A Metal frame window system for high indoor humidities

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A Metal frame window system for high indoor humidities

Bowen, R. P.; Solvason, R. P.

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de recherches Canada

A METAL FRAME WINDOW SYSTEM FOR HIGH INDOOR HUMIDITIES

by R.P. Bowen and K.R. Solvason

ANALYZED

Reprinted from

Proceedings, Second CSCE Conference on Building Science and Technology

November 10 81 _{1 1 1983, Waterloo, Ontario} p. 127 - 147

DBR Paper No. 1171

Division of Building Research

Price $2.00 OTTAWA NRCC 23094

Ris&

A f i n d'augmenter l a temp'erature de s u r f a c e d e s f e n s t r e s b encadrement dt a l l i q u e d'une p a r t i e d'un nouveau b k iment (moins d e 5 p. c e n t du t o t a l d e s f e n s t r e s ) e n vue d ' o b t e n i r une humidit'e r e l a t i v e d e 50 p. c e n t 3 une temp'erature d e 2 2 O C s a n s c o n d e n s a t i o n , l e s t e c h n i q u e s s u i v a n t e s o n t 6t'e examinees: u t i l l s a t i o n d ' u n v i t r a g e t r i p l e cons t i t u s d'un v i t r a g e d o u b l e s c e l l ' e d a n s l ' e n c a d r e m e n t e x t ' e r i e u r e t d ' u n v i t r a g e s i m p l e d a n s l'encadrement i n t ' e r i e u r ; p r e s s u r i s a t i o n d e l a lame d ' a i r e n t r e l e s c h b s i s e x t ' e r i e u r e t i n t ' e r i e u r a v e c d e l ' a i r s e c b _{l a} temp'erature ambiante; a p p o r t suppl'aaentaire d e c h a l e u r a u n i v e a u d e l ' a p p u i du c h b s i s i n t ' e r i e u r . L ' b n e r g i e t o t a l e r e q u i s e p o u r l a p r e s s u r i s a t i o n d e l ' a i r e t l ' a p p o r t I s u p p l h e n t a i r e d e c h a l e u r au c o u r s d e l a p s r i o d e s ' b t e n d a n t d ' o c t o b r e 1981 3 a v r i l 1982 a Bt'e d e 166 M p a r m a t r e d e l a r g e u r d e v i t r a g e . C e t t e c o n c e p t i o n p e u t s'av'erer economlque l o r s q u ' u n e p a r t i r - 9 s s p S c i f i q u e s l i ' e e x i g e l ' a m b l i o ~ -- -- -

--.

- -- -- +

A

METAL

FRAME WINDOW SYSTEM FOR HIGH INDOOR HUMIDITIES

R.P. Bowen and K.R.

Solvason*

ABSTRACT

To increase the surface temperature of a metal window system for a

portion of a new building (less than

5%

_{of the windows) in order to}

permit 50% relative humidity at 22OC without condensation, the following

techniques were investigated: triple glazing by use of a doublesealed

unit in an exterior frame and single-glazed Interior frame;

pressurization of the air space between the inner and outer frames, with

dry air at room temperature; and provision of supplementry heat to the

sill of the inner single-glazed frame. The total energy consumption for

the pressurizing air and sill heater for the winter period October 1981

to April 1982 was 166

MJ

per meter of window width. It is suggested that

such an approach might prove economical in situations where a small

portion of a building has unique occupancy requirements, or where an

occupancy change requires retrofitting of a window system.

*Division of Building Research, National Research Council of Canada,

Ottawa, Canada,

KIA OR6

A METAL FRAME WINDOW SYSTEM FOR HIGH INDOOR HUMIDITIES R.P. Bowen* and K.R. Solvason*

1. INTRODUCTION

The Thermal P r o p e r t i e s S e c t i o n of t h e Division of Building Research, N a t i o n a l Research Council Canada (DBRINRCC), a s s i s t e d i n t h e d e s i g n arld l a b o r a t o r y e v a l u a t i o n of a n o v e l window system f o r a h i g h r i s e o f f i c e complex. A p o r t i o n of t h e b u i l d i n g t o be used f o r housing a c o q u t e r f a c i l i t y had t o be conditioned t o 50% r e l a t i v e humidity a t a room

temperature of approximately 22OC. No condensation could

be

t o l e r a t e d

6a

t h e window system, which was t o u t i l i z e a metal frame and sash. The windows f o r t h e computer f a c i l i t y r e p r e s e n t less t h a n 5 % of t h e t o t a l e x t e r i o r g l a z i n g a r e a of t h e building.

The l o c a t i o n h a s a January d e s i g n temperature of -31°C (23% b a s i s ) and annual degree days of 5345 (The Supplement t o t h e National B u i l d i n g Code of Canada 1980). F i g u r e 1 i s a histogram shawing t h e number of hours i n a s e l e c t e d range of temperatures f o r October 1981 t o A p r i l 1982 (Monthly Meteorological Summary 1981-82).

