Relative Humidities at which Condensation will Occur on the Inner Pane of Double Windows

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Publisher’s version / Version de l'éditeur:

Technical Note (National Research Council of Canada. Division of Building Research), 1958-07-01

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Relative Humidities at which Condensation will Occur on the Inner Pane of Double Windows

Nowak, E. S.; Wilson, A. G.

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DIVISION OF BUILDING RESEARCH

NATIONAL RESEARCH COUNCIL OF CANADA

T ECIHIN II CAlL

NOTJE

ｾ

No.

258

NOT FOR PUBLICATION

PREPARED BY EN and AGW CHECKED BY

FOR INTERNAL USE APPROVED BY RFL PREPARED FOR Northern Section, DBR DATE July 1958

SUBJECT Relative Humidities at which Condensation

will Occur on the Inner Pane of Double Windows

The levels of relative humidity at which condensation will occur on the inside surfaces of the inner panes of double and triple windows are given in Table I. These are based on temperatures that were calculated assuming one-dimensional heat flow, that is, no variation in temperature in any vertical plane. The inside air temperature used in ｴｾ･ｳ･ calculations was 70°F.

The U values used were those given in the 1958 ASH&AE Guide (p.2l2) for double and triple windows having I-inch air spaces. The method and data on which these U values are based are given in an ASHVE Research Paper (1). The following coef-ficients are used in arriving at these U values.

Outside film, fo

=

6.6 Btu/hr sq ft 0p Inside film, fi = 1.5 Btu/hr sq ft 0p Air space conductance = 0.94 Btu/hr sq ft OF

The outside film coefficient is based on an outside air tempera-ture of OOp, clear skies and no solar radiation, and a convection coefficient of 4.0 Btu/hr sq' ft OF. The value for the outside convection coefficient corresponds to, the average value for a smooth surface 10 feet long when swept by a parallel air stream at 15 mph.

The values of relative humidity for double windows given in Table I do not corresppnd exactly with those obtained from Fig.

4

(p. 229) of the 1958 ASH&AE Guide sl.nee the latter are based on a U value of 0.55 and an inside film coefficient of 1.65. The U value of 0.55 corresponds to that given in the Guide for windows with a i-inch air space.

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2

-It has been mentioned that one-dimensional heat flow was assumed in calculating the surface temperatures in Table ｉｾ Measure-ments on double windows installed in the Building Services cold room of the Division have shown that substantial vertical temperature gra-dients occur in the air space between the panes, with temperatures at the bottom being lower than at the top. The temperature variations in the air space result in similar variations of lesser magnitude in the temperature of the inside surface of the inner pane. These surface temperature variations depend not only on the air space temperatures but also on the temperature, velocity and direction of the room air moving over the window. The type and location of the heat distribution equipment is therefore a factor.

In measurements on a double window 2 feet high with a

i-inch air space, it was observed that some condensation occurred over the lower part of the inner pane at ,relative humidities as low as ｾＶＬ

30

and 20 per cent at cold room temperatures of 20°F, OOF and -20 F. Temperature measurements indicated that condensation would occur over

almost the entire !nside surface of the inner pane at the relative humi-dities given in Table I. In this study the vertical air temperature gradient in the warm room was quite small although. the heat was intro-duced on the wall opposite the window. Smaller vertical surface

temperature gradients and higher mean surface temperatures would be expected with heating outlets located dlrectly beneath the windows. Reference

1. Parmelee, G.V. and W.W. Aubele. Heat flow through unshaded glass (design data for use in load calculations). Trans. A.S.H.V.E., Vol.

56, 1950.

p.

371-398.

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e

TABLE I

Relative Humidities at which Condensation

will _{Form on Inner Panes of Double and Triple Windows}

e

\

.,

•

I

Deub Ie :;.Jindow ("Ufl Value of

0.53)

Triple 'IVindoH (liD" Value of

0.34)

I Outside

I

Temnerature OF LLO 'Temperature on insid e sur face of inner pane OF

59

Inside relati ve humidity at which condensation will occur on the

ins ide surface

%

ReE. 68 I II

II

Temperature on inside surface of inner pane 0H'

.

63

Inside relative humidity at which

I

condensa t i (In Hi11

I

occur 8n irsiae

I

surface ,

'1;

R.P •

78 I

I I

r

I

,

I

l 20 I) -21) -L,a

52 uS

38

31

53 hI

32

23

59

54

50

45

68