Should I insulate my cavity walls?

Cavity insulation has a mixed reputation, and the reason is that the same measure is excellent in most houses and damaging in a minority. Nearly all of the difference is established before installation, by a survey most people do not know to insist on.

Safety

Step by step

1. Find out whether you have a cavity at all.Houses built after roughly the 1920s usually have cavity walls; earlier ones are typically solid. A wall over about 260mm thick suggests a cavity, and the brick pattern helps — stretcher bond with no header bricks generally means a cavity.
2. Check whether it is already insulated.Many houses were done under earlier schemes without the current owner knowing. A borescope inspection through a small hole settles it, and the property's EPC often records it.
3. Assess exposure to driving rain.This is the single biggest risk factor. Properties in severe exposure zones — coastal, high ground, west-facing in wet regions — are much more likely to have problems, because wet outer leaf plus a filled cavity lets moisture bridge to the inner wall.
4. Check the condition of the outer wall.Cracked render, failed pointing, damaged brickwork and blocked weep holes all let water in. Those must be repaired before insulating, not afterwards, and a good survey will say so.
5. Check the cavity is clear and wide enough.Debris, mortar snots and wall ties bridging the gap all cause problems. A borescope survey looks at this directly and is the part that separates a survey from a sales visit.
6. Insist on a proper survey before any quote.Borescope inspection, exposure assessment, condition of the outer leaf and confirmation of cavity width. A quote given without one is not a basis for a decision.
7. Use a guarantee-backed installer.Registered with a recognised guarantee scheme so the work carries a long-term guarantee that transfers with the property. Keep the paperwork; it is asked for when selling.
8. Understand the failure mode.When it goes wrong, moisture crosses the cavity to the inner wall and shows as damp patches on internal walls, mould in corners, and cold spots. It typically appears within a year or two and can be difficult to attribute.
9. Weigh the payback honestly.In a suitable house, cavity insulation is among the shortest-payback measures available — often a few years. That is why it is worth doing properly rather than not at all.
10. Do the cheaper things first.Loft insulation and draught-proofing cost less, carry no risk, and often deliver more. Cavity insulation belongs after them rather than before.
11. Check what schemes currently apply.Grant funding for insulation exists and its scope changes. Confirm the current position and eligibility before assuming a cost.
12. Know what to do if problems appear.Contact the installer and the guarantee scheme promptly. Extraction of failed insulation is possible and expensive, and a documented guarantee is what makes it someone else's cost rather than yours.

Tips

• Ask specifically whether a borescope inspection is included before booking a survey. That single question separates the surveys from the sales visits.
• If a house nearby with the same construction and exposure has had problems, take that seriously — exposure is very local.
• Keep the guarantee certificate with the house paperwork. Buyers' solicitors ask for it, and a missing one causes delays.

Common mistakes

• Insulating before repairing the outer wall — Cracked render or failed pointing lets water into a cavity that can now carry it across to the inner leaf. The repairs come first, not afterwards.
• Accepting a quote without a borescope survey — Cavity width, debris and bridging wall ties all determine suitability and none of them is visible from outside. Without that inspection nobody knows whether the house is suitable.

Questions people ask