Work out home energy audit savings instantly with clear inputs, formula shown and shareable results.
Energy saving measures multiply rather than add: 18% from insulation and 6% from draught proofing give 22.9% combined, not 24%, because the second measure works on the already reduced consumption. Payback is measure cost divided by that annual saving.
Combined saving
combined = 1 - (1 - s1)(1 - s2); payback = cost / annual saving
Each measure reduces the same heat loss, so the second one has less to work with. This applies to every combination of efficiency measures.
Draught proofing and loft insulation, often within two to three years. Solid wall insulation and glazing take a decade or more.