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.

What is the Home Insulation Payback Calculator?

Choose an insulation type — loft, cavity wall, solid wall or underfloor — set your home size, heating fuel and installation cost to compute annual energy and bill savings, CO2e avoided, and the simple payback period. Based on EST and CIBSE benchmark savings. It runs free in your browser on Gera Tools, with nothing uploaded.

Home Insulation Payback Calculator

Name: Home Insulation Payback Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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.

The four main insulation types compared

Understanding what each measure targets helps you interpret the results and prioritise the right upgrade for your home.

Loft insulation is the most accessible and typically fastest-payback measure. Heat rises, so an uninsulated or thinly insulated loft loses a significant share of a home’s heat. Insulating to the recommended depth (around 270 mm of mineral wool) is straightforward work, and costs for a typical semi-detached home are modest compared to the lifetime savings. If you already have some loft insulation but it is thin or old, topping up delivers a smaller saving than starting from bare — the first 100 mm provides most of the benefit.

Cavity wall insulation fills the gap between the inner and outer leaves of cavity-wall construction, common in UK homes built after around 1920. Installers drill small holes, inject mineral fibre or blown bead, and fill the holes. When the cavity is in good condition and the wall is suitable, this is one of the most cost-effective insulation investments available.

Solid wall insulation applies to older properties with single-brick walls that have no cavity to fill. These walls lose proportionally more heat, so the annual energy saving from insulating them is the largest of any measure — but the installation cost is considerably higher, whether done internally (reducing room size slightly) or externally (rendering the outside). Payback is longer, but the long-run saving is substantial, and external insulation also improves a building’s weather resistance.

Underfloor insulation targets suspended timber ground floors, where draughts through floorboards and heat loss into the sub-floor space can be surprisingly significant in older homes. Solid concrete floors do not benefit in the same way. Savings are generally smaller than loft or cavity-wall measures, making underfloor insulation a good complement after the higher-priority upgrades.

Heating fuel matters for bill and carbon savings

The kWh saved by insulation is roughly the same regardless of fuel — you need less heat, so you burn less fuel. But what that means in pounds and CO2 varies considerably. Electric heating costs more per kWh than mains gas in most cases, so the same kWh saving converts to a larger bill reduction when you heat with electricity. Carbon-factor differences also mean that insulating an electrically heated home avoids more CO2 per kWh saved than insulating a gas-heated one, especially as the electricity grid decarbonises.

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.

Use this calculator to compare measures side by side and identify which one gives the shortest payback for your specific fuel and home size before committing to an installation quote.