The short answer
The main signs are damp patches on the inside of external walls (often worst on the wall facing the weather), mould that keeps coming back, walls that feel cold or show cold bands, and damp that began a year or two after the insulation went in. Outside, look for saturated brickwork that stays dark long after rain. Only a camera inside the cavity confirms it.
On this page
Signs inside the house
Damp patches on external walls
The most common sign. Patches appear on the inside face of outside walls, frequently at skirting level, below windows or in a band across the wall. If they are worse after windy, wet weather, and worst on one side of the house, the cavity is a strong suspect.
Mould that comes back
Black mould behind furniture, in corners and around window reveals is often condensation. But if it returns after the room is well ventilated and heated, and it sits on outside walls only, cold spots in the wall may be the cause. Slumped or patchy fill leaves areas uninsulated, and those run cold enough to attract condensation.
Walls that feel cold
Put a hand on the inside of an outside wall on a cold day. If one area is noticeably colder than the rest, the fill there may have slumped or never been installed properly. A thermal camera shows this clearly.
Damp that started after the insulation
Timing matters. If you can date the insulation and the first damp patch, and they are a year or two apart, write that down. It is one of the most useful facts for a surveyor or a guarantee claim.
Decorating that will not stay put
Wallpaper lifting, paint blistering or salts blooming on plaster along outside walls.
Signs outside the house
Brickwork that stays wet
Walls that stay dark for days after rain, especially on the weather side, are holding water. Saturated brick passes more water to the fill.
Drill holes in the mortar
Filled cavities show a regular pattern of filled holes in the mortar joints, roughly a metre apart. Wide spacing, missed areas or holes only on some walls can mean an incomplete fill.
Cracking in a horizontal pattern
Horizontal cracks in mortar joints, repeating every few courses, can mean wall ties are corroding. Wet fill makes that worse. See wall tie replacement.
Which signs matter most
| Sign | Weight | Why |
|---|---|---|
| Damp on outside walls, worse on the weather side | High | Matches water crossing the cavity |
| Damp began after insulation | High | Strong timing link |
| Cold bands or cold spots | Medium | Suggests slumping or voids |
| Mould only in kitchen and bathroom | Low | More often condensation |
| Tide mark up to a metre | Low | More often rising damp or ground levels |
| Horizontal joint cracking | Separate | Wall ties; check alongside |
Confirming it
A borescope survey is the only way to know. Small holes are drilled through mortar joints on each wall and a camera looks at the fill: dry or wet, full or slumped, and whether anything is bridging the gap. Historic England notes that installers should check cavities with a borescope before filling; the same tool tells you whether it has gone wrong since.
Common causes once confirmed
- The wall faces severe wind-driven rain. BRE guidance maps exposure zones for this reason.
- A narrow cavity, under about 50 mm.
- Loose fill without binder that has settled.
- Mortar or rubble in the cavity bridging it.
- Poor pointing or leaking rainwater goods letting in more water than the wall can shed.
What to do next
- Write down what you see, where and since when. Photograph it.
- Fix gutters, downpipes and obvious pointing failures.
- Look for a guarantee. The Welsh Government guide gives CIGA as the main provider and explains the claim route.
- Have the cavity inspected.
If the survey confirms the fill has failed, the removal cost calculator shows typical prices for your house. Tell us what the walls are doing and we will find a specialist who can look.