What aspect of building performance measures the efficacy of insulation?

The efficacy of insulation is primarily measured by its R-value. R-value quantifies the thermal resistance of insulation materials; the higher the R-value, the better the material insulates. This measurement reflects the material's ability to resist heat flow, which is essential for maintaining comfortable indoor temperatures and improving energy efficiency in buildings.

In contrast, U-value measures the rate of heat transfer through a building element, such as walls or windows, and is inversely related to insulation efficacy—the lower the U-value, the better the insulating value. Thermal mass refers to the ability of a material to absorb and store heat, which impacts energy performance but does not directly measure insulation efficacy. Conductivity measures the material's ability to conduct heat; while it does play a role in determining both R-value and U-value, it is not a direct measure of insulation performance itself. Thus, R-value is the most appropriate metric when assessing how well insulation performs in a building.