The R-value, also known as thermal resistance, is a quantity determined by the temperature difference, at steady state, between two defined surfaces of a material or construction that induces a unit heat flow rate through unit area (R = _T/q). R-value is the reciprocal of thermal conductance. A unit of thermal resistance used for comparing insulating values of different materials, for the specific thickness of the material. The higher the R-value number, a material, the greater its insulating properties and the slower the heat flow through it. This R-value does not include the interior and exterior air film coefficients.