R-value (insulation)

The R-value is a measure of therm, specifical insulation lly 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, in the context of construction. The higher the R-value, the more insulating the material is. Higher R-values can reduce heating bills in cold weather and cooling bills in hot weather.

Because R-value can be measured with either US or metric units, it is sometimes denoted RSI-value if the SI units are used. An R-value can be given for a material (e.g., for polyethylene foam), or for an assembly of materials (e.g., a wall or a window). In the case of materials, it is often expressed in terms of R-value per metre, or per inch in the US. R-values are additive for layers of materials. R-value is defined as 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, over a unit area.

The U-factor or U-value is the overall heat transfer coefficient and can be found by taking the inverse of the R-value. It is a property that describes how well building elements conduct heat per unit area across a temperature gradient. The elements are commonly assemblies of many layers of materials, such as those that make up the building envelope. It is expressed in watts per square metre kelvin. The higher the U-value, the lower the ability of the building envelope to resist heat transfer. A low U-value, or conversely a high R-value, usually indicates high levels of insulation. 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.