How is the payback period calculated?

Simple payback divides the installation cost by the annual bill saving. If insulation costs 600 and saves 150 per year, the payback is four years. It ignores fuel-price inflation and discounting, so real payback is usually a little faster as energy prices rise.

Why does solid wall insulation cost more but save more?

Solid walls lose far more heat than insulated cavity walls, so insulating them delivers the largest energy saving per home — but it is also the most expensive and disruptive measure, which lengthens the payback despite the bigger annual saving.

Where do the saving benchmarks come from?

The annual kWh savings are based on Energy Saving Trust and CIBSE benchmark figures for a typical UK home, scaled by home size. Your actual saving depends on your home's current insulation, heating habits, and climate, so treat the result as indicative.

Does the heating fuel change the result?

Yes. The energy saved is roughly the same kWh regardless of fuel, but the bill saving and CO2 avoided depend on the fuel's unit price and carbon factor. Electric and oil heating give larger bill savings than mains gas for the same kWh saved.

Is loft insulation still worth it if I have some already?

Topping up thin loft insulation to the recommended depth gives a smaller saving than insulating a bare loft, because most of the benefit comes from the first 100 mm. This tool assumes insulating from a poorly insulated baseline, so reduce the figure if you are only topping up.

Home Insulation Payback Calculator

This calculator estimates the financial and carbon return on a home insulation measure. You pick the type of insulation, your home size, heating fuel and the quoted cost, and it returns the annual energy saved, the money and CO2e saved each year, and how long the measure takes to pay for itself.

How it works

Each measure has a benchmark annual energy saving for a typical home, scaled by home size. That kWh saving is converted to money and carbon using your fuel’s price and emission factor, and payback is cost divided by annual saving:

kWh_saved   = benchmark_kWh × size_factor
bill_saving = kWh_saved × fuel_price_per_kWh
co2_saved   = kWh_saved × fuel_carbon_factor
payback_yrs = install_cost / bill_saving

Benchmarks follow Energy Saving Trust and CIBSE figures: solid wall insulation saves the most energy per home, followed by cavity wall, loft, and underfloor.

Tips and example

Cavity wall insulation in a typical semi-detached gas-heated home saves around 2,500 kWh a year — roughly 175 in bills and 0.5 tonnes of CO2 — so a 1,000 installation pays back in about six years and keeps saving for decades afterwards. Solid wall insulation saves more energy but costs far more, so its payback is longer; loft insulation from bare is usually the fastest payback of all.