MIL-HDBK-1003/19

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

following formula:

RWALL

=

1/3 (Ae/Af) RWALLo

(Equation 4.2)

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:

a.

It is cheaper to insulate the ceiling than the wall.

b.

Stratification causes larger heat loss rates per unit area of

ceiling than per unit wall area.

c.

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

(Equation 4.3)

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

(Equation 4.4)

RBASE = 0.75 RWALL

(Equation 4.5)

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

(Equation 4.6)

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.

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