cycle cost. Roof thermal resistance should meet or exceed agency
criteria and requirements for Federal buildings.
Documents. The NRCA Energy Manual and the 1985
American Society of Heating, Refrigerating and Air Conditioning
Engineers (ASHRAE) 1985 Handbook - Fundamentals, Chapter 20,
contain discussions on heat-flow calculations and economic
insulation thickness for building envelopes.
Variations. High levels of thermal resistance are not
always life cycle cost effective. Refer to the discussion on
"Buildings With Dominant Internal Loads" in Chapter 20 Errata of
the ASHRAE 1985 Handbook - Fundamentals.
Evaluation. An energy evaluation should be conducted
for each building to determine if the roof insulation thermal
resistance should be higher or lower than set forth above.
ASHRAE Methods. Chapter 28 of the ASHRAE 1985 Handbook
- Fundamentals, provides a number of methods for estimating
energy consumption associated with roof insulation. Hour by
hour, detailed computer simulation methods are preferred. At
least three levels of thermal resistance should be evaluated for
the roof over each type of space in each building.
Life Cycle Cost. Economic evaluation of roof
insulation should be carried out in accordance with NAVFAC P-442,
Economic Analysis Handbook, or equivalent.
a) Where some space heating is accomplished by means
of heat recovery, or where some space cooling is accomplished by
free cooling methods such as economy cycle, or cooling tower, or
cooling resulting in lower purchased energy requirements, lower
levels of thermal resistance can be justified on a life cycle
cost basis. Where energy is expensive, higher levels can be
b) Each area of a building may vary. The level of
thermal resistance for each area of the building does not
necessarily have to be the same throughout. Quite often, areas
with long hours of operation and high internal heat gains will
require less insulation than areas with little or no internal
heat gain and/or short hours of operation. Compensate for
reductions in R-value due to fasteners and structural members.