Heat loss
Heat loss through an element (Q = U × A × ΔT)
- where
- heat loss (W)
- U-value (W/m²K)
- net area of the element (m²)
- internal design temperature (°C)
- external design temperature (°C)
Explain this equation
Works out the heat flowing out through one wall, roof, floor or window at design conditions. Use it to show the effect of insulation in watts or to size heating.
U is the watts lost through one square metre for each degree of difference between inside and outside. Multiplying by the area and the temperature difference gives the total watts.
Steady-state conduction: heat flow is proportional to area, U-value and temperature difference.
Sense check: 20 m² of wall at U = 0.3 with 24 °C difference loses about 144 W; the same wall uninsulated at 2.0 loses 960 W.
Watch out: use the net area with windows and doors taken out, and add those separately with their own U-values.
Steady-state fabric heat loss through one wall, roof, floor or window on a design day, for comparing before and after insulation or sizing emitters.
Enter your values to see the result.
Good to know
This is the transmission term of the BS EN 12831 heat-load method. Use the net area (gross wall minus windows and doors, which you calculate separately with their own U-values). External design temperatures depend on location and are published in CIBSE Guide A and the MCS heat-pump tools, typically between about −1 °C and −5 °C in the UK. Living rooms are usually designed at 21 °C, bedrooms at 18 °C. Ventilation heat loss and thermal bridges at junctions are extra.
Common mistake
Using gross wall area and then adding windows too, so the window area is counted twice.
Limits
Steady state, one element, no thermal bridging at junctions (ψ-values), no ventilation or infiltration loss. For floors on the ground use the ISO 13370 U-value with the external temperature, as an approximation.
Sources
- BS EN 12831-1:2017 Energy performance of buildings – Method for calculation of the design heat load (transmission heat loss Φ = U A (θint − θe))
- BS EN ISO 6946:2017 Building components and building elements – Thermal resistance and thermal transmittance – Calculation methods (surface resistances; combined method; Annex F corrections for air voids and mechanical fasteners)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow BS EN ISO 6946, BS 5250, PAS 2035, Building Regulations Part L and manufacturers’ data. Whole-house retrofit needs a qualified retrofit assessor and coordinator.