2. BACKGROUND

Water vapour condenses on a s u r f a c e whenever t h e e u r f a c e temperature f a l l s below t h e dew-point temperature of t h e a i r s t r e a m

i n

c o n t a c t w i t h it. The s u r f a c e temperatures on a window system depend on b o t h i n t e r i o r and e x t e r i o r environmental c o n d i t i o n s and on r e s i s t a n c e t o h e a t flow through t h e system. The two main h e a t flow p a t h s t h a t a f f e c t t h e o v e r a l l c o e f f i c i e n t of h e a t t r a n s m i s s i o n (U-value) of a window a r e through t h e g l a z i n g system and frame (Fig. 2).

Considering t h e g l a s s p o r t i o n of t h e g l a z i n g system f i r s t , t h e w i n t e r U-value recognized by ASHRAE (Handbook of Fundamentals 1981)

f o r

*Division of B u i l d i n g Research, N a t i o n a l Research Council Canada, Ottawa, Canada, KIA OR6

double and t r i p l e g l a z i n g

4s

shown i n T a b l e 1. The v a l u e s were c a l c u l a t e d u s i n g t h e s u r f a c e f i l m r e s i s t a n c e s assumed f o r n a t u r a l convection on t h e room s i d e and a wind v e l o c i t y of 6.7 m / s on t h e weather s i d e . Also t a b u l a t e d a r e t h e s u r f a c e temperatures and maximum r e l a t i v e h u m i d i t i e s t h a t can b e maintained w i t h o u t any condensation a t t h e c e n t r a l r e g i o n of t h e g l a s s f o r -3S°C o u t s i d e temperature and 22OC i n s i d e temperature. For a double-glazed u n i t w i t h 13-m a i r s p a c e t h e room s i d e g l a s s temperature w i l l be approximately 3OC. A dew-point

temperature of 3OC corresponds t o a r e l a t i v e humidity of 28%. S i m i l a r l y , f o r t r i p l e g l a z i n g w i t h two 131mn a i r spaces t h e maximum i n d o o r r e l a t i v e humidity w i t h o u t condensation on t h e c e n t r a l r e g i o n of t h e g l a s s i s 47%. It should be noted t h a t t h e i n f o r m a t i o n provided i n Table 1 a p p l i e s o n l y f o r c o n d i t i o n s where t h e s u r f a c e c o e f f i c i e n t s a r e e q u a l t o t h o s e of t h e U-value c a l c u l a t i o n s . With a lower i n s i d e f i l m r e s i s t a n c e ( f o r c e d convection) and a h i g h e r o u t s i d e f i l m r e s i s t a n c e , i n s i d e s u r f a c e

temperatures would be h i g h e r and h i g h e r h u m i d i t i e s could be maintained. The v a l u e s i n T a b l e 1 f o r t h e maximm r e l a t i v e h u m i d i t i e s f o r

I

1

i

d i f f e r e n t window u n i t s provide only values. It i s u n l i k e l y t h a t t h e y could be maintained even a t t h e c e n t r a l a g e n e r a l i n d i c a t i o n of t h e l i m i t i n g I r e g i o n of t h e g l a s s . F i r s t l y , t h e r e w i l l be a temperature g r a d i e n t on

t h e g l a s s from t o p t o bottom, t h e bottom b e i n g c o l d e r t h a n t h e top. I n a s e a l e d g l a z i n g u n i t t h e space between t h e panes of g l a s s w i l l decrease a s t h e temperature i n t h e a i r s p a c e drops, s o t h a t t h e temperature of t h e c e n t r a l r e g i o n of t h e i n n e r pane w i l l a l s o decrease. Heat l o s s a t t h e p e r i m e t e r of a window i s g r e a t l y a f f e c t e d by t h e g l a z i n g d e t a i l and, f o r a s e a l e d u n i t , by t h e metal spacer. F i g u r e 3 i l l u s t r a t e s t h e temperature

1

_{g r a d i e n t s caused by b o t h g l a s s d e f l e c t i o n and t h e m e t a l edge of a s e a l e d}

u n i t .

The i n s i d e s u r f a c e t e m p e r a t u r e s of most m e t a l window frames a r e u s u a l l y lower t h a n t h e temperatures on most of t h e g l a s s a r e a . This i n d i c a t e s t h a t h e a t l o s s through t h e frame i s r e l a t i v e l y high. Although metal has many c h a r a c t e r i s t i c s t h a t make i t a u s e f u l m a t e r i a l f o r window frame and s a s h members, c a r e f u l d e s i g n i s r e q u i r e d when i t i s used f o r high humidity l o c a t i o n s . I f i t s h i g h c o n d u c t i v i t y i s n o t p r o p e r l y accounted f o r i n t h e d e s i g n of t h e frame and s a s h , and i n t h e d e s i g n of t h e b u i l d i n g envelope i n which i t i s t o be i n s t a l l e d , i t can l e a d t o

, condensation on t h e m e t a l yindow p a r t s b e f o r e i t appears on t h e g l a s s . A metal frame can a l s o lower t h e temperature a t t h e edges of t h e glazing.

