In its most practical form it must be inverted, so an R-value of 4 becomes 1 / 4, and that is 0.25 watts being lost, for each degree Celsius temperature difference, through each square metre of area.įor example, in winter it might be 2 ☌ outside and 20 ☌ inside, making a temperature difference of 18 ☌. This relates to the technical/constructional value. 16.2.1 Non-reflective surface R-values for air films.13.3 The limitations of R-values in evaluating radiant barriers.13.2 Surface temperature in relationship to mode of heat transfer.13.1 Thermal conductivity versus apparent thermal conductivity.
It is intended as a simple, non-technical rating which consumers can understand. Insulating camping products such as sleeping mats and foam mattresses are often given a rating such as R-7 or R24, which may be called "the R Value" but is not the same. 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. Īn 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. 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.