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Last edited 23 Apr 2019
Thermal conductivity (sometimes referred to as k-value or lambda value (λ)) is a measure of the rate at which temperature differences transmit through a material. The lower the thermal conductivity of a material, the slower the rate at which temperature differences transmit through it, and so the more effective it is as an insulator. Very broadly, the lower the thermal conductivity of a building's fabric, the less energy is required to maintain comfortable conditions inside.
The U value of an element of a building can be calculated from sum of the thermal resistances (R-values) of the layers that make up the element plus its internal and external surface resistances (Ri and Ro).
U-value = 1 / (ΣR + Ri + Ro)
The standards for the measurement of thermal conductivity are BS EN 12664, BS EN 12667 and BS EN 12939. In the absence of values provided by product manufacturers following thermal conductivity tests, the thermal conductivity data obtained from BS EN 12524 Building materials and products. Hygrothermal properties.
 Thermal conductivity of typical building materials
Thermal conductivity values of typical building materials shown below.
|glass foam aggregate (dry)||0.08|
|phenolic foam (PIR)||0.020|
|polyurethane foam (PUR)||0.025|
|steel||16 - 80|
|stone (granite)||1.7 - 4.0|
|straw bale||0.055 - 0.065|
|timber (hardwood - commonly used)||0.14 - 0.17|
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