T h i s problem was recognized by t h e window i n d u s t r y a number of y e a r s ago and r e s u l t e d i n t h e u s e of "thermal breaks." These a r e l o r

c o n d u c t i v i t y m a t e r i a l s used t o s e p a r a t e t h e i n n e r and o u t e r frame o r s a s h components. To a s s e s s t h e performance of t h e thermal break

a

test was developed f o r i n c l u s i o n i n t h e metal windw s t a n d a r d s (Canadian General Standards Board 1979). Unfortunately, n e i t h e r passing t h e t e s t nor t h e presence of a thermal break i s s u f f i c i e n t t o ensure t h a t

condensation w i l l not occur on t h e frame o r g l a s s . B r i e f l y , t h e test

involves s u b j e c t i n g t h e window t o a 55 deg temperature d i f f e r e n c e , w i t h zero a i r p r e s s u r e d i f f e r e n c e a c r o s s t h e u n i t . Thermocouples a r e used t o measure t h e room s i d e g l a s s temperature a t t h e q u a r t e r h e i g h t , h a l f

h e i g h t and t h r e e q u a r t e r h e i g h t and t o determine t h e lowest temperature on t h e warm-side metal s u r f a c e s .

The p a s s i n g c r i t e r i a f o r a window c o n t a i n i n g a s e a l e d u n i t r e q u i r e s t h a t t h e c o l d e s t p o i n t on t h e frame be no more than 10 deg l e s s than t h e average of t h e t h r e e g l a s s temperatures. This means t h a t f o r t h e double u n i t w i t h 131mn a i r space and g l a s s temperatures a s i n Table 1 t h e frame temperature can be a s low a s -7'C and s t i l l comply w i t h t h e standard. A dew--point temperature of -7*C corresponds t o a r e l a t i v e humidity of only 1 2 % when t h e a i r temperature i s 22OC. For t r i p l e g l a z i n g t h e

corresponding humidity would be 22%. This i s n o t t o imply t h a t a l l metal windows j u s t meet t h e standard. The dilemma f a c e d by t h e d e s i g n e r s of metal windows i n i n c o r p o r a t i n g a low-conductivity m a t e r i a l i n t h e frame

i s t o maintain t h e s t r u c t u r a l i n t e g r i t y of t h e frame and p o s i t i o n t h e thermal break i n t h e plane of t h e glazing. Systems t h a t s u b s t a n t i a l l y exceed t h e minirmm requirements may be more expensive o r u n s u i t e d f o r

a

p a r t i c u l a r a p p l i c a t i o n .

3. PROTOTYPE DESIGN

To maintain t h e indoor c o n d i t i o n s on t h e computer f l o o r s a t 2Z°C and 50%

RH

without s u r f a c e condensation on t h e window system i t i s c l e a r t h a t t h e s u r f a c e temperature of t h e g l a s s and frame m e t n o t be below l l ° C . The Division was approached f o r advice only a f t e r t h e main window frame

design f o r t h e major p a r t of t h e b u i l d i n g had been f i n a l i z e d . T h i s meant t h a t t h e r e were s t r o n g i n c e n t i v e s t o i n c o r p o r a t e i t i n t o t h e design f o r

t h e computer f l o o r . F i g u r e 4

i s

a schematic of t h e main frame t h a t s u p p o r t s a s e a l e d double-glazing u n i t .

The f i r s t o p t i o n o f t e n considered i n t h i s t y p e of s i t u a t i o n i s t o change from a double-glazed t o a t r i p l e - g l a z e d s e a l e d u n i t . I n t h e o r y , and a s e v i d e n t from t h e e a r l i e r t a b l e , t h e c e n t r e of t h e g l a s s could w i t h s t a n d c l o s e t o 50%

RH

i f t h e r e were no d e f l e c t i o n of t h e pane. The pane of g l a s s w i l l d e f l e c t , however, and, more i m p o r t a n t , t h e r e i s s t i l l a high conduction p a t h a t t h e perimeter of t h e s e a l e d u n i t because of t h e m e t a l spacer. The h e a t flow p a t h s t h a t mst be improved i n c l u d e t h e through-the-glass p o r t i o n , t h e s e a l e d u n i t s p a c e r , and t h e main window frame (Fig.

4).

The approach t a k e n was t o provide t r i p l e g l a z i n g by use of a s e a l e d double u n i t i n t h e main frame p l u s a s i n g l e - g l a z e d s a s h and frame. The i n n e r and o u t e r frames could be s e p a r a t e d by a m a t e r i a l w i t h lower c o n d u c t i v i t y t h a n metal.

