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Roof temperatures and ice dams
NATIONAL RESEARCH COUNCIL OF CANADA DIVISION OF BUILDING RESEARCH
DBR INTERNAL REPORT NO. 426
ROOF TEMPERATURES AND I C E DAMS by J . K . L a t t a
Checked by: M. C
.
B.
Approved by: C.
B . C.
Date: A p r i l 1976PREFACE
I c i c l e s and a s s o c i a t e d i c e a l o n g t h e r o o f edge a r e o f t e n a r e s u l t o f d e f e c t s i n roof and c e i l i n g c o n s t r u c t i o n . They p r e s e n t a danger t o p e o p l e and p r o p e r t y below b u t t h e w a t e r backed up by t h i s i c e may a l s o cause damage t o t h e r o o f , c e i l i n g , and w a l l . These i c e dams
form when t h e snow i n c o n t a c t w i t h t h e r o o f s u r f a c e m e l t s and r u n s down a s l o p i n g r o o f u n t i l i t emerges a t t h e edge o f t h e snow c o v e r where i t
r e f r e e z e s . The r e s u l t i n g l a y e r o f i c e may be v e r y t h i c k and t h e volume o f w a t e r t h a t o v e r t o p s t h e r o o f s h i n g l e s may b e l a r g e .
F r e q u e n t l y i n an a t t e m p t t o s t o p t h e w a t e r from e n t e r i n g t h e b u i l d i n g , a continuous band o f impervious m a t e r i a l i s l a i d a l o n g t h e edge o f t h e r o o f and i n extreme c a s e s h e a t i n g c a b l e s a r e used t o melt d r a i n a g e p a t h s through t h e i c e . Both measures t r e a t t h e symptom r a t h e r t h a n t h e cause. I t i s p r e f e r a b l e t o reduce t h e amount of snow m e l t i n g s o a s t o l i m i t t h e accumulation o f i c e . Thus i t f o l l o w s t h a t h e a t
s h o u l d b e c o n t a i n e d i n t h e b u i l d i n g a s much a s p o s s i b l e and such h e a t a s does escape should b e c a r r i e d away by v e n t i l a t i o n b e f o r e i t warms t h e r o o f deck.
I n o r d e r t o o b t a i n some i n f o r m a t i o n about t h e weather condi- t i o n s n e c e s s a r y f o r t h e formation o f i c e dams, thermocouples were i n s t a l l e d a t v a r i o u s p o i n t s on t h e roof a t t h e time of an e x t e n s i v e r e b u i l d i n g of t h e a u t h o r ' s h o u s e . The roof t e m p e r a t u r e s under changing weather c o n d i t i o n s were recorded and t h e e f f e c t s o f r e f l e c t i v e i n s u l a -
t i o n i n s t a l l e d on t h e u n d e r s i d e o f t h e roof deck were a l s o o b s e r v e d . S i m i l a r r e a d i n g s were t a k e n i n summer t o determine t h e maximum s h i n g l e t e m p e r a t u r e which might o c c u r .
The r e s u l t s o f t h e s e o b s e r v a t i o n s a r e p r e s e n t e d i n t h i s r e p o r t . One p o s s i b l e means o f a l l e v i a t i n g t h e problem o f i c e dams i s proposed a l t h o u g h f u r t h e r i n v e s t i g a t i o n i s needed t o e s t a b l i s h i t s e f f e c t i v e n e s s .
Ottawa
A p r i l 1976 C . B. Crawford, D i r e c t o r , DBR/NRC.ROOF TEMPERATURES AND ICE DAMS
b Y
J . K . L a t t a
INTRODUCTION
I t i s u s u a l l y a c c e p t e d t h a t i c e dams form on r o o f s when t h e snow i n c o n t a c t w i t h t h e r o o f m e l t s , r u n s down t h e r o o f a n d f r e e z e s on emerging from under t h e snow b l a n k e t a t t h e e a v e . F o r t h i s t o o c c u r , i t i s n e c e s s a r y f o r t h e t e m p e r a t u r e o f t h e r o o f s u r f a c e t o r i s e above 32°F a t a time when t h e ambient a i r t e m p e r a t u r e i s below f r e e z i n g . T h i s can happen when t h e r o o f s u r f a c e i s warmed b y h e a t l o s s from t h e b u i l d i n g below and a l s o by s o l a r r a d i a t i o n which p e n e t r a t e s t h e snow.
I t i s d o u b t f u l t h a t much can b e done t o modify t h e h e a t g a i n from t h e s u n o t h e r t h a n by s h a d i n g t h e r o o f w i t h t a l l t r e e s o r i n some s i m i l a r manner. Work c a r r i e d o u t e l s e w h e r e showed t h a t snow t e m p e r a t u r e s immediately above an aluminum-covered r o o f and an a s p h a l t - s h i n g l e d r o o f were s i m i l a r [CRREL, Q u a r t e r l y R e p o r t , April-May-June, 19731. T h i s would
i n d i c a t e t h a t t h e r e f l e c t i v i t y o f t h e r o o f s u r f a c e h a s l i t t l e e f f e c t on r o o f s u r f a c e t e m p e r a t u r e under a c o v e r o f snow.
The c o n v e n t i o n a l method o f d e a l i n g w i t h t h e problem i s t o i n s u l a t e t h e c e i l i n g below t h e r o o f and t o v e n t i l a t e t h e a t t i c s o a s t o c a r r y away t h e h e a t which would o t h e r w i s e warm t h e r o o f . I n o r d e r t o s t o p t h e f o r m a t i o n o f i c e dams, t h e r o o f s h o u l d i d e a l l y be formed l i k e a n u m b r e l l a under which t h e wind c o u l d blow f r e e l y . For v a r i o u s r e a s o n s , such a s snow b e i n g blown o n t o t h e i n s u l a t i o n , t h i s i s n o t a p r a c t i c a l form o f c o n s t r u c t i o n , and one i s n o r m a l l y r e s t r i c t e d t o v e n t i l a t i o n t h r o u g h g a b l e ends and t h r o u g h t h e eave s o f f i t .
