R-value. Thermal resistance
Lambda values (λ), combined with information on the thickness
of material, represent the thermal resistance of individual materials used in the building envelope, or their R-value.
Thickness measured in m2·°C/W
λ
The higher the R-value, the better it resists the heat transfer.
The cumulative thermal resistance of a mixed structure is comprised of individual R-values of the layers in the structure.
R=
R-Value (Thermal Resistance) --- Thermal resistance is the ability of a material to prevent the passage of heat. It’s the thickness of the material (in metres) divided by its conductivity. This is expressed as m2K/W.
If the material consists of several elements, the overall resistance is the total of the resistances of each element. The higher the R-value, the more efficient the insulation.
Example:
PIR Board: 0.022 W/mK and 100mm thick; R-value = 0.1 metres ÷ 0.022 = 4.54 m2K/W
Glass Fibre Roll: 0.044 W/mk and 100mm thick; R-value = 0.1 metres ÷ 0.044 = 2.27 m2K/W