This relates to the technical/constructional value. They are useful as it is a way of predicting the composite behaviour of an entire building element rather than relying on the properties of individual materials. A low U-value, or conversely a high R-Value usually indicates high levels of insulation. The higher the U-value, the lower the ability of the building envelope to resist heat transfer. It is expressed in watts per square metre kelvin: W/(m 2⋅K). The elements are commonly assemblies of many layers of materials, such as those that make up the building envelope. It is a property that describes how well building elements conduct heat per unit area across a temperature gradient. The U-factor or U-value is the overall heat transfer coefficient and can be found by taking the inverse of the R-value. R-values are additive for layers of materials, and the higher the R-value the better the performance. In the case of materials, it is often expressed in terms of R-value per metre.

for polyethylene foam), or for an assembly of materials (e.g. An R-value can be given for a material (e.g. The R-value is the building industry term for thermal resistance 'per unit area.' It is sometimes denoted RSI-value if the SI units are used. The measure is therefore equally relevant for lowering energy bills for heating in the winter, for cooling in the summer, and for general comfort.

R-value is the temperature difference per unit of heat flux needed to sustain one unit of heat flux between the warmer surface and colder surface of a barrier under steady-state conditions. In the context of construction, the R-value is a measure of how well a two-dimensional barrier, such as a layer of insulation, a window or a complete wall or ceiling, resists the conductive flow of heat. Installed faced fiberglass batt insulation with its R-value visible (R-21)

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