The short answer
Not on its own. Properly installed in a suitable, well maintained wall, cavity fill should keep the inner wall dry. It causes damp when the wall was unsuitable (very exposed to driving rain, or a narrow cavity), when the installation was poor (gaps, slumping, debris left in the cavity), or when the outer brickwork lets in more water than the fill can cope with.
On this page
Why the cavity exists
A cavity wall is two walls with a gap between them. The outer leaf of brick is expected to get wet: in a storm, rain driven against it soaks in and some reaches the inside face of the brick. Without fill, that water runs down the back of the brick and out at the base, and the gap stops it reaching the inner leaf. Metal ties hold the two leaves together, and each one has a drip in the middle so water falls off rather than tracking across.
Filling the cavity takes that gap away. The fill is made to repel water, so in a sheltered wall that is in good order it works: the inner wall stays dry and much warmer. Problems start when one of the conditions it relies on is missing.
The four ways fill leads to damp
1. The wall is too exposed
Some walls face so much wind-driven rain that water reaches the fill in quantity. BRE guidance on insulating masonry cavity walls treats exposure to driving rain as one of the principal risks and includes a map of exposure zones for the UK. Coastal houses, hilltops and gable ends facing the prevailing south-westerly wind are the usual problems. Fill in these walls can stay wet for long periods and pass water through to the inside.
2. The cavity is narrow or irregular
Older cavity walls are often narrow. Historic England notes that early cavities range from under 50 mm to about 100 mm, that insulating cavities under 50 mm is generally not worthwhile, and that narrow cavities carry a greater risk of water crossing the insulation. Irregular cavities, with brick headers or rubble bridging the gap, are worse.
3. The installation was poor
- Voids. Drilling points missed or spaced too widely leave gaps, which run cold and attract condensation.
- Slumping. Loose beads blown in without a binder settle over time, leaving an empty band at the top of the wall.
- Debris. Mortar dropped on ties when the house was built, or rubble at the base of the cavity, makes a bridge that the fill then holds damp against.
- Blocked weep holes or trays. Fill packed into cavity trays over windows stops them shedding water.
4. The outer wall lets in too much water
Open mortar joints, cracked render, leaking gutters and downpipes, and ground or paving built up against the wall all put more water into the brick than it was ever meant to take. Even a well installed fill struggles under a constantly overflowing gutter.
How to tell it apart from other damp
| What you see | Points towards | Check first |
|---|---|---|
| Patches on outside walls, worse after wind and rain, worst on one elevation | Cavity fill or rain penetration | Pointing, gutters, then a borescope |
| Damp appeared a year or two after insulation | Cavity fill | Date of installation, guarantee |
| Black mould in corners and behind furniture, worse in winter | Condensation, possibly cold spots from voids | Ventilation and heating; thermal image |
| Tide mark up to about a metre high | Rising damp or high ground levels | Damp-proof course, outside ground level |
| Damp under one window | Failed cavity tray or sill | Tray and sill detail |
None of these is proof on its own. The way to know is to look inside the cavity with a borescope survey.
If it is the fill
Check for a guarantee before paying for anything; the GOV.UK consumer guide says to go to the installer first and the guarantee provider second. See CIGA guarantee claims. If the fill does need to come out, extraction is done from outside, typically in one to three days, and our removal cost calculator gives a budget for your house.
If it is not the fill
Fix the cheap things: clear gutters, repair downpipes, repoint open joints, lower soil or paving that has built up against the wall, and improve ventilation in kitchens and bathrooms. Watch the damp through a winter before deciding the cavity is to blame.
Not sure which it is? Tell us what you are seeing and we will find a cavity wall insulation removal company near you to look inside the wall.
Where our information comes from
- BRE Good Building Guide 44 Part 2: Insulating masonry cavity walls, principal risks and guidance (2001)
- Historic England: Insulating early cavity walls (HEAG083, April 2016)
- GOV.UK (BEIS): Cavity wall insulation, consumer guide to issues arising on installation (October 2019)
- GOV.UK: Approved Document C, resistance to moisture