THERMAL RESISTANCE: A materialâ€™s ability to resist heat flow via the three heat transfer mechanisms: Conduction (measured as R-Value), Convection, and Radiation
IMPACT: Most Insulation Systems only consider conductive heat losses when evaluating insulation performance (R Value). Radiative and convective losses can amount to as much as 85% of a building envelopeâ€™s heat loss/gain(1)
Insulation is designed to effectively reduce the rate of heat loss (winter) and heat gain (summer) through a building envelope. Heat transfer can occur by the the following three mechanisms:
CONDUCTION: Transfer of energy from hot to cold regions of a substance by molecular interaction (direct contact). Measured as â€œR Valueâ€ in insulating systems.
CONVECTION: The conveying of heat through a liquid or gas by motion of its parts. It is conduction in a fluid as enhanced by the motion of the fluid.
IMPACT: Convective losses can account for up to 35% of a building envelopeâ€™s overall heat losses(1)
RADIATION: Tansfer of thermal energy in the form of electromagnetic waves travelling at the speed of light.
IMPACT: Radiative losses can account for up to 55% of a building envelopeâ€™s overall heat losses (1)
The process of heat transfer is a very complex topic, which is often oversimplified so as to help the consumer understand and make good insulation purchases. The problem is that oversimplification (such as using the R-Value system alone) can lead to errors in energy calculations and may result in a building being under or over-insulated.
As such, the amount that each heat transfer mechanism contributes to the overall heat loss from a structure will depend on the configuration of the building envelope, as well as other environmental factors, including but not limited to air infiltration, moisture, and extreme temperatures.
(1) CBD-149 â€œThermal Resistance of Building Insulationâ€, Canadian Building Digest, NRC