316 Electrical Power Systems Technology
and doors of buildings. The periodic opening of doors also has a consid-
erable effect on heat loss. A building must have sufficient insulation to
reduce heat loss; otherwise, electrical heating and air conditioning sys-
tems will be very inefficient. The heat loss of a building depends primar-
ily upon the building construction, and upon the design temperature dif-
ference factor in the area where the building is located. Buildings made of
concrete have a different amount of heat loss than those of a wood-frame
construction. Heat loss will occur through the walls, floors, windows, and
ceilings. Each of these must be considered when estimating the heat loss
of a building.
The following sample problem will help you to understand the im-
portance of adding insulation to a building.
Sample Problem:
Given: a building constructed to provide the following thermal resis-
tance (R) factors:
(a) Exterior shingles are R = 0.80
(b) Plywood sheathing is R = 0.75
(c) Building paper used is R = 0.04
(d) Wall structure is R = 0.85
(e) Wall plaster is R = 0.35
(f) Insulation is R = 11.0
Find: the total thermal resistance (R), the coefficient of heat transfer
(U), and the watts (W) of heat loss both with and without the insulation.
Solution (without insulation):
R = a + b + c + d + e
= 0.80 + 0.75 + 0.04 + 0.85 + 0.35 = 2.79
1
U = —
R
1
= ——
2.79
= 0.3584 Btu per ft^2 per hour per °F