INSTALLATION
For t h e p u r p o s e o f o b t a i n i n g s p e c i f i c i n f o r m a t i o n about r o o f t e m p e r a t u r e s , thermocouples w e r e i n s t a l l e d i n v a r i o u s p l a c e s on t h e r o o f when t h e o p p o r t u n i t y p r e s e n t e d i t s e l f a t t h e time o f a major r e n o v a t i o n
and e x t e n s i o n o f t h e a u t h o r ' s h o u s e . During t h e c o u r s e o f t h i s renova- t i o n , t h e r o o f , which had b e e n a s i m p l e r i d g e r o o f w i t h g a b l e e n d s , was changed t o a gambrel r o o f e x t e n d i n g down t o t h e second f l o o r l e v e l . The u p s t a i r s windows were c o n v e r t e d i n t o dormer windows and v e n t i l a t i o n was p r o v i d e d t h r o u g h t h e g a b l e ends o f t h e s e dormers a s w e l l a s t h r o u g h t h e main g a b l e ends o f t h e h o u s e . An e a v e s o f f i t s t r i p v e n t i l a t o r was
i n s t a l l e d a t t h e r e a r , where a s h e d dormer was p r o v i d e d o v e r t h e bathroom. I n a d d i t i o n t o t h e s e c o n v e n t i o n a l forms o f v e n t i l a t i o n , a v e n t i l a t i o n p a s s a g e was p r o v i d e d up t h e l o w e r s l o p e o f t h e gambrel r o o f and i n t o t h e a t t i c . T h i s s p a c e was formed by p l a n t i n g 2 by 2 ' s on t o p o f t h e
s h e a t h i n g , t o which t h e s h i n g l e s were n a i l e d , was t h e n added o v e r t h e 2 by 2 ' s . The 1 - 1 / 2 by 1 4 - 1 / 2 i n . s p a c e formed i n t h i s way c o n n e c t e d t h c
a t t i c w i t h a c o n t i n u o u s s t r i p v e n t a t t h e 2nd s t o r e y f l o o r l e v e l . The dormers i n t e r r u p t e d t h e s e p a s s a g e s and t h e r e f o r e t h e r e was no v e n t i l a t i o n up t h e lower r o o f / w a l l where t h e y o c c u r r e d . The c e i l i n g o f t h e o r i g i n a l h o u s e was i n s u l a t e d w i t h 5 - 1 / 2 i n . o f rock-wool-paper f a c e d i n s u l a t i o n which f i l l e d t h e 2 by 6 j o i s t s p a c e s . There was an open s p a c e d b o a r d
f l o o r o v e r t h e c e n t r e 8 f t . The i n s u l a t i o n i n t h e e x t e n s i o n was R.20 g l a s s f i b r e i n s u l a t i o n w i t h no p a p e r e n c l o s u r e , b u t t h e c e n t r e 8 f t . s t r i p was c o v e r e d w i t h s h e e t s o f 3 / 8 i n . plywood l o o s e l y l a i d i n p l a c e . F i g u r e s 1 t o 8 i l l u s t r a t e f r o n t and r e a r e l e v a t i o n s , a c r o s s s e c t i o n o f t h e h o u s e , some d e t a i l s o f c o n s t r u c t i o n , and t h e l o c a t i o n o f t h e t h e r m o c o u p l e s . These were i n s t a l l e d a t t h e t i m e o f c o n s t r u c t i o n on t h e o u t s i d e o f t h e r o o f deck o r r o o f / w a l l s h e a t h i n g b e f o r e t h e a p p l i c a - t i o n o f t h e u n d e r l a y and s h i n g l e s . The l e a d s were r u n down t o t h e b a s e - ment where t h e y were c o n n e c t e d t o a 16 p o i n t speedomax r e c o r d e r . The f r o n t o f t h e h o u s e f a c e d 12" w e s t o f n o r t h . The s l o p e o f t h e u p p e r r o o f was 25 1/2O, and t h a t o f t h e back ( s o u t h f a c i n g ) r o o f / w a l l was 67O from t h e h o r i z o n t a l . (The s l o p e o f t h e f r o n t r o o f / w a l l was s l i g h t l y s t e e p e r . ) The s u n ' s r a y s , t h e r e f o r e , would s t r i k e t h e u p p e r r o o f a p p r o x i m a t e l y a t r i g h t a n g l e s i n mid-summer and t h e r e a r r o o f / w a l l a p p r o x i m a t e l y a t r i g h t a n g l e s i n m i d - w i n t e r .
Thermocouples were p r o v i d e d t o measure o u t s i d e and a t t i c a i r t e m p e r a t u r e s , and were i n s t a l l e d on b o t h t h e n o r t h a n d s o u t h s i d e s o f t h e h o u s e i n t h e r o o f / w a l l v e n t i l a t i o n s p a c e , u n d e r t h e r o o f / w a l l s h i n g l e s and t h e upper r o o f s h i n g l e s 1 f t from t h e b r e a k i n t h e r o o f l i n e a n d 4 f t from t h e r i d g e . Because i t was n o t p o s s i b l e t o i n s t a l l t h e t h e r m o c o u p l e s o t h e r t h a n a t t h e t i m e o f c o n s t r u c t i o n , t h e i n s t a l l a - t i o n i n t h e m o d i f i e d h o u s e was d u p l i c a t e d i n t h e e x t e n s i o n t o e n a b l e a comparison o f d i f f e r e n t v e n t i l a t i o n o r i n s u l a t i o n s y s t e m s t o b e made l a t e r i f i t was f e l t d e s i r a b l e t o do s o . Of t h e 19 t h e r m o c o u p l e s i n s t a l l e d , o n l y 16 c o u l d b e c o n n e c t e d t o t h e r e c o r d e r and 3 h a d t o b e l e f t u n c o n n e c t e d . Because o f t h e d u p l i c a t i o n , t h e measurement o f t h e a t t i c and two u p p e r r o o f t e m p e r a t u r e s 4 f t from t h e r i d g e i n t h e e x t e n - s i o n was o m i t t e d . I t was f e l t t h a t t h e r e would b e l i t t l e d i f f e r e n c e between t h e s e t e m p e r a t u r e s and t h e c o r r e s p o n d i n g o n e s i n t h e m o d i f i e d h o u s e .
The i n s t a l l a t i o n was completed on 4 t h J a n u a r y 1 9 7 3 a n d , e x c e p t f o r some s h o r t p e r i o d s when t h e r e c o r d e r was s w i t c h e d o f f , t h e tempera- t u r e o f e a c h p o i n t was r e c o r d e d a u t o m a t i c a l l y a t i n t e r v a l s o f a p p r o x i - mately 16 nlin u n t i l 1 6 t h A p r i l 1973. By t h i s t i m e a l l t h e snow a n d i c e h;id m e l t c d o f f t h e r o o f . F u r t h e r r e a d i n g s were t a k e n between 1 2 t h J u n e and 1 1 t h -July 1973 i n o r d e r t o check t h e maximum s h i n g l e t e m p e r a t u r e s i n summer
.
I n o r d e r t o p r e v e n t i c e damming, t h e r o o f t e m p e r a t u r e must b e k e p t a s low a s p o s s i b l e ; h e a t t r a n s f e r t o t h e r o o f from t h e a t t i c must
be minimized. The e f f e c t o f r e f l e c t i v e i n s u l a t i o n (aluminum f o i l ) a p p l i e d t o t h e u n d e r s i d e o f t h e r o o f deck was t h e r e f o r e i n v e s t i g a t e d i n t h e f a l l o f 1973 i n t h e o r i g i n a l house a t t h e l o c a t i o n o f thermocouples No. 4 and 5 . The r e c o r d e r c o n n e c t i o n s were changed s o a s t o measure o u t - s i d e a i r t e m p e r a t u r e and o n l y t h e a t t i c and u p p e r r o o f t e m p e r a t u r e s i n t h e o r i g i n a l house and t h e e x t e n s i o n . Readings were t h e n t a k e n i n r e r - m i t t e n t l y from 29th October 1973 t o 4 t h J a n u a r y 1974. F u r t h e r r e a d i n g s were t a k e n from 5 t h t o 23rd J u l y 1974 i n o r d e r t o d e t e r m i n e i f t h e u s e o f aluminum f o i l reduced t h e amount of h e a t l o s s from t h e s h i n g l e s t o t h e a t t i c i n summer s o much a s t o r a i s e t h e maximum s h i n g l e t e m p e r a t u r e s d a n g e r o u s l y h i g h .
I n March 1974 a 4 f t s q u a r e a r e a o f t h e n o r t h r o o f o f t h e
e x t e n s i o n was i n s u l a t e d w i t h R.20 g l a s s f i b r e b a t t s . There were no t h e r - mocouples a t t h i s l o c a t i o n b u t t h e r a t e a t which t h e snow m e l t e d from t h e i n s u l a t e d a r e a was compared w i t h t h a t o f t h e snow on t h e r e m a i n d e r o f t h e r o o f .
