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A third of an uninsulated home's heat leaves through the walls: filling the cavity (blown mineral wool or beads through drilled holes, half a day's work) is one of the biggest single savings in the book: only loft insulation rivals its payback. The checks that matter: a borescope survey confirms the cavity is clear and dry, exposed/driving-rain locations (western coasts) need proper assessment (damp risk is real when done badly), and CIGA guarantees on the work are standard: insist. Grant schemes (ECO4, Great British Insulation Scheme) cover many households: check before paying full price.
What The Cavity Wall Insulation Calculator Works Out
- Typical saving: an Energy Saving Trust style estimate of the yearly gas saving for your house type: about GBP 145 for a bungalow, GBP 180 for a mid-terrace, GBP 255 for a semi and GBP 380 for a detached house. Roughly a third of an uninsulated home's heat leaves through its walls.
- Payback: the typical install cost (GBP 700 to GBP 2,500 by house type, half a day's drilling and blowing in mineral wool or bead) divided by the yearly saving, landing at four to seven years. The saving then continues indefinitely.
- When it was built: the eligibility gate. Homes from the 1920s to the mid-1990s usually have an unfilled cavity; post-1995 homes were generally insulated at build; pre-1920 homes mostly have solid walls, which need a different and far dearer measure.
- Your installer quote (advanced): replaces the typical cost with a real one. Leave at 0 to keep the house-type default.
- Grant or supplier contribution (advanced): ECO4 and the Great British Insulation Scheme route supplier money into exactly this measure, taking installs to free or heavily subsidised for income- and property-band-eligible households. The net cost card and payback use the figure after the grant.
- Years you will stay and net saving (advanced): total savings over your stay minus the net cost. A negative figure means you move before it repays, though the measure still lifts the EPC and helps the sale.
- Gas price change and heating fuel (advanced): scale the saving for higher or lower gas prices, or for oil (about 10% more per kWh) and electric heating (about 3.3x gas), where the same heat loss costs far more.
- Gas saved and CO2 saved (advanced): the gas estimate converted to kilowatt-hours at your unit rate, defaulting to the current Ofgem price cap, and the CO2 avoided at 0.183 kg per kWh.
- Monthly saving (advanced): the yearly saving divided by twelve. Before booking: a borescope survey to confirm a clear, dry cavity, proper exposure assessment in driving-rain postcodes, and a CIGA guarantee on the work.