The next i m p o r t a n t f a c t o r t o be considered i s t h e l o c a t i o n of t h e s e a l e d u n i t : i n t e r i o r o r e x t e r i o r . The i n t e r i o r i s u s u a l l y t h e

p r e f e r r e d l o c a t i o n s i n c e i t a l l o w s t h e c a v i t y between t h e s i n g l e and double u n i t s t o be vented t o o u t s i d e . As w e l l , t h e s e a l e d u n i t would be s u b j e c t e d t o a s m a l l e r temperature v a r i a t i o n from extremes t h a n would be t h e c a s e f o r an e x t e r i o r p o s i t i o n , and i t would be s h e l t e r e d from t h e r a i n and snow. Both of t h e s e f a c t o r s would t e n d t o promote a l o n g e r l i f e f o r t h e s e a l e d u n i t . Cleaning of t h e o u t e r s u r f a c e of t h e s e a l e d u n i t and t h e single-glazed s a s h from t h e e x t e r i o r would, however, be d i f f i c u l t and t h u s expensive. This i s a major p r a c t i c a l o b j e c t i o n from a

maintenance p o i n t of view. Another disadvantage of t h e i n t e r i o r p o s i t i o n f o r t h e s e a l e d u n i t , a l l o t h e r t h i n g s being e q u a l , i s t h a t f o r c e d

convection from t h e h e a t i n g u n i t s would i n c r e a s e t h e r i s k of thermal breakage. A s a r e s u l t , i t was decided t o l o c a t e t h e s i n g l e - g l a z e d frame and s a s h on t h e room s i d e (Fig. 5). Allawing u s e of t h e same d o u b l e glazed main window frame a s f o r t h e remainder of t h e b u i l d i n g .

By p l a c i n g t h e s e a l e d u n i t on t h e o u t s i d e , a compromise i s made i n t h e l o c a t i o n of t h e primary a i r s e a l i n t h e g l a z i n g system. Normally, one would p l a c e t h e primary a i r s e a l on t h e i n n e r s a s h o r frame and v e n t t h e space t o t h e o u t s i d e . This would r e s u l t i n having t h e s i n g l e - g l a z e d i n n e r s a s h t a k e a l l t h e wind l o a d and t h e r e would be some h e a t l o s s a s s o c i a t e d w i t h v e n t i n g of t h e a i r space. It was decided, t h e r e f o r e , t o s e a l t h e double-glazing u n i t and main frame j u s t a s f o r t h e r e s t of t h e

b u i l d i n g , making i t t a k e t i e m a j o r i t y of t h e wind load. As t h e a i r s p a c e between t h e double-glazed main frame and t h e s i n g l e - g l a z e d s a s h cannot be h e r m e t i c a l l y s e a l e d , t h e s i n g l e - g l a z e d s a s h had t o b e openable f o r

cleaning. A dry g a s k e t was used t o provide t h e a i r s e a l between t h e single-glazed s a s h and t h e frame.

S i n c e t h e i n s i d e s u r f a c e of t h e s e a l e d u n i t and i t s frame w i l l b e below t h e dew-point of t h e room a i r , d r y a i r from some o t h e r s o u r c e had t o be supplied. Supplying t h e d r y a i r under p r e s s u r e w i l l a l s o r e d u c e t h e amount of room a i r e n t e r i n g t h e space. Outside a i r h e a t e d t o room temperature b e f o r e i n j e c t i o n i n t o t h e s p a c e i s a n adequate s o u r c e of p r e s s u r i z i n g a i r . Its dew-point temperature w i l l be w e l l below t h e temperatures on s u r f a c e s i n s i d e t h e space.

Having gone through t h e conceptual s t a g e , t h e manufacturer designed and b u i l t a u n i t f o r t e s t i n g i n t h e Environmental T e s t F a c i l i t y of

DBR/NRCC.

4.

TEST SPECIMEN

The window u n i t c o n s i s t e d of two openings s e p a r a t e d by a m u l l i o n (Fig. 6). Glazing f o r each opening comprised a s e a l e d double-glazing u n i t on t h e e x t e r i o r and a s i n g l e - g l a z e d s a s h and frame on t h e i n t e r i o r . The s i n g l e - g l a z e d frame was connected t o t h e main frame by PVC

connectors. Two s e c t i o n s of p r e c a s t c o n c r e t e w i t h a t t a c h e d e x t r u d e d p o l y s t y r e n e i n s u l a t i o n and e x t e r i o r g r a n i t e f a c i n g s were used a s t h e l i n t e l and s i l l . The window frame, c o n c r e t e l i n t e l and s i l l were connected by a wood frame and mounted i n t h e opening of a polystyrene- l i n e d s t e e l t e s t frame. The p e r i m e t e r , on b o t h t h e room s i d e and

weather s i d e of t h e assembly, was s e a l e d t o t h e p o l y s t y r e n e surround t o p r e v e n t a i r l e a k a g e round t h e specimen. The main p a r t of t h e assembly was s e a l e d u s i n g f i e l d i n s t a l l a t i o n procedures.