ROOF TEMPERATURES I N THE ABSENCE OF SOLAR RADIATION
M e t e o r o l o g i c a l r e c o r d s were examined t o l o c a t e c o l d b u t o v e r - c a s t days i n an a t t e m p t t o d e t e r m i n e t h e e f f e c t o f h e a t l o s s from t h e house a l o n e on t h e t e m p e r a t u r e d i s t r i b u t i o n between t h e v a r i o u s p o i n t s on t h e r o o f and r o o f / w a l l . No very c o l d days c o u l d be found t h a t were n o t a l s o c l e a r sunny ones and i t was found t h a t t h e a i r t e m p e r a t u r e was u s u a l l y +15"F o r h i g h e r under o v e r c a s t s k i e s . During one such p e r i o d
( 6 t h and 7 t h February 1973) when t h e a i r t e m p e r a t u r e was + 1 4 " ~ , th e t e m p e r a t u r e s measured a t t h e o t h e r l o c a t i o n s were no more t h a n 6 F deg h i g h e r e x c e p t f o r t h e a t t i c which was 29°F. T h i s would i n d i c a t e t h a t
a l l o u t s i d e s u r f a c e t e m p e r a t u r e s t e n d e d t o follow t h e a i r t e m p e r a t u r e v e r y c l o s e l y a s might b e e x p e c t e d f o r w e l l - i n s u l a t e d b u i l d i n g s . Unfor- t u n a t e l y , no r e c o r d o f t h e d e p t h o f snow on t h e r o o f was a v a i l a b l e b u t t h e r e was u n l i k e l y t o have been more t h a n a few i n c h e s owing t o mild t e m p e r a t u r e s and t h e l a c k o f heavy s n o w f a l l s i n J a n u a r y .
S i m i l a r l y , on 28th J a n u a r y t h e a t t i c was a t 29 t o 30°F a l l day w i t h a i r t e m p e r a t u r e a t 19°F. The t e m p e r a t u r e s o f t h e remaining p o i n t s were between 2 2 and 2 7 " ~ .
[ l u r i n g t h e p e r i o d from 2 8 t h February t o 2nd March, f o l l o w i n g a sunny day on 28th February, i t became o v e r c a s t o v e r n i g h t and i t was snowing by 0700 h r on 1 s t March. The a i r t e m p e r a t u r e remained more o r
l e s s c o n s t a n t a l l n i g h t a t 16 t o 17°F w h i l e t h e a t t i c t e m p e r a t u r e dropped s t e a d i l y from 3 4 " ~ a t 2000 h r t o 30°F a t 0600 h r t h e f o l l o w i n g morning.
A l l o t h e r p o i n t s e x c e p t one were between 1 7 and 24°F and d i d n o t
f l u c t u a t e more t h a n 1 o r 2 F deg. The one e x c e p t i o n was t h e t e m p e r a t u r e i n t h e v e n t i l a t i o n s p a c e on t h e s o u t h s i d e o f t h e e x t e n s i o n which
t h i s s u g g e s t e d t h a t t h e r e was an a i r l e a k from t h e house i n t o t h i s s p a c e . During 1 s t March, t h e a i r t e m p e r a t u r e r o s e s l i g h t l y t o 2 1 ' ~ by 0900 h r , t h e n remained s t e a d y , and t h a t o f t h e a t t i c r o s e a s i m i l a r amount t o 3 5 " ~ b u t d i d n o t r e a c h t h i s p o i n t u n t i l 1430 h r . The d i f f e r e n c e i n t e m p e r a t u r e between t h e o t h e r p o i n t s i n c r e a s e d s l i g h t l y d u r i n g t h e day which may have been due t o t h e i n s u l a t i n g e f f e c t o f t h e b l a n k e t o f f r e s h snow ( 8 . 6 i n . f e l l d u r i n g t h e d a y ) . Some s u p p o r t i s g i v e n t o t h i s s u g g e s t i o n by t h e f a c t t h a t t h e t e m p e r a t u r e s o f t h e s h i n g l e s on t h e upper r o o f where t h c snow would c o l l e c t wcre somewhat h i g h e r t h a n t h o s e on t h e lower r o o f / w a l l . l l c d i f f e r c n c c was n o t g r e a t , howevcr, and t h e r e was some o v e r l a p i n t e m p e r a t u r e between t h e two s e t s . A t a b o u t 2200 h r , f o r example, t h e t e m p e r a t u r e s o f t h e r o o f s h i n g l e s were i n t h e range o f 25 t o 32OF and t h e w a l l s h i n g l e s were i n t h e range o f 22 t o 2 6 ' ~ .
I t a p p e a r s , t h e r e f o r e , t h a t f o r a r e a s o n a b l y w e l l - i n s u l a t e d h o u s e , t h e t e m p e r a t u r e o f t h e r o o f s h i n g l e s a r e u n l i k e l y t o r i s e above 32OF on a d u l l day, even w i t h t h e i n s u l a t i n g e f f e c t o f a b l a n k e t o f f r e s h snow, when t h e a i r t e m p e r a t u r e i s i n t h e low 2 0 ' s . With l e s s snow on t h e r o o f , t h e a i r t e m p e r a t u r e would have t o be i n t h e u p p e r 2 0 ' s f o r m e l t i n g t o o c c u r .
ROOF TEMPERATURES ON A COLD AND SUNNY DAY
C o n s i d e r t h e c o n d i t i o n s on 6 t h , 7 t h and 8 t h J a n u a r y 1973. There i s no r e c o r d o f t h e d e p t h o f snow on t h e r o o f b u t a photograph t a k e n on 24th December 1972 ( F i g . 9) shows a b o u t 6 i n . on t h e n o r t h r o o f ; a second one t a k e n on 3rd J a n u a r y 1 9 7 3 ( F i g . 10) shows s l i g h t l y more snow.
A t h i r d p h o t o g r a p h , a l s o t a k e n on 3 r d J a n u a r y 1973 ( F i g . l l ) , shows a b o u t
3 i n . o f snow on t h e s o u t h r o o f . The w e a t h e r r e c o r d s f o r Ottawa I n t e r -
n a t i o n a l A i r p o r t show t h a t December 1972 was a month of more t h a n a v e r a g e s n o w f a l l . The f o l l o w i n g s n o w f a l l s were r e c o r d e d between 24th December and 6 t h J a n u a r y : 26th December 1972 2 . 7 i n . 27th December 1972 1 . 3 i n . 28th December 1972 0 . 9 i n . 30th December 1972 4 . 9 i n . 4 t h J a n u a r y 1973 2.0 i n . 5 t h J a n u a r y 1973 3 . 6 i n . f o r a t o t a l o f 15.4 i n . The r e l a t i v e l y s m a l l q u a n t i t y o f snow on t h e s o u t h r o o f on 3 r d J a n u a r y and t h e f a c t t h a t t h e r o o f was a l r e a d y b a r e o f snow a b o u t 2 o r 3 i n . from t h e e a v e i n d i c a t e d t h a t t h e snow d i d n o t remain l o n g on t h a t s i d e (2nd and 3 r d J a n u a r y were sunny d a y s ) . One c o u l d n o t r e a s o n a b l y assume, t h e r e f o r e , t h a t t h e r e was more t h a n a b o u t
8 i n . o f snow on t h e s o u t h r o o f on t h e morning o f t h e 6 t h ( 3 i n . +
2 . 0 i n . + 3.6 i n . = 8 . 6 i n . ) f o l l o w i n g t h e s n o w f a l l s on t h e 4 t h and 5 t h .
u n l i k e l y t o have been a s much a s t h e 6 i n . p l u s t h c 15 i n . snowfnl I . A
f i g i l r c o f 1 2 t o 15 i n . would bc more l i k e l y .
