Signs of failed cavity wall insulation
Failed or unsuitable cavity fill does not always look dramatic. The usual clues are damp-related, and they overlap with other common problems – so treat this as a checklist for investigation, not a self-diagnosis.
Common signs
- Random damp patches on external walls – especially after wind-driven rain, and not limited to one obvious leak line.
- Black mould or musty smells on the inner face of outside walls, often in corners or behind furniture.
- Cold spots on walls that feel colder than neighbouring sections of the same elevation.
- Peeling wallpaper, blown plaster or tide marks on the inner leaf of a filled wall.
- Evidence through a borescope – wet, slumped, compacted or bridged insulation in the cavity (something a specialist survey can show).
Damp has many causes. Condensation is the most common cause of damp in homes (GOV.UK). RICS also warns that "rising damp" is often misdiagnosed and that failure of an original damp-proof course is extremely rare (RICS). Rule out gutters, roof leaks, plumbing and bridging at ground level before paying for extraction.
When cavity fill is more likely to be involved
- Symptoms line up with elevations that were filled
- Problems started or worsened after insulation was installed
- The house is in an exposed position, or the walls were never suitable for full-fill
- A surveyor has specifically mentioned cavity insulation, cavity bridging or wet fill
What to do next
- Gather paperwork – installer name, date, CIGA or other guarantee, any ECO/scheme letters.
- Get an impartial look at the cavity – a damp or insulation survey with a borescope, from someone who isn't only there to sell extraction. Our sister site covers independent damp surveys.
- Use the guarantee route if you have one – report to the installer first; involve CIGA if needed (CIGA).
- Only then compare extraction quotes – if independent advice says the fill should come out.