Comparison of R-values of various insulating substances.
The R-value is a measure of the capacity of a material to resist heat transfer. It is the reciprocal of (i.e. one over) the conductivity of the material (U-value). The higher the R-value, the greater the insulating properties of the material. It is important to be aware that R-values depend not only on thickness but also on material type and and installed weight per unit area. All materials with the same R-value, regardless of type, thickness, or weight, have the same insulating power. To obtain the R-value of a multilayered assembly, such as a wall or ceiling, the R-values of the individual layers and components must be added together.
The effectiveness of an insulation's resistance to heat flow also depends on how and where the insulation is installed. For example, insulation that is compressed will not provide its full rated R-value. The overall R-value of a wall or ceiling will be somewhat different from the R-value of the insulation itself because some heat flows around the insulation through the studs and joists. Therefore, it's important that the insulation be properly installed to achieve the maximum R-value.
See also wood R-value.