conditions of no wind, t h e r e was a positive a i r p r e s s u r e difference due t o s t a c k effect equal t o a p p r o x i -
2
m a t e l y 3 . 7 5 N/m
.
T h i s i c t e d to p r o m o t e a i r m o v e m e n t up through the ceilings. T h i s difference d e c r e a s e d a t h i g h e r out- side t e m p e r a t u r e s and u n d e r wind conditions but not t o the point w h e r e r e v e r s a l o c c u r r e d f o r any a p p r e c i a b l e period of t i m e .3
Operation of the 0.038 m / s exhaust fan i n the one unit reduced t h i s p r e s s u r e difference by almost 2 . 5 N / m 2 and t h u s could lead t o r e v e r s a l when outside t e m p e r a t u r e s r o s e above - 1 2 ° C . In the f a l l of 1969, the roof space p r e s s u r i z a t i o n s y s t e m was t r a n s f e r r e d to the unit in t h e a d ~ a c e n t block that had experienced ceiling condensation difficulties and a l l of the s i x u n i t s i n the t e s t building w e r e r e t u r n e d t o n o r m a l operation. D u r i n g the winter of 1969-70 r e p o r t s of ceiling condensation w e r e received
8 - T a r n u r a , K u e s t e r and Handegord
f r o m the o c c u p a n t s of f o u r of the u n i t s i n the o r i g i n a l t e s t build- ing. The unit in the adjacent block with roof space p r e s s u r i z a - tion did e x p e r i e n c e a n i s o l a t e d c o n d e n s a t i o n p r o b l e m i n a c l o s e t a d j a c e n t t o a p a r t y w a l l . A p r e s s u r e difference t r a v e r s e a c r o s s the c e i l i n g indicated t h a t roof space p r e s s u r i z a t i o n d e c r e a s e d f r o m the c e n t r e and t h a t r e v e r s a l o c c u r r e d n e a r both p a r t y w a l l s , i n d i c a t i n g the e f f e c t of i n t e r c o n n e c t i o n to the roof s p a c e s of a d j a c e n t u n i t s . I t w a s a l s o o b s e r v e d that some s u r f a c e con- d e n s a t i o n had o c c u r r e d on a n i n t e r i o r w a l l n e a r t h e soffit supply plenum w h e r e roof s p a c e p r e s s u r e w a s high which r e s u l t e d in a flow of cold a i r downward f r o m the roof s p a c e i n t o the w a l l . During the w i n t e r s of 1970-71 and 1971-72 the roof s p a c e p r e s s u r - i z a t i o n s y s t e m w a s m o d i f i e d by opening t h e p r e v i o u s l y s e a l e d soffit vent on the s i d e of t h e unit o p p o s i t e the a i r s u p p l y plenum s o t h a t t h e s y s t e m would o p e r a t e a s a roof space f o r c e d ventil- ation s y s t e m d e l i v e r i n g a p p r o x i m a t e l y 0. 095 m 3 / s of outside a i r continuously. O b s e r v a t i o n of conditions i n the roof s p a c e indi- c a t e d t h a t heavy f r o s t i n g of the u n d e r s i d e of the roof sheathing o c c u r r e d i n two of the s p a c e s blocked by insulation a n d n e a r the vent opening of t h e e x h a u s t soffit. D u r i n g both w i n t e r s , h o w e v e r , no condensation w a s v i s i b l e f r o m inside the housing unit.
D i s c u s s i o n
Although conclusive e v i d e n c e a s t o t h e d e g r e e of s u c c e s s of the r e m e d i a l m e a s u r e s in preventing c o n d e n s a t i o n p r o b l e m s i n a l l s u c h u n i t s w a s not obtained f r o m the s t u d y , the e x p e r i e n c e , ob- s e r v a t i o n s and m e a s u r e m e n t s m a d e provided a b a s i s f o r p r a c t i - c a l a n a l y s i s a s t o t h e c a u s e s of the p r o b l e m s and t h e potential of c o n t r o l m e a s u r e s . T h e o b s e r v a t i o n of "as-built" d e t a i l s in c o n -
s t r u c t i o n i n t h i s and o t h e r p r o j e c t s had shown t h a t a s u b s t a n t i a l n u m b e r of leakage o p e n i n g s o c c u r in t h e ceiling s y s t e m s of C a n a d i a n w o o d - f r a m e h o u s e s . M e a s u r e m e n t of t h e a c t u a l l e a k - age o p e n i n g s i n s i n g l e - f a m i l y h o u s e s of s i m i l a r c e i l i n g c o n s t r u c - tion h a s s i n c e i n d i c a t e d a n a g g r e g a t e opening a r e a , 0 . 0 2 per c e n t of t h e t o t a l c e i l i n g a r e a . B a s e d on t h i s l e a k a g e opening and m e a s u r e d a i r p r e s s u r e d i f f e r e n c e s c a u s e d by s t a c k effect, with i n s i d e conditions of 30 p e r c e n t r e l a t i v e h u m i d i t y a n d a t e m p e r a t u r e of 2 1 ° C , a t a n outside t e m p e r a t u r e of - 1 8 ° C t h e r e i s a p o t e n t i a l f o r t r a n s f e r of w a t e r i n t o t h e roof s p a c e of 6.8 kg p e r d a y . In f l a t r o o f s y s t e m s s u c h a s d e s c r i b e d , t h i s m o i s t a i r c o m e s in c o n t a c t a l m o s t i m m e d i a t e l y with t h e cold u n d e r s i d e s u r f a c e of t h e roof w h e r e the m o i s t u r e can c o n d e n s e a s f r o s t . T h i s i s i n m a r k e d c o n t r a s t t o t h e situation in a sloping roof with a t t i c