I n p a s s i n g , i t may b e o b s e r v e d t h a t t h e t u n n e l u n d e r t h e snow, which can b e s e e n i n F i g . 9, was c a u s e d by an a i r l e a k which a l I o w e d h o t a i r t o flow up t h e r a f t e r s p a c e and warm t h e r o o f s h e a t h i n g a l o n g t h i s
l i n e a s i s shown by t h e p a t t e r n o f f r o s t on t h e r o o f s e e n i n F i g . 1.
Upper Roof
A s t u d y of t h e r e c o r d e d r o o f t e m p e r a t u r e s d u r i n g t h e snow c o n d i t i o n s d e s c r i b e d p r e v i o u s l y y i e l d e d t h e f o l l o w i n g :
(1) S h i n g l e s 1 f t . from e a v e o n t h e n o r t h s i d e ( F i g . 1 2 ) . T h e r e was no g r e a t d i f f e r e n c e between t h e tempera- t u r e s o f t h e s h i n g l e s on t h e o r i g i n a l h o u s e and t h e e x t e n s i o n , t h e maximum d i f f e r e n c e b e i n g l e s s t h a n 4 F deg between 0800 and 0900 h r on t h e 6 t h . The t e m p e r a t u r e s were between 0 and 10 F deg warmer t h a n t h e o u t s i d e a i r . They a l s o l a g g e d b e h i n d t h e a i r t e m p e r a t u r e by 1 t o 2 h r and s o t e n d e d t o b e c l o s e s t t o i t s h o r t l y a f t e r noon when t h e a i r t e m p e r a t u r e was a t i t s maximum. S h i n g l e s 1 f t . from e a v e on s o u t h s i d e ( F i g . 1 3 ) . The t e m p e r a t u r e o f t h e s h i n g l e s r o s e w e l l above t h e ambient a i r t e m p e r a t u r e and, i n f a c t , above t h e a t t i c t e m p e r a t u r e on b o t h t h e o r i g i n a l h o u s e and t h e e x t e n s i o n . Those on t h e o r i g i n a l house r e a c h e d a maximum o f 43°F a t 1330 h r on t h e 7 t h , compared t o a maximum a t t i c t e m p e r a t u r e o f 2 4 " ~ which was n o t r e a c h e d u n t i l 1600 h r . The d i f f e r e n c e s i n tempera- t u r e were n o t s o g r e a t on t h e e x t e n s i o n , b e i n g 3°F above a t t i c t e m p e r a t u r e a t 1500 h r . No d e f i n i t e e x p l a n a t i o n can b e o f f e r e d f o r t h e d i f f e r e n c e be- t w e e n t h e two t h e r m o c o u p l e s . I t may b e n o t e d , how-
e v e r , t h a t t h e v e n t i l a t i o n s p a c e i n t h e w a l l o f t h e o r i g i n a I h o u s e i s c o n n e c t e d t o t h e a t t i c j u s t below t h e l o c a t i o n o f t h e t h e r m o c o u p l e . I t i s p o s s i b l e t h a t a i r i n t h i s s p a c e was warmed by t h e s u n and r o s e t o flow o v e r t h e u n d e r s i d e o f t h e r o o f b o a r d s . The maximum t e m p e r a t u r e r e c o r d e d i n t h e v e n t i l a t i o n s p a c e was a b o u t 5 t o 10 F deg c o o l e r t h a n t h e maxi- mum s h i n g l e t e m p e r a t u r e , b u t i t p r e c e d e d t h e
s h i n g l e t e m p e r a t u r e s s l i g h t l y . The p h o t o g r a p h t a k e n on 3 r d J a n u a r y ( F i g . 11) shows t h a t t h e r e was more i c e a l o n g t h e c o n v e n t i o n a l e a v e of t h e e x t e n s i o n where t h a t thermocouple was l o c a t e d t h a n on t h e
o r i g i n a l h o u s e . The s u n , t h e r e f o r e , may n o t have been a b l e t o warm t h e r o o f a t t h a t p o i n t a s e a s i l y a s on t h e o r i g i n a l h o u s e . I t may b e n o t e d a l s o t h a t t h e maximum t e m p e r a t u r e r e c o r d e d e a c h day f o r t h e s h i n g l e s on t h e o r i g i n a l h o u s e r o s e s l i g h t l y day by day d e s p i t e a f a l l i n g a i r t e m p e r a t u r e . The peak t e m p e r a t u r e s o f t h e a t t i c and t h e s h i n g l e s a t t h e o t h e r l o c a t i o n s t e n d e d t o f a l l .
I t would seem t h a t c o n s i d e r a b l e b e n e f i t i n t h e con- t r o l o f i c e dams c o u l d b e o b t a i n e d on s o u t h e r l y
e x p o s u r e s i f t h e edge o f t h e r o o f c o u l d b e k e p t c l e a r o f snow. The p r e s e n c e o f an e a v e s t r o u g h t o c o n t a i n t h e snow would p r o b a b l y p r e v e n t t h i s edge c l e a r i n g o c c u r r i n g by n a t u r a l means a n d a l s o h i n d e r any a t t e m p t by t h e owner t o c l e a r t h e r o o f edge manually. I t i s u n l i k e l y t h a t a s i m i l a r c l e a r i n g on t h e n o r t h e a v e would have any e f f e c t . I n an e n d e a v o u r t o v e r i f y t h e e f f e c t o f a s n o w - f r e e edge on t h e r o o f , a s m a l l
s t e e p l y - s l o p i n g e x t e n s i o n t o which t h e snow would n o t s t i c k was b u i l t o n t o h a l f o f t h e e a v e o v e r t h e b a t h - room and t h e snow and i c e a c c u m u l a t i o n s on t h e two h a l v e s were compared d u r i n g t h e w i n t e r o f 1974-75.
I t was found t h a t t h e i c i c l e f o r m a t i o n below t h e
e x t e n s i o n was much g r e a t e r t h a n t h a t on t h e u n m o d i f i e d e a v e ( F i g . 1 4 ) . I t was found l a t e r t h a t t h e r e was no i c e on t h e r o o f above t h e e x t e n s i o n ( F i g . 15) whereas a s m a l l i c e dam h a d s t a r t e d t o form on t h e o t h e r h a l f ( F i g . 1 6 ) . I n t h e e a r l y p a r t o f t h e w i n t e r , however, t h e c o n d i t i o n s were s u c h t h a t many i c i c l e s were formed b u t l a t e r c o n d i t i o n s were n o t c o n d u c i v e t o t h e forma- t i o n o f l a r g e i c e dams. The e v i d e n c e i s t h e r e f o r e n o t c o n c l u s i v e . E a r l i e r s t u d i e s c o n d u c t e d d u r i n g t h e w i n t e r o f 1972-73 on t h e w e s t f a c i n g e a v e s o f o t h e r h o u s e s i n d i c a t e d t h a t o n l y s l i g h t b e n e f i t c o u l d b e o b t a i n e d from s i m i l a r e a v e e x t e n s i o n s ( F i g . 1 7 ) . (3) S h i n g l e s 4 f t . from t h e r i d g e ( o r i g i n a l h o u s e ) ( F i g . 1 8 ) . The t e m p e r a t u r e s o f t h e s e r o o f s h i n g l e s were between t h o s e o f t h e o u t s i d e a i r and t h e a t t i c . They f l u c t u a t e d i n sympathy w i t h t h e changes i n t h e o t h e r two a n d t h a t on t h e s o u t h s i d e r e s p o n d e d a l s o t o t h e e f f e c t s o f s o l a r r a d i a t i o n
The n o r t h s i d e s h i n g l e f l u c t u a t e d i n t e m p e r a t u r e l e s s t h a n e i t h e r t h e o u t s i d e a i r o r t h e a t t i c , v a r y - i n g l e s s t h a t 5 F deg compared w i t h 8 o r 9 F deg f o r t h e a t t i c and 15 t o 25 F deg f o r t h e o u t s i d e a i r .
