Fabric heat loss
Conduction only.
Method
Q = UAΔT. Worked case: U 0.28, 40 m², 20 K is 224 W.
Not a heat-loss calculation for Building Regulations compliance.
Logic: how this figure is built.
How to read a fabric heat loss
This page answers a steady-state question: how many watts leave through an area of fabric at a temperature difference, if you already know the U-value. It is Q = UAΔT. It is not a Building Regulations compliance calculation and not a room heat-loss design.
U is watts per square metre per kelvin. Area is the area that U applies to. ΔT is the air-to-air difference the U-value was defined for. U 0.28 W/m²K, 40 m² and 20 K is 224 W. Change any one of those and the watts change in proportion.
A U-value already includes the layers and the surface resistances it was calculated with. Do not multiply by another surface factor. Do not add ventilation, thermal bridges or intermittent heating. A wall U-value on a roof area is the wrong fabric. Degrees Celsius and kelvin are the same size for a difference, so 20°C difference is 20 K.
Use it as a first screen beside a model when someone has quoted a U-value and an area and you want the order of the fabric loss. A compliance calculation needs the approved method, the actual elements and the ventilation allowance. This page does none of that.
- Related: sensible heat
- weights and heat
Last checked 3 October 2026. See the disclaimer.