The upgrades cut fabric heat loss by 49% and save about 7,220 kWh of fuel (722 €) per year, paying back in ~19.2 years. Best value: Walls (6.3-year payback).
Money saved per year
€722
7,220 kWh of fuel per year
Payback (all measures)
19 yr 2 mo
cost 13,850 €
Heat loss before
13,192kWh/yr
fabric only, 298 W/K
Heat loss after
6,693kWh/yr
125 W/K
Reduction in fabric loss
49.3%
CO₂ saved per year
1.32t
30-year savings
€34,350
with 3%/yr price growth
Design heat loss cut
4.14kW
at 21 °C in / −3 °C out, so a smaller boiler or heat pump will do
Heating degree days
1,844K·d (base 15.5 °C)
London, UK
Savings per measure
| Element | Area (m²) | U before → after | kWh fuel saved | Saved (€/yr) | Cost (€) | Payback |
|---|---|---|---|---|---|---|
| Walls | 110 | 1.5 → 0.55 | 4,369 | 437 | 2,750 | 6.3 yrs |
| Roof / loft | 60 | 0.7 → 0.15 | 1,380 | 138 | 1,200 | 8.7 yrs |
| Windows & doors | 22 | 2.8 → 1.2 | 1,472 | 147 | 9,900 | 67.3 yrs |
| Ground floor | 60 | 0.7 → 0.7 | 0 | 0 | 0 | — |
- Degree days: London, UK (1991–2020 normals, approximate), base 15.5 °C.
- Ground floor losses use a 0.7 factor because the ground is warmer than the outdoor air in winter. Ventilation and air-leakage losses are not included here.
- A 15% performance gap is applied to all savings (thermal bridges, workmanship, warmer indoor temperatures after insulating).