R-value (insulation)


In building and construction,the R-value is a measure of how well an object, per unit of its exposed area, resists conductive flow of heat: the greater the R-value, the greater the resistance, and so the better the thermal insulating properties of the object. R-values are used in describing effectiveness of insulation and in analysis of heat flow across assemblies (such as walls, roofs, and windows) under steady-state conditions. Heat flow through an object is driven by temperature difference '(e.g. T2-T1) between two sides of the object, and the R-value quantifies how effectively the object resists this drive: T2-T1 divided by the R-value and then multiplied by the surface area of the object’s side gives the total rate of heat flow through the object (as measured in Watts or in BTUs per hour). Moreover, as long as the materials involved are dense solids in direct mutual contact, R-values are additive; for example, the total R-value of an object composed of several layers of material is the sum of the R-values of the individual layers.Note that the R-value is the building industry term for what is in other contexts called ″thermal resistance per unit area.″ It is sometimes denoted RSI-value if the SI (metric) 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 unit length (e.g. per inch of thickness). The latter can be misleading in the case of low-density building thermal insulations, for which R-values are not additive: their R-value per inch is not constant as the material gets thicker, but rather usually decreases.

R-value should not be confused with the intrinsic property of thermal resistivity and its inverse, thermal conductivity. The SI unit of thermal resistivity is K·m/W. Thermal conductivity assumes that the heat transfer of the material is linearly related to its thickness. In calculating the R-value of a multi-layered installation, the R-values of the individual layers are added as shown in the equation.

To account for other components in a wall such as framing, first calculate the U-value (=1/R-value) of each component, then the area-weighted average U-value. The average R-value will be 1/(this average U-value).

Related formulas


RvaluetotalR-value total (m2*kelvin/watt)
RvalueoutsideairfilmR-value outside air film (m2*kelvin/watt)
RvaluebrickR-value of brick (m2*kelvin/watt)
RvaluesheatingR-value of sheating (m2*kelvin/watt)
RvalueinsulationR-value of insulation (m2*kelvin/watt)
RvalueplasterboardR-value of plasterboard (m2*kelvin/watt)