s p a c e above t h e i n s u l a t i o n . H e r e t h e m o i s t a i r r i s i n g through l o c a l i z e d openings c a n m i x with t h e l a r g e r volume of a i r , m o s t of w h i c h e n t e r s f r o m o u t s i d e a t a l o w m o i s t u r e c o n t e n t , offering
I
a p o t e n t i a l f o r dilution b e f o r e it c o m e s i n contact with c o n - d e n s i n g s u r f a c e s . M o s t of the s u r f a c e s a v a i l a b l e a r e sloped ' t o t h e e a v e s and u n d e r subsequent m e l t i n g conditions, t h e m o i s t u r e c a n m i g r a t e by g r a v i t y t o t h e e a v e s , o r e v a p o r a t e in t h e p r o c e s s , r a t h e r t h a n dripping d i r e c t l y onto t h e c e i l i n g a s would be t h e c a s e i n a f l a t roof. T h e m o i s t u r e r e m o v a l potential of a f l a t roof of the type d e s c r i b e d c a n t h u s be m u c h l e s s than t h a t of t h e t r a d i t i o n a l s l o p e d roof w i t h a n a t t i c s p a c e .
If the influence of a i r p r e s s u r e d i f f e r e n c e s due t o s t a c k effect i s a c c e p t e d a s t h e p r i m a r y m e c h a n i s m a c t i n g t o m o v e w a t e r v a p o u r i n t o t h e roof s p a c e s of h o u s e s i n w i n t e r , a n y s y s t e m t h a t a c t s t o d e c r e a s e t h i s p r e s s u r e d i f f e r e n c e , such a s roof space
10
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T a r n u r a , K u e s t e r a n d Handegordp r e s s u r i z a t i o n o r living s p a c e e x h a u s t , should help t o r e d u c e o r s o l v e t h e p r o b l e m . T h i s a p p r o a c h , h o w e v e r , has d i s a d v a n t a g e s , e . g . , i n c r e a s e d e l e c t r i c a l p o w e r and m a i n t e n a n c e r e q u i r e m e n t s , p o t e n t i a l h i g h e r heating c o s t s , and p o s s i b l e s u r f a c e c o n d e n s a - t i o n p r o b l e m s within t h e h o u s e w h e r e r e v e r s a l of a i r flow o c c u r s . I n c r e a s e d v e n t i l a t i o n of f l a t r o o f s by i n j e c t i o n of o u t s i d e a i r t e n d s t o r a i s e the p r e s s u r e i n the roof s p a c e t o a d e g r e e d e p e n d e n t on t h e r e s i s t a n c e t o "out-flow" and i n c r e a s e s t h e r a t e of r e m o v a l of m o i s t u r e . A p p l i c a t i o n of t h i s m e t h o d t o t h e unit d e s c r i b e d ap- p e a r e d t o be a s e f f e c t i v e a s p r e s s u r i z a t i o n without c r e a t i n g the p r o b l e m of l o c a l i z e d c o l d i n t e r i o r s u r f a c e s .
The m o s t obvious a p p r o a c h t o solving t h e roof s p a c e c o n d e n s a - t i o n p r o b l e m i s t o i n c r e a s e t h e a i r t i g h t n e s s of c e i l i n g c o n s t r u c - t i o n s . F o r new c o n s t r u c t i o n , e f f o r t s in t h i s d i r e c t i o n h a v e been taken i n C a n a d a ( 2 ) . O t h e r c h a n g e s i n p r a c t i c e with r e s p e c t to a p p l i c a t i o n of b a r r i e r s t o a i r flow in c o n s t r u c t i o n o p e r a t i o n s o r r e l o c a t i o n of e l e c t r i c a l d i s t r i b u t i o n a n d ~ x r i r l a g could be w a r r a n t e d . A c k n o w l e d g e m e n t s T h e a u t h o r s acknowledge t h e a s s i s t a n c e of R , G . E v a n s a n d L. P. Chabot i n t h e f i e l d t e s t s . T h i s p a p e r i s a c o n t r i b u t i o n f r o m the DBR/NRC, C a n a d a and i s published w i t h t h e a p p r o v a l of t h e D i r e c t o r of t h e Division. R e f e r e n c e s (1) A. G. Wilson and G. T . T a n u r a . A i r L e a k a g e a n d P r e s s u r e M e a s u r e m e n t s in Two Occupied H o u s e s , ASHRAE J o u r n a l , Dec.1963, p. 65-73. (NRC 7758).
( 2 ) M e a s u r e s t o r e d u c e t h e i n c i d e n c e of d a m a g e f r o m u n d u e c o n d e n s a t i o n . C e n t r a l M o r t g a g e a n d Housing C o r p . B u i l d e r s ' B u l l e t i n No. 228. O t t a w a , 3 J u l y 1973.
