Condensation performance of metal-framed double windows with and without thermal breaks

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Specification Associate, 13, 1, pp. 25-31, 1971-04-01

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Condensation performance of metal-framed double windows with and

without thermal breaks

Sasaki, J. R.

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COLD-WEATHER CONSIDERATIONS:

Condensation performance of

metal-framed double windows

with and without thermal breaks

by

J.

R. Sasaki

Ressarclr Officer, Diuisiorz of Builtling Rsssurck, Natiorzal Research Courrcil of Carzada

Condensation occurs on any surface that has a temperature below the dew point temperature of the adjacent air. As the lowest surface temperature inside a building in cold weathel- normally occurs on the windows, the window thermal performance governs the relative humidity that can be maintained in a building without excessive condensation (1, 2).

The t r a d i t i o n a l method for minimizing heat loss and inside surface condensation of windows has been t o attach a storm unit t o the outside of the existing window. Although this form of double window is still used, a

large proportion of double windows today is the double vertical or horizontal sliding type.

Regardless of type, double windows are characterized by a glass-enclosed air space, and a frame that is in contact with the air in the space and with the air inside and outside of the w i n d o w . T h e c o n d e n s a t i o n performance of metal-f ramed windows is normally determined by the thermal performance of the frame.

r

S Y N O P S I S : T h e f a c t o r s a f f e c t i n g t h e i n s i d e s u r f a c e - t e m p e r a t u r e o r condensation pzrformance of metal-framed double windows in cold weather are discussed. The t h e r m a l performance of a number of idealized double windows was investigated in laboratory tests and the results are presented. The thermal p e r f o r m a n c e s o f d o u b l e w i n d o w s with solid frame members of aluminum, stainless steel a n d polystyrene are compared, and the condensation performance of an aluminum double window with a number o f different "thermal-break" arrangements are described. A set of design recommendations for metal-framed windows is given, based on the results of the laboratory investigation.

A

SURFACE TEMPERATURE

PERFORMANCE

The factors affecting the inside surf ace temperatures of double windows with metal frames are shown in Figs 1 and 2. Fig 1 shows a frame with no thermal break and Fig. 2, a

metal frame with thermal break: (a) Solid metal frame with no thermal break (Fig. 1)

The inside frame temperature, t ~ , is determined by the heat balance on the

.

frame. Convective heat transfer from the ~ n s ~ d e air tends to warm the frame while convective heat transfer t o the air space and outside air tend t o cool the frame; heat is lost from the ins~de f r a m e exposure by conduct~on through the frame.

The inside frame temperature w ~ l l be maxlmum when the conductance of the inner frame exposure ( f w

.

L i ) is maximum, and the conductances of the frame (2kxILi + L,), air-space frame exposure (fs

.

L s ) and outer frame exposure ( f o

.

Lo) are min~mum.

The window designer can control only the frame exposures ( L i L s and L o and t h e thickness and conductivity of the frame mater~al ( k x ) ~ . The inside frame temperature will tend t o be high when Li/Lo is large and ( k x ) ~ is small. The effect of the space exposure length (L,) on t~ is difficult t o predict because a large L S reduces frame conductance but increases the conductance to the air space.

(b) Metal frame with thermal break (Fig. 2)

A thermal break is any material introduced in a metal frame to reduce the conduction heat loss from the inside frame exposure. To be effective, its resistance, RTB= ( L l k x ) ~ ~ must be significantly greater than that of a

corresponding section of metal frame. T h e f a c t o r s a f f e c t i n g t h e temperature of the frame section on

the ins~de of the thermal break are similar to those for a metal frame with no thermal break. I n general, t~ will tend to be high when L i and ( L l k x ) ~ ~ are large, and ( k x ) ~ , Lsi and Lo are small.

If the thermal break effect~vely reduced conduction heat loss through the frame t o negligible proportions, t~

would then depend pr~marily on the convective heat transfer a t the inner frame exposure and in the air space. Inside frame temperature would tend t o be high when Li/Lsi is large; the geometry of the outer frame section would be unimportant.

Laboratory Investigation

The study was conducted in the DBRINRCC cold-room facility(3) with the window speclmen in a part~tion separating the cold and warm rooms. The inside surface of the window faced the warm room.