A f a n / c o i l a i r - c o n d i t i o n i n g u n i t of t h e t y p e used i n t h e rest of t h e b u i l d i n g was mounted on t h e t e s t assembly t o s i m u l a t e f i e l d c o n d i t i o n s as

c l o s e l y a s p o s s i b l e . The f a n / c o i l u n i t was l o c a t e d under t h e l e f t hand window, w i t h t h e convector cover e x t e n d i n g a c r o s s t h e f u l l w i d t h of t h e t e s t specimen. Thermocouples were a p p l i e d t o t h e assembly

as

shown o n Fig. 7.

5. TEST PROCEDURE t

The s t e e l t e s t frame w i t h window system i n s t a l l e d was mounted - between t h e weather s i d e and room s i d e chambers of t h e DBR Environmental T e s t F a c i l i t y (Fig. 8). The s p a c e between t h e s e a l e d u n i t s and t h e single-glazed s a s h u n i t s was p r e s s u r i z e d t o 10 Pa above t h e room s i d e chamber pressure. Air from t h e c o l d chamber was heated t o approximately room temperature and s u p p l i e d t o t h e space. The flow r a t e r e q u i r e d t o maintain t h e 10 Pa p r e s s u r e d i f f e r e n c e was about 0.5 dm3/s.

The room s i d e a i r temperature was maintained c l o s e t o 22°C by e i t h e r t h e room s i d e a i r - c o n d i t i o n i n g u n i t o r t h e f a n / c o i l u n i t mounted on t h e test assembly. A t v a r i o u s s t a g e s moisture was introduced t o t h e room s i d e chamber and condensation on t h e window system was observed. The weather s i d e a i r temperature was maintained a t -35°C (unless o t h e r w i s e

i d e n t i f i e d ) f o r each t e s t . A multi-nozzle wind machine, which d i r e c t e d a i r perpendicular t o t h e window assembly, was used t o provide a uniform s u r f a c e conductance of approximately 25 w / ~ * * K on t h e weather s i d e .

A number of tests were conducted w i t h v a r i o u s modifications of b o t h assembly and c o n d i t i o n s . The following a r e t h e key items t o be

discussed.

1) Throughout t h e t e s t i n g s e r i e s o b s e r v a t i o n s were made of condensation w i t h i n t h e window space, w i t h and without p r e s s u r i z a t i o n of t h e space. I n most t e s t s t h e p r e s s u r e d i f f e r e n c e a c r o s s t h e assembly was c l o s e t o zero, but some t e s t s were made w i t h a p r e s s u r e d i f f e r e n c e of about 50 Pa

from room s i d e t o weather s i d e .

2 ) The e f f e c t s of f a n speed i n t h e f a r r c o i l u n i t (low and high) and t h e d i r e c t i o n of t h e d i s c h a r g e g r i l l (away from o r towards window) on window s u r f ace temperatures were i n v e s t i g a t e d . T e s t s r e l a t e d t o i d e n t i f y i n g t h e e f f e c t s of t h e f a n were c a r r i e d o u t a t 22'C room s i d e and -35°C weather s i d e temperatures.

3 ) As w i l l be e v i d e n t from t h e t e s t r e s u l t s , supplementary h e a t was necessary t o maintain t h e metal frame and s i n g l e g l a z i n g above t h e a i r dew-point. Two techniques were t r i e d : a h e a t pipe, and h e a t i n g cables.'

The h e a t pipe, c o n s t r u c t e d a s shown i n Fig. 9, was a t t a c h e d t o t h e s i l l of t h e i n t e r i o r frame and extended a c r o s s t h e f u l l width of t h e window. The r e s e r v o i r of t h e h e a t p i p e extended i n t o t h e space housing t h e a i r -

Q

4)

The second t e c h n i q u e t o p r o v i d e supplementary h e a t t o t h e frame u s e d e l e c t r i c h e a t i n g cable. It had a nominal c a p a c i t y of 26 W/m and was a t t a c h e d t o t h e f u l l w i d t h of t h e single-glazed frame a t t h e l o c a t i o n shown on Fig. 10. The c a b l e was looped t o g i v e two rows f o r a t o t a l of 52 W/m of window width. A temperature c o n t r o l l e r w i t h a c o p p e r

c o n s t a n t a n thermocouple s e n s o r was i n s t a l l e d t o a d j u s t t h e "on" t i m e o f t h e h e a t e r i n r e l a t i o n t o o u t s i d e a i r temperature. The thermocouple s e n s o r was placed i n t h e o u t s i d e a i r and t h e c o n t r o l l e r set w i t h a

p r o p o r t i o n a l band of about 14' K ( z e r o r a t e and r e s e t ) add

ti

c y c l e t i n e o$

approximately 24 s. The c o n t r o l p o i n t was s e t t o g i v e 100% "on" t i m e a t about -31°C.