Maximum t e m p e r a t u r e s were a l s o r e a c h e d a t d i f f e r e n t tirncs: 1400 t o 1500 hr f o r t h c o i ~ t s i c l c a i r , 1500 t o 1700 h r f o r t h e a t t i c and 1800 t o 2200 h r f o r t h e s h i n g l e , which remained a t o r n e a r i t s maximum t e m p e r a t u r e f o r l o n g e r p e r i o d s o f t i m e . On t h e s o u t h s i d e , t h e s h i n g l e t e m p e r a t u r e f l u c t u a t e d much more t h a n t h a t on t h e n o r t h s i d e owing t o t h e i n f l u e n c e o f t h e s u n . I t v a r i e d through a range o f 20 t o 25 F deg and a l t h o u g h i t
peaked a f t e r t h e o u t s i d e a i r t e m p e r a t u r e , t h i s peak sometimes preceded t h a t o f t h e a t t i c a i r t e m p e r a t u r e . A t no time d u r i n g t h e p e r i o d under c o n s i d e r a t i o n d i d i t exceed t h e a t t i c t e m p e r a t u r e i n d i c a t i n g t h a t , w i t h a b o u t 8 i n . o f snow on t h e r o o f , t h e h e a t flow was always o u t w a r d s . I n o t h e r words, no h e a t was t r a n s f e r r e d t o t h e a t t i c through s o l a r
r a d i a t i o n p e n e t r a t i n g t h e r o o f . I t s h o u l d a l s o be n o t e d t h a t o f t h e s i x r o o f s h i n g l e s whose tempera- t u r e s were r e c o r d e d , o n l y t h e t e m p e r a t u r e o f t h e s h i n g l e s 1 f t from t h e eave on t h e s o u t h s i d e o f t h e o l d house r o s e above a t t i c t e m p e r a t u r e t o any
a p p r e c i a b l e d e g r e e and f o r any l e n g t h o f t i m e . Thus t h e d i r e c t s o l a r h e a t g a i n t o t h e a t t i c through t h i s depth o f snow on t h e r o o f may w e l l b e minimized. The r i s e i n a t t i c t e m p e r a t u r e would t h e r e f o r e seem t o b e t h e r e s u l t o f a b a l a n c e between a more o r l e s s c o n s t a n t h e a t l o s s from t h e house and v e n t i l a t i o n w i t h o u t s i d e a i r . I n t h e c a s e o f t h i s p a r t i c u l a r house w i t h i t s somewhat unusual r o o f and w a l l con- s t r u c t i o n i t i s p o s s i b l e t h a t s o l a r h e a t g a i n through t h e s h i n g l e s on t h e lower r o o f / w a l l i n t o t h e v e n t i l a - t i o n s p a c e a l s o a f f e c t e d t h e a t t i c t e m p e r a t u r e . T h i s s p a c e , however, covered o n l y 1 3 f t on t h e s o u t h s i d e o u t o f a t o t a l l e n g t h o f 39 f t .
Roof/wall and V e n t i l a t i o n Space Temperatures
Roof/Wall s h i n g l e s on t h e n o r t h s i d e ( F i g . 1 9 ) . The s h i n g l e s on b o t h t h e o l d house and t h e e x t e n - s i o n f o l l o w e d a l m o s t i d e n t i c a l t e m p e r a t u r e p a t t e r n s , which were a l s o v e r y s i m i l a r t o t h o s e o f t h e o u t s i d e a i r . During t h e day, t h e s h i n g l e t e m p e r a t u r e r o s e more r a p i d l y t h a n t h e a i r t e m p e r a t u r e and was two o r t h r e e d e g r e e s warmer a b o u t midday when t h e a i r tem- p e r a t u r e was c l o s e t o O°F. O v e r n i g h t , t h e r e was very
l i t t l e d i f f e r e n c e between t h e s h i n g l e and a i r tempera- t u r e s a l t h o u g h t h e s h i n g l e s tended t o b e a d e g r e e o r s o c o l d e r .
(2) Roof/Wall s h i n g l e s on t h e s o u t h s i d e ( F i g . 2 0 ) . A s o n e would e x p e c t , t h e s i t u a t i o n was v a s t l y d i f f e r e n t from t h e n o r t h s i d e s i n c e t h e s u n ' s r a y s were a p p r o x i m a t e l y normal t o t h e s h i n g l e s u r f a c e a b o u t midday i n m i d - w i n t e r . A t noon, b o t h s h i n g l e t e m p e r a t u r e s were o v e r 80 F deg above t h e a i r tem- p e r a t u r e . I n d e e d , t h e t e m p e r a t u r e o f t h e s h i n g l e s on t h e o r i g i n a l h o u s e r o s e t o a h i g h of 9 5 F deg on 8 t h ,January 1 9 7 3 f o r a g a i n o f a l m o s t 100 F d e g o v e r t h e a i r t e m p e r a t u r e which was s l i g h t l y below
0"17. I t may b e n o t e d t h a t t h e wind on t h e 8 t h h a d d r o p p e d t o 5 mph a s compared t o a b o u t 15 mph on t h e 0 t h a n d 7 t h . I n a l l c a s e s , t h e peak s h i n g l e t e m p e r a - t u r e o c c u r r e d a n h o u r o r s o b e f o r e t h e maximum a i r t e m p e r a t u r e . T h i s c o u l d h a v e b e e n due t o t h e o r i e n - t a t i o n o f t h e h o u s e w i t h t h e r e a r f a c i n g s l i g h t l y e a s t o f s o u t h . O v e r n i g h t , t h e s h i n g l e t e m p e r a t u r e on t h e o r i g i n a l h o u s e d r o p p e d s l i g h t l y below a i r t e m p e r a - t u r e owing t o r a d i a t i o n t o a c l e a r n i g h t s k y . I t v a r i e d b e t w e e n 1 a n d 5 F deg on t h e d a t e s u n d e r c o n s i d e r a t i o n . No r e c o r d o f t h e d e g r e e o f c l a r i t y on t h e d i f f e r e n t n i g h t s was a v a i l a b l e , however. A s n o t e d e a r l i e r , t h e t e m p e r a t u r e o f t h e s h i n g l e s o n t h e n o r t h s i d e d i d n o t d r o p below t h e a i r tem- p e r a t u r e t o t h e same e x t e n t a s d i d t h a t o f t h e s h i n g l e s on t h e s o u t h s i d e . I t i s p r o b a b l e t h a t t h i s d i f f e r e n c e b e t w e e n t h e n o r t h e r n a n d s o u t h e r n e x p o s u r e s was due t o t h e g r e a t e r number o f l a r g e t r e e s o n t h e n o r t h s i d e e v e n though t h e y a r e d e c i d u o u s t r e e s and b a r e o f l e a v e s i n w i n t e r . The s h i n g l e s on t h e s o u t h s i d e o f t h e e x t e n s i o n d i d n o t , however, f i t i n t o t h e same p a t t e r n , f o r a l t h o u g h t h e t e m p e r a t u r e r o s e a b o u t 80 F deg above t h e a i r t e m p e r a t u r e d u r i n g t h e d a y , i t r e m a i n e d 7 t o 10 F deg above i t d u r i n g t h e n i g h t . I t i s p o s s i b l e t h a t t h e p o s i t i o n o f t h e s e s h i n g l e s i n a r e c e s s b e t w e e n two dormers may h a v e h a d a n e f f e c t on t h e i r t e m p e r a t u r e , b u t i t i s more p r o b a b l e t h a t t h e r e was an a i r l e a k from t h e h o u s e i n t o t h e ven- t i l a t i o n s p a c e b e h i n d them. The t e m p e r a t u r e i n t h a t s p a c e was c o n s i s t e n t l y h i g h e r t h a n i n s i m i l a r s p a c e s e l s e w h e r e on t h e h o u s e .
