HEAT EXCHANGES DUE TO INFILTRATION THROUGH CRACKS IN THE WINDOW FRAMING
According to MEEB, the calculation for the sensible heat loss or gain by the flow of outdoor air into spaces is as follows:
Qinfil = (V) (1.08) (tchange)
where:
Qinfil = sensible heat exchange due to infiltration (Btu/h)
V = volume flow rate, in cubic feet per minute (cfm) of outdoor air introduced
1.08 = a constant derived from the density of air at 0.075 lb/ft3 under average conditions, multiplied by the specific heat of air (heat required to raise one lb of air 1° F), which is 0.24 Btu/lb3 h, and by 60 min. The units of this constant are Btu-min/ ft3 h°F.

To obtain (V), we multiplied the average air speed through the cracks in the windows by the area of the cracks. (The average air speed was determined by anemometer readings at the cracks.) To obtain the area of the cracks, the cracks on the bottom row of the window bay were measured by tape measure while the windows were closed. Because the two window rows above are the same type and size, the area of cracks of the bottom window is multiplied by three to obtain the total area of cracks in one window bay.
V = (A)(v)
A = area of cracks in window = 0.375 ft2
v = air speed (velocity) through cracks = 63.5 ft/m
V = (0.375 ft2) (63.5 ft/m) = 23.8 cfm
To obtain (tchange), we assumed an interior design temperature of 68°F, and used table A.1 in MEEB to obtain the winter design dry-bulb temperature of 22°F.
t = 68°F - 22°F = 46°F
Qinfil = (V) (1.08) (tchange)
Qinfil = (23.8 ft3/min) (1.08 Btu-min/ ft3 h°F)(46°F)
Qinfil = 1,182.4 Btu/h
There are 7 bay windows in the sunspace. Therefore, the total heat exchange due to infiltration is:
total Qinfil = (Qinfil) 7 = ( 1,182.4 Btu/h )7 = 8,277 Btu/h
total Qinfil = 8,277 Btu/h
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