The R-value is a measure of thermal resistance used in the building and construction industry. Under uniform conditions it is the ratio of the temperature difference across an insulator and the heat flux (heat transfer per unit area per unit time, ) through it or . Thermal resistance varies with temperature but it is common practice in construction to treat it as a constant value. An R-value is a unit thermal resistance for a particular material or assembly of materials (such as an insulation panel). The R-value depends on a solid material’s resistance to conductive heat transfer. For loose or porous material, the R-value accounts for convective and radiative heat transfer through the material. However it does not account for the radiative or convective properties of the material’s surface, which may be an important factor for some applications. R is expressed as the thickness of the material normalized to the thermal conductivity. The unit thermal conductance of a material is the reciprocal of the unit thermal resistance. This can also be called the unit surface conductance. The higher the value of R, the better the building insulation’s theoretical effectiveness. R-value is the reciprocal of U-factor.