(3) Roof/Wall v e n t i l a t i o n s p a c e s on t h e n o r t h s i d e . The t e m p e r a t u r e s i n t h e v e n t i l a t i o n s p a c c s on t h e n o r t h s i d e o f b o t h t h e o r i g i n a l h o u s e a n d t h e e x t e n s i o n were, l i k e t h e s h i n g l e s , e s s e n t i a l l y s i m i l a r . They f o l l o w e d t h e p a t t e r n o f a i r tempera- t u r e b u t were 5 t o 7 F deg warmer.
( 4 ) Roof/Wall v e n t i l a t i o n s p a c e s on t h e s o u t h s i d e . ( a ) O r i g i n a l house
On t h e s o u t h s i d e o f t h e h o u s e , a t o r s h o r t l y a f t e r midday, t h e t e m p e r a t u r e i n t h e v e n t i l a t i o n s p a c e was a b o u t 30 F deg above t h e a i r t e m p e r a t u r e b e c a u s e o f s o l a r r a d i a t i o n . A t t h e same t i m e t h e s h i n g l e s were 80 t o 100 F deg above a i r t e m p e r a t u r e . On t h e n i g h t o f 5 t h / 6 t h J a n u a r y t h e s p a c e tempera- t u r e was a p p r o x i m a t e l y 1 5 F deg warmer t h a n t h e a i r t e m p e r a t u r e which was f a l l i n g a t t h a t t i m e . I t was 10 F deg warmer on t h e n i g h t o f 6 t h / 7 t h J a n u a r y . These o v e r n i g h t t e m p e r a t u r e s were a b o u t 2 t o 5 F deg warmer t h a n t h o s e i n t h e c o r r e s p o n d i n g s p a c e on t h e n o r t h s i d e a t a t i m e when t h e t e m p e r a t u r e o f t h e r o o f / w a l l s h i n g l e s on t h e s o u t h s i d e were e i t h e r t h e same a s o r s l i g h t l y c o l d e r t h a n t h a t of t h e s h i n g l e s on t h e n o r t h s i d e . On t h e n i g h t o f 7 t h / 8 t h J a n u a r y , t h e t e m p e r a t u r e d i f f e r e n c e between t h e two s p a c e s was l e s s pronounced a n d between 0600 and 0800 h r , t h e s o u t h s p a c e was s l i g h t l y c o l d e r t h a n t h e n o r t h o n e . During t h e n i g h t o f 8 t h / 9 t h J a n u a r y , t h e two r o o f / w a l l s p a c e t e m p e r a t u r e s were v i r t u a l l y t h e same. The wind s p e e d and d i r e c t i o n r e c o r d s f o r NRC ( s i t u a t e d a p p r o x i - m a t e l y 2 m i l e s e a s t of t h e h o u s e ) showed t h a t on t h e n i g h t s o f 5 t h / 6 t h a n d 6 t h / 7 t h J a n u a r y , t h e wind was p r e d o m i n a n t l y from t h e n o r t h w e s t a t a s p e e d o f between 14 and 19 mph. On t h e n i g h t of t h e 7 t h / 8 t h Y t h e d i r e c t i o n was s ti 11 p r e d o m i n a n t l y n o r t h w e s t b u t t h e s p e e d h a d dropped t o between 8 and 10 mph. On 8 t h J a n u a r y , t h e wind s p e e d dropped t o 5 mph from t h e n o r t h w e s t and 6 mph from t h e w e s t a t 0700 and 0800 h r s r e s p e c t i v e l y . The s o u t h r o o f / w a l l s p a c e was s l i g h t l y c o o l e r t h a n t h a t on t h e n o r t h s i d e a t t h i s t i m e . By t h e n i g h t o f t h e 8 t h / 9 t h , t h e s p e e d h a d moderated
s t i l l f u r t h e r t o between calm and 5 mph and i t s d i r e c t i o n h a d changed t o s o u t h w e s t . The r e c o r d s f o r t h e v e n t i l a t i o n s p a c e t e m p e r a t u r e s , t h e wind s p e e d s and d i r e c t i o n s f o r t h e p e r i o d s o f 2 8 t h , 2 9 t h , and 3 0 t h J a n u a r y ( F i g . 21) and 9 t h , 1 0 t h and 1 1 t h F e b r u a r y 1 9 7 3 show s i m i l a r p a t t e r n s . One may c o n c l u d e , t h e r e f o r e , t h a t a flow o f a i r i s promoted i n t h e r o o f / w a l l
v e n t i l a t i o n s p a c e s from one s i d e o f t h e house t o t h e o t h e r when t h e wind impinges more o r l e s s d i r e c t l y on one f a c e . The wind speeds around t h e house were n o t measured b u t because o f t h e s h e l - t e r i n g e f f e c t o f s u r r o u n d i n g houses t h e y would be c o n s i d e r a b l y l e s s t h a n t h o s e r e c o r d e d a t NRC
.
(b) Extension The t e m p e r a t u r e s r e c o r d e d i n t h e v e n t i l a t i o n s p a c e on t h e s o u t h s i d e o f t h e e x t e n s i o n d i d n o t correspond t o t h o s e i n t h e s p a c e on t h e o r i g i n a l house. They were c o n s i s t e n t l y h i g h e r by 10 o r even 20 F deg d u r i n g t h e n i g h t and peaked a t about 60°F d u r i n g t h e day a s compared t o 30 t o 3S°F i n t h e o r i g i n a l house. T h i s , a s n o t e d e a r l i e r , l e a d s one t o t h e c o n c l u s i o n t h a t t h e a i r i n t h i s s p a c e on t h e e x t e n s i o n was b e i n g warmed t o some e x t e n t by a c o n v e c t i v e leakage o f a i r from i n s i d e t h e house. T h i s t h e o r y is sup- p o r t e d by t h e absence o f f r o s t on t h e r o o f t o t h e r i g h t o f t h e l e f t dormer a s seen i n Fig. 2.MAXIMUM ROOF SHINGLE TEMPERATURES IN
SUMMER
Because i t had been found t h a t t h e r o o f / w a l l s h i n g l e s r o s e 80 o r even 100 F deg above ambient a i r t e m p e r a t u r e on a sunny day i n mid- w i n t e r when t h e s u n ' s r a y s were approximately normal t o them, i t was d e c i d e d t o i n v e s t i g a t e t h e maximum t e m p e r a t u r e which might be reached by t h e r o o f s h i n g l e s i n mid-summer. The s l o p e o f t h e r o o f was such t h a t t h e s u n ' s r a y s would be normal t o t h e r o o f s u r f a c e s h o r t l y b e f o r e and a f t e r t h e summer s o l s t i c e . The t e m p e r a t u r e s were t h e r e f o r e r e c o r d e d between 1 2 t h June and 1 1 t h J u l y 1973, i . e . , a p e r i o d s t r a d d l i n g t h e s o l s t i c e .
