Has my cavity wall insulation caused damp, and can it be taken out?

Filling a cavity with insulation is one of the best-value energy measures there is in the right house. In the wrong house — badly exposed, with defective pointing or render, or with a cavity full of mortar debris — the fill becomes a path for water across the gap that was there to stop exactly that. The result is damp that appears one or two winters after the work, in a house that was previously dry.

What you'll need

• Binoculars
• Installation paperwork and guarantee certificate
• Independent surveyor with a borescope (optional)

Step by step

1. Establish the timeline.Find out when the cavity was filled and whether the damp predates it. Installation paperwork, an EPC, or the guarantee certificate will date it. Damp that began afterwards, in a house that was dry before, is the strongest single piece of evidence you will get.
2. Check the exposure of the house.Cavity fill is not suitable everywhere. Severely exposed locations — coastal, hilltop, open country facing the prevailing wind — were always meant to be excluded or to require a different product. If your house is in one, the suitability assessment is the question.
3. Look at the pattern of the damp.Irregular patches, on the weather elevation, at any height including upper floors, worse after several days of driving rain and better in a dry spell. That is water crossing the cavity, not rising and not condensing.
4. Rule out the ordinary causes first.Gutters, downpipes, pointing, render cracks, sills, flashings and ground level. Every one of these produces the same symptoms and all of them are cheaper to fix. A surveyor will check them before blaming the fill, and so should you.
5. Get a borescope inspection.A small hole drilled in a mortar joint and a camera pushed through shows what is actually in the cavity — the fill's condition, whether it has slumped, whether it is wet, and whether there is debris on the ties. This is the evidence.
6. Go to the guarantee first.Most UK cavity fill installed under the main schemes carries a long guarantee from an independent agency rather than from the installer. Find the certificate and use their complaint procedure — it exists precisely for this and it survives the installer going out of business.
7. Understand what extraction involves.Bricks or mortar joints are removed at intervals, the fill is vacuumed out through the openings, and the wall is made good. It is dusty, disruptive, leaves visible making-good, and it does not by itself repair whatever defect let the water in.
8. Fix the fabric before refilling anything.If the wall is refilled after extraction, the pointing, render, sills and rainwater goods must be sound first, otherwise the same thing happens again.

Tips

• Keep every document: the original installation certificate, the guarantee, the survey, dated photographs of the damp and of the weather. A claim is an evidence exercise.
• Cold-call companies offering free cavity extraction surveys are the mirror image of free damp surveys — same incentive structure, same caution required.
• If the fill has slumped, the top of the wall will be noticeably colder than the bottom, which sometimes shows as mould appearing along the upper part of the room only.

Common mistakes

• Blaming the fill without checking the gutters and pointing — Defective rainwater goods and failed joints produce the same weather-following damp, and they are far more common. If a claim is made on the fill and the real cause is a gutter, the claim fails and the damp continues.
• Paying a company to extract before exhausting the guarantee — Guarantee schemes can require remedial work to be carried out by their own approved route. Having someone else do it first may void the cover you already have.

Questions people ask