R-value is a measure of a material’s ability to resist heat flow. Before 1945, the U-factor measured this, with lower U-factors being better at resisting heat.
The rated R-value is a regulated number. The regulation requires insulation to be labeled with a rated R-value derived from one of four approved test methods. Real-world R-value is a dynamic number that changes depending on various factors.
- Material R-Value - Center-of-Cavity R-Value - Clear-Wall R-Value - Whole-Wall R-Value - Overall R-Value
R-value is important, but air leakage is also crucial in determining insulation performance. The R-value test can measure the effects of convective loops within insulation but not the amount of air that may leak through the wall assembly once installed.