The h i g h e s t t e m p e r a t u r e s were r e c o r d e d f o r t h e thermocouple under t h e s h i n g l e s 4 f t . from t h e r i d g e o f t h e r o o f on t h e s o u t h s i d e o f t h e o r i g i n a l house. The maximum s h i n g l e temperature was 172OF a t 1230 h r EST on 9 t h J u l y when t h e a i r t e m p e r a t u r e was 9S°F, a day which t h e Uplands weather o f f i c e r e c o r d e d a s "mainly sunny and warm". T h i s was a r i s e o f
77 F deg above a i r t e m p e r a t u r e . The maximum r i s e above a i r t e m p e r a t u r e was 88 F deg r e c o r d e d a t 1145 h r EST on 1 3 t h June when t h e s h i n g l e was
168OF and t h e a i r t e m p e r a t u r e was 80°F. I t can be s e e n , t h e r e f o r e , t h a t t h e r i s e i n s h i n g l e t e m p e r a t u r e because of s o l a r r a d i a t i o n i n summer was v e r y s i m i l a r t o t h a t o f t h e r o o f / w a l l s h i n g l e i n w i n t e r . The s u n ' s r a y s would be more o r l e s s normal t o t h e s h i n g l e i n each c a s e .
The s h i n g l e s 1 f t from t h e eave were c o n s i d e r a b l y c o o l e r t h a n t h e ones n e a r t h e r i d g e . T h i s may have been due t o some v e n t i l a t i o n o c c u r r i n g i n t h e r o o f / w a l l s p a c e , which would c o o l t h e s h i n g l e s n e a r t h e
eave, whereas t h e s h i n g l e s 4 f t . from t h c ri dgc miiy hnvc hccn o v c r an almost s t i l l pocket o f a i r . There was l i t t l e o r no d i f f e r e n c e i n temper- a t u r e between t h e s h i n g l e s n e a r t h e eave o f t h e o l d house and t h o s e on t h e e x t e n s i o n d e s p i t e t h e f a c t t h a t t h e s h i n g l e s on t h e e x t e n s i o n were n e a r a c o n v e n t i o n a l l y v e n t i l a t e d eave r a t h e r t h a n above t h e s p a c e i n t h e
r o o f / w a l l , which would be e x p e c t e d t o produce more c o n v e c t i v e v e n t i l a t i o n . The s h i n g l e s n e a r t h e e a v e had a maximwn t e m p e r a t u r e o f 157OF on 9 t h J u l y 1973 a s compared t o 17Z°F f o r t h o s e 4 f t . from t h e r i d g e ; t h e r o o f / w a l l s p a c e was a t 116OF and t h e a t t i c r e a c h e d a maximum of 130°F about two h o u r s l a t e r . On 1 3 t h J u n e , when t h e s h i n g l e s 4 f t . from t h e r i d g e e x p e r i e n c e d t h e i r maximum r i s e above ambient a i r t e m p e r a t u r e and r e a c h e d 168"F, t h e e a v e s h i n g l e s r o s e t o 14g°F, t h e r o o f / w a l l s p a c e t o 9h°F, and t h e a t t i c t o l l S ° F . For t h e n i n e sunny days examined, t h e maximum and a v e r a g e r i s e above a i r t e m p e r a t u r e f o r t h e two p o s i t i o n s o f r o o f s h i n g l e s , t h e a t t i c , and t h e r o o f / w a l l v e n t i l a t i o n s p a c e s were a s f o l l o w s :
Rise Above A i r Temperature (F deg)
Maximum Average Roof s h i n g l e s 4 f t from r i d g e 88 77 Roof s h i n g l e s 1 f t from eave 6 9 5 8
A t t i c 4 7 3 4
Roof/Wall v e n t i l a t i o n s p a c e s 25 21
EFFECT OF INSULATION PLACED UNDER THE ROOF DECK
An examination o f t h e t e m p e r a t u r e r e c o r d s t a k e n i n t h e f a l l and e a r l y w i n t e r o f 1973-74 i n d i c a t e d t h a t t h e aluminum f o i l i n s u l a t i o n had no s i g n i f i c a n t e f f e c t on t h e s h i n g l e t e m p e r a t u r e s . P r i o r t o i n s t a l l a t i o n of t h e f o i l , t e m p e r a t u r e d i f f e r e n c e s o f up t o 4 F deg were r e c o r d e d
between c o r r e s p o n d i n g thermocouples i n t h e o r i g i n a l house and i n t h e e x t e n s i o n a l t h o u g h t h e two a t t i c a i r t e m p e r a t u r e s remained w i t h i n 2 F deg of one a n o t h e r , t h e o l d house b e i n g s l i g h t l y warmer.
A f t e r i n s t a l l a t i o n o f t h e f o i l t h e d i f f e r e n c e i n a i r t e m p e r a t u r e i n t h e two h a l v e s o f t h e a t t i c remained unchanged. The e f f e c t o f t h e f o i l on t h e s h i n g l e t e m p e r a t u r e s was n o t c l e a r . There was some s l i g h t i n d i - c a t i o n t h a t when t h e h e a t flow was outward, i . e . , when t h e a t t i c was warmer t h a n t h e s h i n g l e s , t h e s h i n g l e s w i t h t h e f o i l below them were
s l i g h t l y c o o l e r than t h e o t h e r s . When t h e h e a t flow was inward because o f s o l a r r a d i a t i o n warming t h e s h i n g l e s , t h e r e v e r s e appeared t o be t r u e . One might conclude, t h e r e f o r e , t h a t t h e aluminum f o i l d i d r e s t r i c t t h e h e a t flow s l i g h t l y b u t t h e change i n s h i n g l e t e m p e r a t u r e was a t most
7 F deg. There was, moreover, no n o t i c e a b l e d i f f e r e n c e i n t h e r a t e a t which t h e snow melted from t h e a r e a s w i t h aluminum f o i l under them a s compared t o t h e remainder o f t h e r o o f . The R. 20 g l a s s f i b r e i n s u l a t i o n , on t h e o t h e r hand, r e t a r d e d t h e snow melt c o n s i d e r a b l y . R . 2 0 i n s u l a t i o n was used because i t was r e a d i l y a v a i l a b l e b u t i t i s p o s s i b l e t h a t b e n e f i t i n t h e c o n t r o l o f i c e damming c o u l d be o b t a i n e d from s m a l l e r t h i c k n e s s e s .
I n summer, t h e evidence a s t o t h e e f f e c t of r e f l e c t i v e i n s u l a - t i o n was once a g a i n i n c o n c l u s i v e . The h e a t flow under s o l a r r a d i a t i o n was always inward on b o t h t h e n o r t h and s o u t h s l o p e s of t h e r o o f b u t o n l y t h e s h i n g l e s on t h e s o u t h s i d e , 4 f t . from t h e r i d g e w i t h f o i l under them, showed evidence o f becoming warmer than t h o s e w i t h no f o i l . The d i f f e r e n c e was always l e s s t h a n 10 F deg however.
CONCLUSIONS
An i c e dam forms when t h e snow i n c o n t a c t w i t h t h e r o o f i s warmed t o about 3 2 ' ~ a t a time when t h e ambient a i r t e m p e r a t u r e i s l e s s t h a n 32OF. The n e c e s s a r y h e a t can be s u p p l i e d t o t h e snow b o t h from s o l a r r a d i a t i o n and h e a t l o s s from t h e house. When a t t e m p t i n g t o c o n t r o l t h e formation o f i c e dams l i t t l e can be done t o modify t h e d i r e c t h e a t g a i n from t h e sun o t h e r t h a n by s h a d i n g t h e r o o f w i t h t a l l t r e e s because roof c o l o u r a p p a r e n t l y has l i t t l e e f f e c t . E f f o r t s must t h e r e f o r e be d i r e c t e d t o r e d u c i n g t h e amount o f h e a t t r a n s f e r r e d from t h e house t o t h e roof by i n c r e a s i n g both t h e amount of i n s u l a t i o n and t h e a t t i c v e n t i l a - t i o n .
