Masonry Magazine February 1981 Page. 32
value of 0.030 Btu/hr/°F/ft²; 8-ft high walls with the entire south wall being a double glazed solar collector having an area of 400 ft² and a U value of 0.690 Btu/hr/°F/ft² and the opaque north, east, and west walls having an average U value of 0.070 Btu/hr/°F/ft²; the area of north, east, and west walls is 15 per cent double glazing: there is 40 ft² of total door area located on the east and west walls; the doors have a U value of 0.360 Btu/hr/°F/ft²: the floor is 1500 ft² and has a U value of 0.070 Btu/hr/°F/ft²: and there is assumed to be one air change per hour. The amount of additional heat needed due to infiltration per °F temperature difference may be determined by multiplying the number of air changes per hour by the volume of the structure times 0.018, which is the Btu required to raise one cubic foot of air one degree in one hour. The heat loss due to infiltration is determined to be:
12 000 ft³ 1 air change/hr x 0.018216 Btu/hr/°F
The heat loss through the components of the structure is:
| Component | U Value Btu/hr/°F/ft² | Area ft² | Heat Loss Btu/hr/°F |
| :------------ | :-------------------- | :------- | :------------------ |
| Roof/Ceiling | 0.030 | × 1500 | 45 |
| South Wall, | | | |
| Double Glazed | 0.690 | X 400- | 276 |
| Opaque Walls | 0.070 | X 708 | 50 |
| Windows | 0.690 | X 132 = | 91 |
| Doors | 0.360 | X 40 = | 14 |
| Floor | 0.070 | × 1500 = | 105 |
| Total | | | 581 |
The total building heat loss is the sum of the heat loss through the building envelope components and the heat loss due to infiltration.
581 Btu/hr/°F + 216 Btu/hr/°F 797 Btu/hr/°F
The total daily building heat loss in Btu/day/F/ft² of floor area, is determined by multiplying the hourly heat loss by 24 hr and then dividing by the floor area of the heated space.
797 Btu/hr/°F x 24 hr
1500 ft² of floor area
12.75 Btu/day/°F/ft² of floor area
From Table 1 of Technical Notes 43, Washington, D.C. is located at 38° 57′ (38.95°) North Latitude and has an average maximum daily temperature of 41.2° F in January, therefore double interpolation is required. First by interpolating between 36° and 40° North Latitude, values of 0.137 and 0.107 are determined for 40° and 45° F respectively. With the second interpolation a value of 0.130 is determined for 41.2° F. For a structure having a total heat loss of 8 Btu/day/F/ft² of floor area, the ratio of south-facing glazing required for the
direct gain system to the floor area is 0.130. The structure in this example has a total heat loss of 12.75 Btu/day/F/ft² of floor area, and thus, the value obtained by interpolation must be adjusted:
0.130 x
12.75 Btu/day/F/ft² of floor area
8.00 Btu/day/°F/ft² of floor area
= 0.207
The size of the direct gain system south-facing area is this adjusted value times the floor area of the structure.
0.207 x 1500 ft² of floor area 311 ft²
Since in the assumption, the size of the south-facing glazing was 400 ft², on a clear winter day the direct gain system should supply more heat than necessary to maintain interior temperatures of 68° to 70° F. Since overheating may occur; shading or ventilation, should be provided so that comfortable temperatures are maintained or the size of the system should be reduced to 311 ft² as determined in the previous calculations.
Now consider reducing the area of the direct gain system to 200 ft² and the rest of the south wall being opaque walls. This reduction of south-facing glazing area reduces the heat loss and the overall heat loss through the components will be:
| Component | U Value Btu/hr/°F/ft² | Area ft² | Heat Loss Btu/hr/°F |
| :------------ | :-------------------- | :------- | :------------------ |
| Roof/Ceiling | 0.030 | X 1500 | 45 |
| South Wall, | | | |
| Double Glazed | 0.690 | X 200 = | 138 |
| Opaque Wall | 0.070 | X 908 = | 64 |
| Windows | 0.690 | X 132 = | 91 |
| Doors | 0.360 | X 40 = | 14 |
| Floor | 0.070 | × 1500 | - 105 |
| Total | | | 457 |
The infiltration remains the same, one air change per hour and the total building heat loss is:
457 Btu/hr/ F + 216 Btu/hr/°F = 673 Btu/hr/°F
The daily heat loss per ft² of floor area is:
673 Btu/hr/°F × 24 hr
1500 ft² of floor area
10.77 Btu/day/F/ft² of floor area
With the latitude and maximum average daily temperatures being the same in the Washington, D.C. area selected, the ratio of south-facing glazing required for the direct gain system to the floor area to maintain interior temperatures of 68° to 70° F when the total heat loss is 8 Btu/day/F/ft² of floor area remains 0.130. By adjusting for the calculated heat loss, the required glazing will be: