Masonry Magazine February 1981 Page. 35
actual building heat loss in Btu/day/°F/ft² of floor area and 8 Btu/day/°F/ft² of floor area. When R-8 night insulation is added to the thermal storage wall system, the adjusted value may be reduced by 30 per cent when the average winter clear day exterior temperature is below freezing (32° F) and 40 per cent when above freezing.
Example. Consider a 50 x 30-ft residential structure with the long dimension on the East-West axis in Washington, D.C. The components of the structure are assumed to be the same as those in the direct gain system example, except the south-facing wall is now a thermal storage wall system instead of just double glazing. The thermal storage wall system consists of double glazing spaced 2 to 4 in. from the exterior face of a 12-in. thick thermal storage wall. The U value of the system is determined by taking the reciprocal of the total R value of the system:
Component
R value
Exterior Air Film
0.17
Double Glazing
1.45
Airspace
0.97
12-in. Brick Masonry
1.32
Interior Air Film
0.68
Total
4.59
U= 1/R = 0.218
The total building heat loss is then calculated the way it was calculated in the direct gain system example. Since the building volumes are the same, and there is assumed to be one air change per hour, the heat loss due to infiltration will be 216 Btu/hr/°F. The heat loss through the building envelope components is:
Component
U value
Btu/hr/°F/ft²
Area
Heat Loss
Btu/hr/°F
Roof/Ceiling
0.030
× 1500 =
45
Thermal Storage
Wall System
0.218
X
400 =
87
Opaque Walls
0.070
X
708 =
50
Windows
0.690
X
132
=
91
Doors
0.360
X
40 =
14
Floors
0.070 × 1500 =
105
Total
392
The total building heat loss is the sum of the heat loss through the components of the building envelope plus the heat loss due to infiltration:
392 Btu/hr/°F + 216 Btu/hr/°F 608 Btu/hr/°F
By interpolating the values in Table 2 for the 38.95° North Latitude, values of 0.342 and 0.257 are determined for 40° and 45° F average winter clear day exterior temperatures, respectively. By interpolating be-
TABLE 2
Area of Thermal Storage Wall System
Required as a Ratio to
the Floor Area"
Average Winter
Clear Day Exterior
Temperature
Degrees North Latitude
36
40
44
48
20 F
0.71 0.75 0.85 0.95
25 F
0.59 0.63 0.75 0.84
30° F
0.50 0.53 0.60 0.70
35° F
0.40 0.43 0.50 0.55
40° F
0.32 0.35 0.40 0.44
45 F
0.25 0.26 0.30 0.33
"Reprinted with permission from Edward Mazria, Edward Mazria and Associates, Inc., Albuquerque, New Mexico 87196.
tween these values, a value of 0.322 is obtained for the 41.2° F average maximum daily exterior temperature for January in Washington, D.C. This value must be adjusted for the difference in the building total heat loss and the 8 Btu/day/F/ft² of floor area base. The building total heat loss in Btu/day/°F/ft² of floor area is calculated by multiplying the heat loss, in Btu/hr/°F, by 24 hr and dividing the product by the floor area in ft².
608 Btu/hr/°F x 24 hr
1500 ft² of floor area
= 9.73 Btu/day/F/ft² of floor area
The ratio of the total heat loss in Btu/day/F/ft² of floor area to the 8 Btu/day/°F/ft² of floor area base is found to be:
9.73 Btu/day/F/ft² of floor area
8.00 Btu/day/°F/ft² of floor area
= 1.22
This value, 1.22 is used to adjust the value interpolated from Table 2.
1.22 0.322 0.393
The collector area required to maintain 68° to 70° F interior temperatures on a clear winter day is the adjusted value times the floor area.
0.393 x 1500 ft² = 590 ft²
The thermal storage wall system is only 400 ft² and thus, less heat than required to maintain interior temperatures between 68° and 70° F on clear winter days will be supplied by the thermal storage wall system. Consider the same structure with R-8 night insulation added to the thermal storage wall. The adjusted value for determining the size of the system is reduced by 40 per cent and the new value is:
0.60 x 0.393 0.236