What is the difference between a U-value and an R-value?

U-values take into account all elements of a construction, including thermal bridges. They can only be worked out by calculation, usually done by the manfacturer or contractors. The R-value, or thermal resistance, is a measure of the insulation board on its own, dividing the thickness (in metres) by the thermal conductivity (lambda value, W/m.K). The R-value can be used to compare how different thicknesses of products with different lambda values perform comparatively.

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