U-values, R-values and conductivities for home extensions
Best Practice insulation specifications are generally expressed in terms of a U-value.This
is the overall rate of energy transfer through 1m2 of a particular building element when the air temperatures on
either side differ by 1ºC.
It is measured in W/m2K. For some refurbishment situations, it is more useful to quote thermal resistances, or
R-values. Thermal resistance depends on thermal conductivity (λ) and thickness (d): R = d/λ and is measured in
m2K/W. Thermal conductivities of insulating materials are published by manufacturers.
The resistances of each component (as well as the resistances of cavities and surfaces) are used to calculate
the overall thermal resistance or R-value for the structural element.
The U-value is the reciprocal of this figure.