Whole-house heat loss
Heat loss parameter HLP
- where
- heat loss parameter (W/m²K)
- heat transfer coefficient (W/K)
- total floor area (m²)
Explain this equation
Works out the heat loss parameter, a size-independent measure of how much heat a home loses. It is used in PAS 2035 and retrofit planning.
HTC is the total heat loss per degree. Dividing by the floor area makes homes of different size comparable: watts per degree per square metre of floor.
A simple normalisation by floor area.
Sense check: 280 W/K over 100 m² is 2.8 W/m²K. Very efficient homes are under 1, typical older homes 2 to 4.
Watch out: small homes and flats with lots of exposed wall tend to score worse, even when well built.
Divides the whole-house heat transfer coefficient by floor area, so homes of different sizes can be compared (SAP line 40).
Enter your values to see the result.
Good to know
A lower HLP means a more efficient envelope. It is a key figure in retrofit assessments and heat pump suitability discussions: older uninsulated homes often sit well above 3 W/m²K and deep retrofits well below 1.5 W/m²K.
Common mistake
Using gross external floor area. SAP uses total internal floor area.
Limits
Depends on the HTC entered. Compares envelopes, not running costs.
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.