What is heat flow?
Heat flow is the amount of heat that passes from warm to cold each second, for example through a wall, roof, floor or window. The unit is the watt (W), or joules per second. Heat always flows from a higher to a lower temperature, by conduction, convection and radiation. Expressed per square metre, it is called heat flux (W/m²).
Calculating heat flow
For a building element, approximately:
Heat flow (W) = U-value × area (m²) × temperature difference (K)
Examples at a 20 degree difference between inside and outside:
- Insulated wall, 10 m² with U = 0.2 W/m²K: 0.2 × 10 × 20 = 40 W.
- Single-glazed window, 2 m² with U ≈ 5.8: about 230 W.
- Modern double-glazed window, 2 m² with U ≈ 1.1: about 45 W.
So that one single-glazed window loses more heat than the whole insulated wall next to it.
Uses
- Heat loss calculation: the sum of all heat flows through the envelope, plus ventilation losses, sets the output a boiler or heat pump must deliver.
- Prioritising: calculate the heat flow per element and insulate where it is largest, taking cost into account.
- Thermal bridges: locally increased heat flow gives cold spots, condensation and mould.
- Thermal imaging: shows heat flow as surface temperature differences; survey in winter with a large temperature difference and no sun on the facade.
Note
The formula gives a snapshot. For annual energy use, work with degree days or ask an energy assessor.
Related terms
- Heat loss area
- Thermal bridge
- Thermal resistance
- Thermal radiation
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