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U Values

The conduction of heat is quantified using the thermal conductivity coefficient, or k-value (W/m.K), of the materials used in its construction. This is the rate at which heat flows through a material between points at different temperatures. The thermal resistance, or R-value (m2K/W), is calculated by dividing the thickness of the material (in metres) by the k-value. From this the thermal transmittance, U-value (W/m2.K) of a building element, is calculated as the inverse of the sum of the R-values of the component parts and adjacent air layers.

The U-value is the measure of heat transmittance through a material and the lower the U value the less heat is transmitted through a construction i.e. the better the insulation quality.

For concrete the k-value depends on its bulk density and the moisture content in service. (CIBSE guide Table A3.1., 1980)

Bulk density


k- value W/m.K

Internal (3% moisture by vol.)

k- value W/m.K

External (5% moisture by vol.)

2000 1.13 1.24
2200 1.45 1.60
2400 1.83 2.00

For further information see U-values: understanding heat movement, published in CONCRETE in March 2003, pp 42–43. Copies are available as a free download from the Members Area of the Concrete Society web site.

Acknowledgement: UKQAA