With a moderately w e l l - i n s u l a t e d house (between R. 15 and R. 2 0 i n s u l a t i o n i n t h e c e i l i n g ) w i t h r e a s o n a b l y good a t t i c v e n t i l a t i o n , i t i s u n l i k e l y t h a t snow w i l l melt on a d u l l day u n l e s s t h e a i r t e m p e r a t u r e r i s e s above 20 o r 2S°F. The p r e c i s e t e m p e r a t u r e w i l l be i n f l u e n c e d by many f a c t o r s i n c l u d i n g t h e depth o f snow on t h e r o o f . On a c o l d (o°F) b u t sunny day w i t h 6 t o 12 i n . o f snow on t h e r o o f , o n l y t h e t e m p e r a t u r e o f t h e s h i n g l e s c l o s e t o t h e eave on t h e s o u t h e r n exposure would be e x p e c t e d t o r i s e above a t t i c t e m p e r a t u r e . Where t h e r e i s l i t t l e snow t h i s t e m p e r a t u r e c o u l d r i s e above t h e f r e e z i n g p o i n t . To minimize i c e damming, t h e r e f o r e , t h e r e may be some advantage i n keeping t h e eave f r e e o f snow on a s o u t h e r n exposure s o t h a t t h e s h i n g l e s can be warmed by t h e sun and s o p r e v e n t t h e formation o f i c e . I t i s d o u b t f u l t h a t much bene- f i t would be o b t a i n e d on o t h e r e x p o s u r e s . A t o t h e r l o c a t i o n s on t h e r o o f and under snow c o v e r , t h e h e a t flow i s always outward and would a p p e a r t o be l a r g e l y a r e s u l t o f t h e b a l a n c e between t h e r a t e of h e a t l o s s from t h e house and t h e amount of v e n t i l a t i o n i n t h e a t t i c . Houses w i t h c a t h e d r a l
c e i l i n g s and no a t t i c s s h o u l d be more prone t o i c c clamming t h a n a r c t h o s e w i t h a t t i c s .
S h i n g l e s on a s t e e p l y p i t c h e d r o o f w i t h a s o u t h e r n e x p o s u r e , which i s f r e e o f snow e x c e p t f o r b r i e f p e r i o d s f o l l o w i n g a snow s t o r m , can e x p e r i e n c e a t e m p e r a t u r e r i s e o f up t o 100 F deg above ambient a i r t e m p e r a t u r e . S h i n g l e s on a l e s s s t e e p l y p i t c h e d r o o f can have a s i m i l a r t e m p e r a t u r e r i s e on sunny days i n summer.
Aluminum f o i l r e f l e c t i v e i n s u l a t i o n p l a c e d on t h e u n d e r s i d e o f t h e r o o f deck does n o t a p p e a r t o r e s t r i c t outward h e a t flow e f f e c t i v e l y enough t o r e d u c e t h e r a t e a t which t h e snow m e l t s . Nor does i t a p p e a r t o r e s t r i c t t h e inward h e a t flow i n summer s u f f i c i e n t l y t o r a i s e t h e r o o f s h i n g l e t e m p e r a t u r e s m a t e r i a l l y .
The r a n g e o f t e m p e r a t u r e s e x p e r i e n c e d by t h e s h i n g l e s and t h e a t t i c d u r i n g c l e a r c o l d w i n t e r c o n d i t i o n s a r e i l l u s t r a t e d i n F i g . 2 2 .
The d i f f e r e n c e from a i r t e m p e r a t u r e f o r e a c h l o c a t i o n i s t h e d i f f e r e n c e a t a s p e c i f i c t i m e b u t t h e t i m e s a t which t h e maximum o r minimum
d i f f e r e n c e s o c c u r a t d i f f e r e n t l o c a t i o n s do n o t n e c e s s a r i l y c o i n c i d e w i t h one a n o t h e r n o r w i t h t h e maximum o r minimum a i r t e m p e r a t u r e .
F I G U R E 1 FROTJT ELEVATION O F HOUSE
O R I G I N A L H O U S E P--- - -- +--- E X T E N S I O N 4 SHINGLE SHINGLE 4 SHINGLE 0 2 ATTIC A I R F I G U R E 4 L O C A T I O N O F T H E R M O C O U P L E S O N F R O N T E L E V A T I O N O F H O U S E OUTSIDE A I R
t
E X T E N S I O N I - O R I G I N A L H O U S E --1
1 3 0 SHINGLE j 5 SHINGLE OUTSIDE A I R I 5 , 3 ATTIC A I R F I G U R E 5 L O C A T I O N T H E R M O C O U P L E S O N R E A R E L E V A T I O N O F H O U S EA
/ SHINGLE SHINGLE SHINGLE FRONT iNORTHl VENTILATION 11 O R I G I N A L H O U S E o E X T E N S I O N F I G U R E 6 C R O S S S E C T I O N OF H O U S E ( T H R O U G H E X T E N S I O N )CJ 0 0 Z 2 4 - in
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3 z 2 g 3 L - 2 0 " m 0 2 Z O - O = OF I G U R E 9 SNOW COVER ON NORTH ROOF 2 4 T H DECEMBER 1 9 7 2
1 " ' ~ ' 1 1 " " 1 ~ ~ " ' 1 ' ~ ' - - - - - - - - A T T I C A I R - - - - - O R I G I N A L H O U S t I I I I I I I l l 1 1 1 1 1 1 l I l I I l I - F I G U R E 1 2 S H I N G L E T E M P E R A T U R E I F O O T F R O M E A V E , N O R T H S I D E I I I I I I I ~ ' I J ' I ~ I I I I l I I I -- - - - - A T T I C A I R - - - - - - - - - I l l I / I l l l l l l l l l l l l l l l O R I G I N A L HOUSE - t I G U R E 1 3 S H I N G I E T E M P E R A T U R E 1 F O O T F R O M E A V E , S O U T H S I D E
F I G U R E 1 4 I C I C L E FORMATION
F I G U R E 16 B E G I N N I N G O F I C E DAM ON CONVENTIONAL EAVE
F I G U R E 1 7 REDUCTION AND I C E ACCUMULATION OVER M O D I F I E D WEST- FACING EAVE
F I G U R E 1 8 S H I N G L E T E M P E R A T U R E S . 4 FEET F R O M R I D G E O N T H E O R I G I N A L H O U S E 1 0 0 I l l l l l l l l l l l l l l l l l l I I 8 0 f l G U H L 1 9 H O O l l W A L L S H I N G L E T L M P E R A T U R E . N O R l H S I D E . O R I G I N A L H O U S E - - - - - - 1 0 0 8 0 6 0 6 0 - - L L - - 4 0 - ATTIC A I R - - N O R l H S I D E O U T S I D L A I R - - 2 0 2 4 4 4 0 - 18 1 2 1 6 2 0 2 4 4 1 1 1 1 1 18 1 2 1 116 12 0 2 4 4 1 1 18 1 2 1 6 2 0 2 4 1 1 1 1 1C 18 1 2 1 1 6 1 1 5 l J A N 1 7 3 6 1 J A N l 7 3 ? ' J A N 1 7 3 8 l J A N 1 7 3 9 l J A N l 7 3 1 , 1 1 , 1 1 1 1 1 1 1 ) 1 1 1 1 1 1 1 1 1 - - - - - - - ATTIC A I R - L L - 4 0 - - 3 ROOFIWALL t 4 - e 2 0 - a -.-.-.-./ ./--'-'-,
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