or heat radiates outward from a
building at night.
The figure “Controlling Heat
Transfer” at right summarizes
strategies for controlling each type
of heat transfer. For conduction,
the unit R-value of the insulation and the total R-value of the
complete roof assembly should
be increased.* To increase unit
R-value, the amount and choice
of insulation are the key factors.
All other things being even, more
insulation means more R-value.
However, different materials
offer different R-values. “Unit
R-Value” below shows the R-value
of various roofing insulation types.
Polyiso and extruded polystyrene
(XPS) offer the highest thermal
values per inch.
To increase the total R-value of
an assembly, consider each component’s R-value, as the value for
each will be different. The values
for each component in an assembly are additive (i.e. the sum of the
values for all components is the
*R-value (thermal resistance) is a measure of the ability to resist heat transfer; U-value (thermal conductance) is a
measure of a material or assembly to transfer heat. The two have a mathematical relationship (U = 1/R). I prefer using
R-values because the numbers are larger and easier to manage, e.g., a U-value of 0.025 is equal to an R-value of 40.
HEAT MOVES through a roof in three ways. Each has a different
impact on building performance and ways to control it.
Controlling Heat Transfer
Type of Heat Transfer Control Strategies
CONDUCTION 1. Increase unit R-value of the primary
2. Increase total R-value of overall roof
3. Eliminate thermal “shorts” in the roof
CONVECTION 1. Install an air barrier to reduce air flow
2. Consider installing a vapor retarder to
3. Eliminate insulation air gaps
RADIATION Install climate-appropriate roof surface
Polyiso XPS EPS Rock Wool Perlite Wood Fiber
TO INCREASE UNIT R-VALUE, the amount and choice of insulation are the key factors. All other things
being even, more insulation means more R-value. However, different materials offer different R-values, as