What does R-value mean?

R-value means resistance to heat flow and is the standard metric used in the building industry to rate how well an insulation material resists conductive heat transfer. The R stands for resistance, and the higher the R-value, the better the material insulates. It is defined as the temperature difference across the material divided by the heat flow through it, so mathematically it is the inverse of thermal conductance. For example, R-19 means the material resists heat flow 19 times better than a material with R-1. R-value is additive when you stack materials, so two layers of R-13 batts give you R-26, though in real assemblies framing and air gaps reduce overall performance. R-value is tested in a laboratory at a specific temperature and does not account for air leakage, thermal bridging through studs, moisture, or radiant heat transfer. It also assumes proper installation without compression, gaps, or voids. While R-value is useful for comparing insulation products, a complete energy efficient assembly also needs air sealing, vapor control, and consideration of thermal mass and radiant barriers for best performance.