U Values
The conduction of heat is quantified using the thermal conductivity coefficient, or kvalue (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 Rvalue (m^{2}K/W), is calculated by dividing the thickness of the material (in metres) by the kvalue. From this the thermal transmittance, Uvalue (W/m^{2}.K) of a building element, is calculated as the inverse of the sum of the Rvalues of the component parts and adjacent air layers.
The Uvalue 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 kvalue depends on its bulk density and the moisture content in service. (CIBSE guide Table A3.1., 1980)
Bulk density
kg/m^{3}

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 Uvalues: 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.