Home efficiency
Cavity-wall insulation: suitability, cost and risks
Filling an empty cavity is one of the cheaper ways to cut heat loss through walls. It suits many homes, but not those with narrow cavities, damaged walls or heavy exposure to driving rain.
Updated 3 October 2026 · energy.org.uk editorial team
The short answer
Energy Saving Trust estimates about a third of the heat lost from an uninsulated home goes through the walls. It puts the cost of cavity-wall insulation for a typical home in Great Britain at around £2,200 (around £1,000 in Northern Ireland), and savings of around £200 a year. Your own saving depends on your home, how you heat it and your energy prices.
Do you have cavity walls?
- Homes built from around the 1920s onwards usually have cavity walls; older homes often have solid walls.
- Cavity walls usually show bricks laid lengthways only. Solid walls often show alternating long and short brick ends.
- Small, evenly spaced holes filled with mortar suggest the cavity has already been filled.
- Your EPC may say whether the walls are insulated, but check it is up to date.
Check
What to check first
- The cavity is at least 50mm wide and clear of rubble.
- The brickwork and pointing are in good condition, with no cracks or leaks.
- The walls are not exposed to driving rain, as on some coastal or hilltop homes.
- The home is not at risk of flooding.
- There is an existing damp problem. Fix that first; see condensation, damp and ventilation.
Do
Practical next steps
- Ask an installer for a survey of the walls before you agree to work.
- Ask what material they will use and whether the work comes with an independent guarantee.
- Check whether you could get help through a scheme in the support finder.
- Keep the home ventilated after the work: trickle vents and extractor fans still matter.
Take care
Important limitations
- Badly installed or unsuitable cavity insulation can lead to damp; a proper survey is the main protection.
- Costs and savings above are Energy Saving Trust typical figures, not quotes.
- Removing failed cavity insulation is costly, so take time over the decision.