6. TEST RESULTS

P r e s s u r i z a t i o n w i t h d r y a i r of t h e s p a c e between t h e i n t e r i o r s a s h and t h e main window condensation i n t h e a i r space. S u r f a c e t e m p e r a t u r e s i n t h e s p a c e were a s low a s -16OC and room s i d e humidity above 50% RH; no condensation formed i n t h e space. Without p r e s s u r i z i n g a i r o r o v e r a l l p r e s s u r e d i f f e r e n c e a s l i g h t amount of condensation

appeared on t h e main window frame and t h e s e a l e d u n i t . Applying a p o s i t i v e p r e s s u r e d i f f e r e n c e a c r o s s t h e assembly i n c r e a s e d t h e

condensation on s u r f a c e s i n t h e space, but i t disappeared a g a i n when t h e s p a c e was p r e s s u r i z e d .

T a b l e 2 summarizes t h e key t e s t r e s u l t s . I n g e n e r a l , t h e c o l d e s t p a r t of t h e assembly was t h e bottom c o r n e r of t h e s i n g f e - g l a z e d s a s h . T e s t 1 was on t h e assembly a s shown i n Fig. 6. Air flow on t h e room s i d e was by n a t u r a l convection ( a i r - c o n d i t i o n i n g u n i t o f f ) . The minimum

s u r f a c e temperature was 5.0°C (Fig. 11). Thus, t h e maxiuum r e l a t i v e humidity w i t h no condensation would be approximately 33%.

With t h e a i r c o n d i t i o n e r o p e r a t i n g and s u p p l y i n g h e a t t o t h e room s i d e ( T e s t s 2-5) t h e s i t u a t i o n improved s l i g h t l y b u t s t i l l r e s u l t e d i n frame t e m p e r a t u r e s f o r which only 35% RH could be maintained. The d i r e c t i o n of t h e f a n d i s c h a r g e g r i l l had no s i g n i f i c a n t e f f e c t on frame s u r f a c e temperature.

From t h e f i r s t f i v e t e s t s I t was obvious t h a t t h e assembly, a s

provided, could n o t p r e v e n t condensation on t h e window system w i t h 50%

RE

on t h e room s i d e . The a d d i t i o n of supplementary h e a t t o t h e frame was, n o t s u r p r i s i n g l y , s u c c e s s f u l i n i n c r e a s i n g s u r f a c e temperatures. The u s e

of t h e h e a t p i p e ( T e s t 6) ; e s u l t e d i n

a

minirmm s u r f a c e temperature of 8.T°C, o r t h e c a p a b i l i t y t o s u s t a i n 43%

_RH.

_{Use of t h e e l e c t r i c h e a t i n g}

c a b l e was more s u c c e s s f u l , a s i s e v i d e n t from T e s t 7, f o r which t h e h e a t e r was on 100% of t h e time. The minimum frame temperature was

10.3°C (Fig. 12), which would m a i n t a i n about 48%

_RE.

_{Table 3 summarizes}

t h e r e s u l t s f o r h e a t i n g c a b l e and wide p r o p o r t i o n a l band c o n t r o l l e r . With h u m i d i t i e s of s l i g h t l y o v e r SOX _{i n t h e room s i d e chamber a}

s l i g h t amount of condensation formed on t h e r i g h t hand s a s h above t h e s i l l , where t h e temperature (10.3'C) was lowest. Temperatures below t h i s a r e a , n e a r t h e c o r n e r of t h e s a s h , were w e l l above t h e dew-point

t e m p e r a t u r e s o t h a t any condensate was re-evaporated. There was n o evidence of d r i p p i n g on t h e convector cover. A s l i g h t amount of

condensation was a l s o e v i d e n t on t h e upper p a r t of t h e c o n c r e t e below t h e s i l l . Again, t h i s would probably re-evaporate and t h u s n o t p r e s e n t a n u i s a n c e problem.

7. DISCUSSION OF RESULTS

Air s p a c e p r e s s u r i z a t i o n and supplementary h e a t i n g of t h e frame were r e q u i r e d t o make t h e window system f u n c t i o n s a t i s f a c t o r i l y . The former was demonstrated t o r e d u c e l e a k a g e of moist room a i r i n t o t h e a i r s p a c e e f f e c t i v e l y . A flow r a t e of approximately 0.5 dm3/s (0.23 dm3/s

p e r meter w i d t h of window system) was r e q u i r e d t o m a i n t a i n a p r e s s u r e of 10 Pa i n t h e window system supplied. I f t h i s flow r a t e were t o

i n c r e a s e s u b s t a n t i a l l y d u r i n g t h e l i f e of t h e system, i t would be a n i n d i c a t i o n of e x c e s s i v e s e a l leakage and c o r r e c t i v e a c t i o n would be necessary.

