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Walls

U-value

The U-value measures how much heat passes through a building element, in watts per square metre per kelvin (W/m²K); the lower the value, the better it insulates.

Also known as U value, thermal transmittance, U-factor, heat loss coefficient, W/m2K

A U-value combines the insulating effect of every layer in a wall, roof or window, including the studs and other thermal bridges. Building codes set maximum U-values for new buildings, and energy-efficient houses go well below them.

In a timber-framed wall, the U-value is improved by thicker insulation, wider stud spacing, an extra insulated layer outside or inside the frame, and by avoiding gaps in the insulation.

Related terms
Mineral woolMineral wool is insulation made from glass or stone fibres, sold as batts, rolls or loose fill and used to fill the cavities between studs and joists. Thermal bridgeA thermal bridge is a local area of the building envelope where heat escapes more easily than through the surrounding construction. Stud spacingStud spacing is the distance from the centre of one stud to the next, typically 600 mm or 450 mm in Europe and 16 or 24 inches in North America.
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