Whole-house heat loss
Heat transfer coefficient HTC (fabric, thermal bridges and ventilation)
- where
- thermal bridging heat loss (W/K)
- thermal bridging factor (W/m²K)
- total exposed element area (m²)
- ventilation heat loss (W/K)
- effective air change rate (per hour)
- internal volume (m³)
- heat transfer coefficient (W/K)
- fabric heat loss, sum of U × A (W/K)
- design heat loss (kW)
- design temperature difference (K)
Explain this equation
Builds up a home's heat transfer coefficient: the total watts it loses for each degree of temperature difference. Use it to compare before and after a retrofit or to check heating size.
UA is the sum of each element's U-value times its area. Junctions between elements leak extra heat; the y-factor approach adds that as a rate per square metre of exposed area. Ventilation and infiltration add 0.33 n V. Adding the three gives HTC. Multiplying by the design temperature difference and dividing by 1000 gives the peak heat loss in kW.
All three heat losses are proportional to temperature difference, so they add.
Sense check: 180 W/K of fabric plus 50 of bridging and 50 of ventilation is about 280 W/K, about 6.4 kW at a 23 K difference.
Watch out: older homes can have high bridging (y of 0.15 is a common default). Treating junctions well can matter as much as adding insulation.
Adds fabric, thermal bridging and air leakage/ventilation losses into one heat transfer coefficient, as in SAP lines (36) to (39).
Enter your values to see the result.
Good to know
ΣU×A is the sum of each element’s U-value times its area (use the fabric calculators). SAP uses y = 0.20 W/m²K when thermal bridging details are unknown; accredited details give lower values. n is the effective air change rate after infiltration, shelter and mechanical ventilation. A 23 K design difference corresponds to 21 °C inside and −2 °C outside.
Common mistake
Forgetting thermal bridges. On a well-insulated house they can be a fifth of the fabric loss.
Limits
Steady-state. Not a room-by-room heat loss calculation for sizing emitters (use BS EN 12831 / MCS for that).
Sources
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.