S h o r t of r e d e s i g n i n g t h e main window frame, t h e only technique a v a i l a b l e f o r i n c r e a s i n g t h e frame and g l a s s s u r f a c e temperatures t o a n a c c e p t a b l e l e v e l would b e t o supply supplementary h e a t t o t h e frame. The h e a t p i p e technique showed p o t e n t i a l , but i t was r u l e d o u t because i t r e q u i r e d a d d i t i o n a l development and was considered more expensive t h a n t h e o t h e r option. The e l e c t r i c h e a t i n g c a b l e w i t h wide p r o p o r t i o n a l band c o n t r o l l e r was e f f e c t i v e i n r e g u l a t i n g t h e supplementry h e a t a s required. With a c t i v a t i o n of t h e h e a t e r , t h e c r i t i c a l temperature on t h e i n s i d e

s u r f a c e s h i f t e d from t h e s i l l , w i t h t h e h e a t e r o f f , t o p a r t way up t h e r i g h t hand s a s h when t h e h e a t e r was on f u l l . A s t h i s would make t h e

s e l e c t i o n of an i n s i d e s u r 5 a c e a c t i v a t i o n t e a p e r a t u r e d i f f i c u l t ,

an

o u t s i d e temperature should be used.

There i s no need f o r a c o n t r o l l e r on each window; one c a n s e r v i c e a bank of windows on t h e same exposure. The a c t i v a t i o n telsperature can be s e l e c t e d a s e i t h e r a n o u t s i d e a i r temperature o r a n o u t s i d e s u r f a c e temperature. Outside s u r f a c e temperature a c t i v a t i o n

i s

p r e f e r a b l e because i t would compensate f o r s o l a r r a d i a t i o n and low o u t s i d e s u r f a c e c o e f f i c i e n t e f f e c t s and t h u s reduce t h e h e a t e r "on" time.

8. COST CONSIDERATIONS

I n t h e f i n a l a n a l y s i s , c a p i t a l and maintenance c o s t s a r e paramountt Once i t i s known t h a t a system cannot meet t h e performance requiremgnts, t h e d e s i g n e r h a s t h e choPce of making a d e s i g a change o r a d o p t i n g a

p a l l i a t i v e s o l u t i o n f o r

small

p o r t i o n s of t h e building.

The

c o u r s e t a k e 9 w i l l depend on c o s t s .

For a b e t t e r assessment of o p e r a t i n g c o s t s t h e d a t a from Fig,

1

and Table 3 can be used t o a r r i v e a t t h e energy consumption f o r t h e s i l l h e a t e r f o r t h e h e a t i n g season 1981182, i.e., approximately 76

HJ,

The energy consumption f o r t h e p r e s s u r i z i n g a i r i s 90

W.

'Phis information, along w i t h c a p i t a l c o s t s , c a n be used t o a s s e s s whether i t i s , i n f a c t , a p r e f e r a b l e a l t e r n a t i v e t o redesigning t h e t o t a l window system f o r t h e s a k e of t h e a r e a i n question.

9. CONCLUSION

The techniques d e s c r i b e d t o e n s u r e &n$mal s u r f a c e condensation a t t h e design c o n d i t i o n s a r e n o t recommended f o r a t o t a l building, They can, however, s a t i s f y s p e c i a l occupancy requirements f o r s m a l l p o r t i o n s of a b u i l d i n g , o r i f a n occupancy changes can be used t o upgrade a n e x i s t i n g window system.

On h i g h - r i s e b u i l d i n g s c l e a n i n g of windows i s a n important

consideration. The 90

NJ

f o r p r e s s u r i z i n g a i r would n o t be r e q u i r e d

I f

c l e a n i n g from t h e e x t e r i o r were possible. This would atlaw t h e s e a l e d u n i t t o be placed on t h e room s i d e , t h e a i r s e a l t o be placed on the room

s i d e , and t h e s p a c e between t h e double-glazed and s i n g l e g l a z e d f

raws

t~ be vented t o t h e outside.

F u r t h e r development wdrk

i s

r e q u i r e d t o improve t h e thermal

r e s i s t a n c e of t h e window frame. The u s e of a composite system may prove t o be t h e b e s t s o l u t i o n t o frame design.

ASHRAE Handbook of Fundamentals 1981, American Society f o r Heating, R e f r i g e r a t i n g and A i r c o n d i t i o n i n g Engineers.

Canadian General Standards Board 1979, Metal Window Standards.

Monthy Meteorological Summary 1981-82, Atmospheric Environment S e r v i c e , Environment Canada.

The Supplement t o t h e N a t i o n a l Building Code of Canada 1980, A s s o c i a t e Committee on t h e National Building Code, National Research Council of Canada, NRCC 17724.

