R-values are a measure of the thermal resistance of a material of a specific thickness, that is, its resistance to the transfer of heat across it. The higher the R-value of a material, the more effective it is as an insulator.
R-values can be used as part of a labelling system to enable comparison of the thermal performance of different materials, such as insulation, or as part of the calculation of heat transfer across the fabric of a building.
R-values can be calculated by dividing the thickness of a material (in metres) by its thermal conductivity (k-value or lambda value (λ) in W/mK). R-values are therefore expressed in m2K/W (or ft2·°F·hr/Btu in the USA). The overall R-value of a multi-layered element can be calculated by adding the R-values of its component materials.
R-values are sometimes described as being the reciprocal of U-values (sometimes referred to as heat transfer coefficients) which describe the thermal conductivity of a building element. However, unlike U-values, R-values do not include surface heat transfers at the boundary of the element by convection and radiation.
U-value = 1 / (the sum of the R-values of the layers of the element + the resistance of the internal and external surfaces of the element)
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