Is the measure of thermal resistance, used in the building and construction industry. The higher the value, the better the insulation effectiveness of a material layer. The R-value is always the property of a material layer in m² K/W, derived from dividing the material layers thickness (m) by the material's thermal conductivity. The R-value is therefore not intelligible without the material layer thickness.

Several indices indicate more detailed information about what sort of R-value is considered. R_T for example indicates the total R-value of the sum of all material layers in a building element. If calculated in accordance with international standard ISO 6946, it also includes repeating thermal bridges.