The warm-side air temperature was controlled at 720F and a surface conductance value of approximately 1.2 Btulhr ft2oF was provided over the inside window surface. The air temperature on the cold side was controlled t o -200F and a surface conductance value of approximately 4.5 Btulhr ft2oF was provided over the outside window surface.

S u r f ace temperatures were measured w i t h t h e r m o c o u p l e s attached t o the metal and glass s u r f a c e s . M e a s u r e d surface temperatures, t , were converted t o temperature indices, I.

METAL FRAMES WlTH

NO THERMAL BREAK

The two metal frames investigated were an aluminum frame having a

t h e r m a l c o n d u c t i v i t y of 1400 Btu-in./hr ft2oF and a thickness of 0.050 in., and a stainless steel frame having a conductivity of 115 Btu-in./hr ft2oF and a thickness of 0.030 in.

The t e s t window was mounted in an insulated wall as shown in Fig. 3. l nside surface temperatures were measured along the vertical centre-line of the window. The two sheets of glass were positioned inside the framed opening giving various frame exposures on the inside, air space and outside. The total frame exposure remained constant at 5 in.

The aluminum and stainless steel frames were tested with the frame exposure conditions listed in Table I. The foam polyst rene surround ( k

₃

=

0.25 Btu-in./hr f t 2 0 ~ ) with no metal

frame was also studied with the same exposure conditions.

The mean glass-centre index, IC remained nearly constant a t 63 in the

tests; it was relatively independent of the frame material and configuration. The minimum glass temperature, IG, the mean frame temperature, IF, and the minimum frame temperature, IF, are listed in Table I for a constant value of IC = 63. Examples of inside surface temperature profiles obtained for the w~ndows framed with metal and insulation are shown in Fig. 4.

The variation in frame temperature with frame configuration i s shown in Fig. 5 for the aluminum and stainless steel frames. The aluminum frame had the lowest frame temperatures ( I F =

24.5 t o 43.5) and the greatest sensitivity t o changes in frame geometry. As anticipated by the heat balance in Fig. 1, I F was lowest when L i /Lo was minimum, and increased as Li/Lo increased (Figure 5b). The air space exposure, LS, did not affect frame temperature when Li/Lo was constant; I F remained unchanged at 33 when L s was increased from 1 t o 3 with Li/Lo = 1.

The minimum frame index of the stainless steel frame varied between 47 and 52; these values were higher than the indices obtained on the aluminum frame. The value of I F appeared to depend primarily on air space exposure; I F was lowest when L s was m i n i m u m and increased as L s increased, except when L i was less than 1 in. In this latter case, the reduction in heat gain area on the inside, lowered the value for IF. The mean frame index, IF, increased with Li.

The tests made on the window with the polystyrene surround indicated the optimum glass and frame temperatures obtainable with a solid frame having a

l o w t h e r m a l conductivity. The minimum frame indices ( I F = 61.5 t o 64) were much higher than those obtained with the metal frames, but the minimum glass indices (IG = 49 to 50.5) were similar to those obtained with the stainless steel frame. I t will be shown in the following section that these values can be exceeded with a

well-designed metal frame with thermal break.

ALUMINUM FRAME WlTH

THERMAL BREAK

The window consisted of two panes of glass, sheet-aluminum jamb and head members, and a replaceable sill

member. The sill member was made with 0.064-in. aluminum. The window was installed in an insulated wall between the cold and warm rooms as

shown in Fig. 6.

The location of the glass panes and the configuration of the jamb and head members were not changed in t h ~ s study; only the sill member varied. Fig. 7a shows the sill members with the thermal-break arrangements that were studied. Each sill member consisted of a short and a long aluminum sectlon separated by the thermal break. Each arrangement was tested f ~ r s t with the thermal break adjacent t o the inner pane and then with the thermal break adjacent to the outer pane (Fig. 7b). An aluminum sill with no thermal break was also investigated for comparison.

T h e m i n i m u m inside-surface temperatures measured with the different thermal-break arrangements are summarized in Table I I; a constant value of I C = 63 for all t e s t s was assumed. Examples of surface temperature prof~les obtained with the various thermal-break arrangements are shown in Fig. 8.

One obvious conclusion from the study i s that a thermal break, even one having a low thermal resistance and located improperly, can significantly improve the thermal performance of an aluminum frame over that of the frame without the thermal break.