TABLE

1. O v e r a l l c o e f f i c i e k t e of h e a t transmission (U-value) of h e r m e t i c a l l y s e a l e d g l a z i n g and s u r f a c e temperatures a t w i n t e r design temperatures Calculated

UY

a l u e s u r f a c e temp* IUI*

*

w / ~ * . K

oc

x

Double Glazing 6-m a i r space 3.3 -0.8 21 13- a i r space 2 .8 3 .O 28 T r i p l e Glazing 6-m a i r space 2.2 6.8 3 8 1 3 - m a i r space 1.8 9.8 47

*Calculated s u r f a c e temperature f o r c e n t r e region, assuming no g l a s s d e f l e c t i o n , 22'C room s i d e temperature and -3S°C weather s i d e

temperature

**Relative humidity corresponding t o a dew-point temperature e q u a l t o t h a c a l c u l a t e d s u r f a c e temperature of t h e g l a s s

-. -

TABLE 2. Summary of test r e s u l t e

Maxilnua

Outside Minimum RH a t

T e s t no. temp D e s c r i p t i o n of t e s t s u r f a c e temp 22OC

O c O c X 1 -35 A s per Fig. 6 5.1 33 A/C o f f 2 -35 A/C f a n on low 5.1 3 3 G r i l l d i r e c t e d t o window 3 -35 A/C f a n on low 5 .1 3 3 G r i l l d i r e c t e d t o room 4 -35 A/C f a n on high 5 -7 3 5 G r i l l d i r e c t e d t o window 1 5 -35 A/C f a n on high 5 -6 3 5 G r i l l d i r e c t e d t o room 6 -35 Heat p i p e on e i l l 8 07 4 3 A/C f a n on high 7 -35 Heating c a b l e on f u l l 10.3 48 A/C f a n on h i g h I

TABLE

3. Test r e s u l t s using heating cable and wide proportional band controller

- - - -

Beating cable Weather side on time a i r temperature

X O c

Maximum room s i d e

RH

f o r no condensation

A

METAL

FRAME WINDOW FOR HIGH INDOOR HUMIDITIES

by

R,P. Bowen and K.R. Solvason

FIGURE CAPTIONS

F i g u r e 1 Number of hours i n temperature range, October-April F i g u r e 2 Schematic showing two main window h e a r flow p a t h s

F i g u r e 3 S u r f a c e temperature of t r i p l e - g l a z e d u n i t i n

DBR

e n v i r o n m n t a l chamber

F i g u r e 4 Schematic of main window frame

F i g u r e 5 Schematic of single-glazed s a s h and frame mounted on main f r a m F i g u r e 6 Schematic of

t c r t

specimen F i g u r e 7 Thermocouple l o c a t i o n s Figure 8 DBR/NRCC environmental t e s t f a c i l i t y F i g u r e 9 Heat p i p e F i g u r e 10 Heating c a b l e on s i n g l e - g l a z e d frame F i g u r e 11 S u r f a c e temperatures, OC, t e s t No. 1 F i g u r e 12 S u r f a c e temperatures, O C , t e s t No. 7

I I I 1 I I f

- 2 . 5 - 7 . 5 -12.5 - 1 7 . 5 - 2 2 . 5 - 2 7 . 5 - 3 2 . 5 TEMPERATURE. "C

F I G U R E 1

ROOM S l DE

GLAZING HEAT LOSS

FRAME

---

--

---

HERMETICALLY SEALED GLAZING U N I T TRIPLE-GLAZED - 6-rnm A I R SPACES WEATHER SIDE: -35°C: ROOM SIDE: 21°C

F I G U R E 3 PLATE HERMETICALLY SEALED

1

1

11

DOUBLE-GLAZ ING U N I T F I G U R E 4 .PRESSURE

S INGLE-GLAZED OPENABLE SASH

, .... - 4 . A .

STEEL TEST FRAME

F I G U R E 7

M

4

-

SUPPLY

Cq-J

WEATHER SIDE CHAMBER R O O M SIDE CHAMBER

I LOWER TEST SPECIMEN

b

REFRIGERATION

POLYSTYRENE SURRWND

STEEL TEST FRAME RETURN

VERTICAL SECTION

152-mm POLYURETHANE FOAM

/ 16-mm COPRR PIPE

/

-1.6-mm ALUMINUM L l - - - I I

y;

-;:,.

FIN TUBES REFRIGERANT - R 11 W l NDOW FRAME \4: F I G U R E 9 S INGLE-GLAZED OPENABLE SASH FAN HEATING CABLE F I G U R E 1 0

T h i s paper, w h i l e being d i s t r i b u t e d i n r e p r i n t form by t h e D i v i s i o n of B u i l d i n g Research, remains t h e c o p y r i g h t of t h e o r i g i n a l p u b l i s h e r . It should n o t be reproduced i n whole o r i n p a r t w i t h o u t t h e p e r m i s s i o n of t h e p u b l i s h e r . A l i s t of a l l p u b l i c a t i o n s a v a i l a b l e from t h e D i v i s i o n may be o b t a i n e d by w r i t i n g t o t h e P u b l i c a t i o n s S e c t i o n , D i v i s i o n of B u i l d i n g R e s e a r c h , N a t i o n a l R e s e a r c h C o u n c i l of Canada, O t t a w a , O n t a r i o , KIA 0R6.