U-value
The thermal transmittance of a building element: heat flow in watts per square metre per kelvin of temperature difference (W/m²K). Lower is better.
The U-value describes how readily heat passes through a wall, roof, floor or window, including surface resistances. It is the reciprocal of the total thermal resistance of the layered assembly (see R-value).
Heat loss Q (W) = U x A x ΔT
Example: 100 m² wall, U = 0.25, ΔT = 20 K: 500 W
Conversion: U (BTU/h·ft²·°F) = U (W/m²K) / 5.678
Here d is layer thickness (m), λ thermal conductivity (W/m·K), and R_si / R_se internal and external surface resistances (typically 0.13 and 0.04 m²K/W for walls).
Typical values (W/m²K):
- Uninsulated solid brick wall: 1.5–2.1
- Modern code wall (EU): 0.15–0.30
- Passive house wall or roof: 0.10–0.15
- Single glazing: about 5.0–5.8
- Double glazing (low-e, argon): 1.1–1.4
- Triple glazing: 0.5–0.8
Mineral wool and EPS have λ around 0.032–0.040 W/m·K, PIR about 0.022, aerogel about 0.015. Real performance is often worse than calculated because of thermal bridges, gaps and moisture. Building codes set maximum U-values per element.
Related terms
All terms →- R-value
Thermal resistance of a material layer or assembly, its ability to resist heat flow; the reciprocal of U-value. Higher is better.
- Thermal bridge
A localised area of a building envelope with significantly higher heat flow than its surroundings, caused by geometry or more conductive materials.
- Passive house (Passivhaus)
A rigorous building performance standard, developed in Germany in the 1990s, achieving very low heating demand through insulation, airtightness and heat-recovery ventilation.
- Heating degree days (HDD)
A measure of how much and for how long outdoor temperature falls below a base temperature, used to estimate and normalise building heating demand.
- Energy Performance Certificate (EPC)
A standardised rating of a building's calculated energy performance, typically on an A–G scale, required in the EU and UK when buildings are sold or rented.