U-value
/ juː ˈvaljuː /
Also known as: thermal transmittance, heat transfer coefficient
Definition
A U-value (thermal transmittance) is a measure of how much heat passes through a building element - wall, roof, floor, or window - per second, per square metre of area, for every degree of temperature difference between inside and outside. It is expressed in W/m²K (watts per square metre kelvin). The lower the U-value, the better the insulating performance of the element.
In practice
U-values are calculated for the whole construction build-up, accounting for each material layer's thermal conductivity (lambda value), thickness, and the effect of thermal bridges. A single-leaf 215mm brick wall has a U-value of approximately 2.0 W/m²K - very poor. A modern cavity wall with 100mm full-fill mineral wool insulation achieves around 0.27 W/m²K. Adding external wall insulation can achieve 0.15 W/m²K or better.
It is important to understand that a calculated U-value assumes perfect construction - no gaps in insulation, no thermal bridges at joist ends, window reveals, or wall ties. In practice, thermal bridging increases the effective heat loss beyond what the U-value alone predicts, which is why Approved Document L now requires psi (linear thermal transmittance) calculations for junctions as well as elemental U-values.
Building Regulations
Approved Document L 2021 sets out both notional U-values (used to define the reference building in SAP calculations) and limiting U-values (the worst permitted for any element). Compliance is demonstrated via a SAP assessment for new dwellings, or an improvement to U-values plus other measures for existing buildings. U-value calculations follow BS EN ISO 6946.
Full Building Regulations guidance