3. Multiply this net wall area by the number of B. T. U.’s which the wall loses per square foot of surface for each hour.
These factors are given in the following table:
| TYPE OF WALL | Zero outside and 70 degrees |
| inside—Number of B. T. U.’s | |
| lost for each square foot of | |
| Brick wall, furred and plastered: | wall surface each hour |
| 8" thick | 21.0 |
| 12" thick | 17.5 |
| Frame wall, sheathed, clapboarded, | 21.7 (with building-paper use |
| and plastered | 20.3) |
Hollow-tile wall and concrete and stone have factors about the same as for the furred brick wall.
SIDE ELEVATION
4. Add to this the number of B. T. U.’s lost per hour through the windows. This is determined by multiplying the area of the windows by the heat loss in B. T. U.’s per hour for each square foot of window, which is 78.8 for single windows, and where storm-windows are added it is 31.5 B. T. U.’s.
5. This total sum is the number of B. T. U.’s lost through walls and windows for each hour.
6. To this must be added the heat lost by leakage through the window-cracks. This is secured by measuring the length of window-cracks on the side which has the greatest length of crack and multiplying this by 168, or the number of B. T. U.’s lost each hour for each linear foot of window-crack. For very tight windows reduce above to 84.