Changes in thermal resistance and location in the frame of the thermal break produced large differences in the inside frame temperature, but little change in the temperature profile of the inner glass. The glass temperature near the sill, being dependent on conduction heat exchange with the frame and convection heat exchange with the air space, changed slightly w i t h changes i n thermal-break arrangement.

The inside frame index varied between 66 and 37.5, depending on the location and on the thermal resistance number, RTB, of the thermal break. I t should be noted that the reported values of I F relate only t o the frame configuration investigated. A smaller inside frame exposure, Li, would have resulted in lower values of IF and a greater change in I F with changes in the thermal break location.

SUMMARY

The only cold-weather thermal performance criterion currently available for metal-framed double windows, is contained in the aluminum window specifications of t h e C a n a d i a n G o v e r n m e n t Specifications Board (4). The criterion for satisfactory thermal performance is that the difference between the inside glass-centre index and the minimum temperature index of the inside surface (glass, sash or frame) must not exceed 14 when the window is tested with forced-convection air flow on the cold side, and natural-convection air flow on the warm side. In terms of the results of the present study, the min- imum temperature index of the inside window surface must be equal t o or g r e a t e r t h a n 4 9 ( l ~ o r l ~ I c - 1 4 )

The study has shown that double windows with stainless steel members can be designed to meet this requirement without a thermal break, if the separation between the inside and outside exposure is large, the metal thickness small and L i not too small. The study has also shown that aluminum windows cannot meet this requirement unless a thermal break i s incorporated in. all metal heat-flow paths leading from the inside to the outside.

For the particular aluminum frame studied, a 318 in. hollow plastic thermal break, or its equivalent or better, provided acceptable thermal

FIG. 7. Simplified heat balance for a

metal-framed double window with no thermal break.

FIG. 2. Simplified heat balance for a metal-framed double window with thermal break.

FIG. 3. Vertical section through window specimen lthermo-couple locations shown).

FIG. 4. lnside surface temperature profiles for frames with no thermal break.

FIG. 5. lnside frame temperature performance of aluminum and stainless steel frames with no thermal break.

FIG. 6. Vertical section through window specimen Ithermo-couple locations shown). S T A I N L E S S STEEL <*-

-.

.'*

--"\.

S T A I N I . E S S STEEL I* ,/ A L U M I N U M

.'

I I I I 0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 L , L

,

5 (31 L o = C O N S T A N T ( b l L,

-

C O N S T A N T ( C l L , = C O N S T A N T F I G U R E 5 I N S I D E F R A M E T E M P E R A T U R E P E R F O R M A N C E O F A L U M I N U M A N D S T A I N L E S S STEEL F R A M E S W I T H N O T H E R M A L B R E A K C O L D S I D E 1, = - 2 0 D F I, = 4 . 5 B t u l h r + - - - --- + - F O A M P O L Y S T Y R E N E S U R R O U N D ( k a O . 2 5 B I U - l n . / h r I ~ ~ " F I - A L U M I N U M

I

\

( T H I C K N E S S = 0 . 0 5 0 l n . 1 1

11

W A R M S I D E

I I

G L A S S T H I C K N E S S = 5 / 3 ~ !I D T H O F G L A Z E D O P E N I N G = 3 6 " I, * M E A N O F T E M P E R A T U R E

I

I

A T AH, f H a n d f H F I G U R E 6 V E R T I C A L S E C T I O N T H R O U G H W I N D O W S P E C I M E N l T H E R M O C O U P L E L O C A T I O N S S H O W N )

performance regardless of i t s location in the frame, while a 114 in. solid plastic thermal break, or i t s equivalent or poorer, did not provide acceptable performance regardless of its location in the frame. The 114 in. wood thermal break provided acceptable thermal performance only when it was located favourably in the frame.

RESULTS

OF

STUDY

The results of the study can be s u m m a r i z e d a s d e s i g n recommendations for metal-framed double windows providing maximum r e s i s t a n c e t o inside surface condensation:

(1) I f an qluminum-framed window i s t o be manufactured without a thermal break, all aluminum members leading from inside to outside should have a greater surface area exposed to the inside environment than to the outside or t o the air space.

FIG. 7. Thermal break arrangements. FIG. 8. Inside surface temperature profiles of aluminum frames with thermal breaks.

(2) I f an aluminum-framed window i s to be manufactured with a thermal break, the resistance of the thermal break should be large, and the inner aluminum frame section should have a greater exposure to the inside than t o the air space.

The thermal-break resistance will be greatest when the heat flow path through the thermal break is long and thin, and has a low conductivity. The relative merits of a number of thermal-break arrangements can be evaluated roughly. by using the resistance number, R T B Metal connections through the thermal break should be minimized because they reduce the effectiveness of the break. Metal members that are exposed t o all environments (inside, air space and outside) should have the thermal break in the air space and adjacent t o the inside pane o f glass.

An aluminum double window that has been properly designed with a thermal break can have inside surface temperatures as high as those on a window made from a low conducrivity material, such as wood.

(3) A double window designed with solid stainless steel members will have

higher inside surface temperatures than an aluminum window of the same configuration, and the need for a thermal break, therefore, is less. A

stainless steel member with no thermal break should be designed so that the greatest possibie length of metal is between the inside and outside metal exposures, provided that the inside exposure length is not less than approximately 1 in.

It should be emphasized that the foregoing recommendations regarding condensation p e r f o r m a n c e are primarily for double windows with frame members exposed t o inside, o u t s i d e and air space. Similar recommendations can be made for sash members of metal windows with removable glazing units.

Frame members of double-glazed windows, and sash members of windows with sealed double-glazing units, have metal exposed to only the inside and outside. The thermal break in these members should be located as close as possible t o the outside t o provide the greatest resistance t o inside surface condensation.

I l l F O A M P O L Y S l J 4 t I k . 0 251' b _{E l !lg&!V( P_IA ST! CAIk}

121 WOOO ! k . O . ? !

I

141 W O O 0 l t . 0 91 L 161 P L A S T I C l k 2) 151 P L A S T I C I t . 2 1

.

k .BIU I " Jhr 1 1 2 " ~ 1 1 X . O 0 6 4 I I I I I I

LLLJ

R . 1 . -1 4 0 0 1 . 0 6 4 1 2 0 I b l C O N F I G U R A T I O N S S l o t

1

T H E R M A L BREAK A O J A C C N T TO -, _ C O L D P A N E pp F I G U R E 1 T H L R M A L B R E A K A R R A N G E M E N T S T E M P E R A T U R E I N D E X , ₍ c x 100 :w. -l lc) F I G U R E 8 (N=,IOL S U R F A C E T E M P E R A T U R E P R O F I L E S O F A L U M I N U M F R A M E S V I I T H T H E R h l A L B R E A K S

NOMENCLATURE

H

=

height of air space (in.)

L

=

frame exposure

=

heat-flow path length (in.) x

=

thickness of conduction heat-flow path (in.) f = film coefficient (Btu/hr ft2 OF)

k = thermal conductivity (Btu-in./hr ft2 OF)

h

=

thermal conductance number

=

1/R (Btu-in./hr ft2 OF Q

=

heat flow (Btulhr)

L

Q(convection/radiation)

=

f . (12 . 1) .At

k

Q(conduction)

=

.

(&

.

1) t

R = thermal resistance number

=

($ l ) / Q (hr ft2 oF/Btu-in.) R(convection/radiation) = l / ( f , L )

R(conduction)

=

L/(kx)

t

=

air and surface temperatures (OF)

t - tC

I

=

surface temperature index

=

(

tw - tc )

.

100

-

IC

=

mean inside centre-glass index IG

=

minimum inside glass index

-

I F

=

minimum inside frame index I F

=

mean inside frame index Subscripts

C = centre glass F

=

frame G

=

glass

TB

=

thermal break

i

=

frame surface exposed t o inside s

=

frame surface exposed t o air space si

=

inner frame surface exposed to air space so

=

outer frame surface exposed t o air space

o = frame surface exposed t o outside

w

=

warm-side air a = air space c

=

cold-side air * v

REFERENCES

Wilson, A. G., Condensation on inside window surfaces. Division of Building Research, National Research Council of Canada, Canadian Building Digest 4, April 1960.

2. Wilson, A.G., and W.P. Brown, Thermal characteristics of double w i n d o w s , D i v i s i o n of Building Research, National Research Council of Canada, Canadian Building Digest 58, Oct. 1964.

3. Brown, W.P., K. R. Solvason, and A.G. Wilson, A unique hot-box cold-room facility. ASHRAE Trans.,

V. 67, 196 1, pp. 56 1-577.

4. Aluminum window specifications. Canadian Government Specifications B o a r d , Department of Defence Production, Ottawa. 63-GP Series.