Larger buildings derive a greater benefit from incidental heating by
internal sources because of the reduced external surface area relative to
the heated floor area. For two story, single family residences, townhouses,
and dormitories or office buildings, the R-values of the wall insulation
should be scaled down from the values in figure 13 according to the
1/3 (Ae/Af) RWALLo
where RWALL is the scaled R-value of the wall insulation and RWALLo is the
reference value for a small, one story building. Furthermore, Ae is the
external surface area of the building (ground level floors are included, for
example, but common walls between townhouse units are not), and Af is the
heated floorspace of the building. Equation 4.2 credits larger buildings
for their more effective utilization of internal source heating during the
winter by allowing reduced levels of wall insulation.
For three reasons, it is common practice to employ higher levels of
insulation in the ceiling than the wall:
It is cheaper to insulate the ceiling than the wall.
Stratification causes larger heat loss rates per unit area of
ceiling than per unit wall area.
Solar gains on roofs during the summer can cause unwanted heating of
the living space beyond that caused by high ambient air temperature.
The total-R-value of the roof structure should therefore be scaled directly
with the wall R-value as follows:
RROOF = 1.5 RWALL
Heat losses through building perimeters and fully bermed basement walls
are limited by contact with the soil so that insulation levels need not be
so high as for exposed external walls. The following formulas yield
reasonable insulation levels for these surfaces:
RPERIM = 0.75 RWALL
RBASE = 0.75 RWALL
Ordinarily, floors are not insulated so as to assure that pipes located
below do not freeze. Because of widely varying conditions beneath ground
level floors, it is difficult to recommend specific insulation levels.
Nevertheless, provided there is no problem with pipes freezing, a reasonable
value might be:
RFLOOR = 0.5 RWALL
The insulation levels recommended above are intended only as starting
point values. Design analysis calculations described in later sections
should be performed before fixing any important design variables.