What is the measure of resistance to heat flow called?

• A. U-factor
• B. R-value
• C. Thermal conductivity
• D. Thermal mass

Answer: (B)

Resistance to heat flow is measured by the R-value. The higher the R-value, the greater the resistance and the less heat moves through the material or wall assembly. You add the R-values of each layer to get a total R-value for a building section, and a higher total means better insulation.

U-factor is the rate of heat transfer, essentially the inverse of R-value, so a smaller U-factor means better insulating performance. Thermal conductivity describes how easily heat moves through a material itself; materials with high conductivity transfer heat quickly and have lower resistance. Thermal mass is about storing heat, not resisting its flow, so it doesn’t directly measure how well a component blocks heat transfer.