Cavity Wall Insulation Problems: Causes and Fixes

The short answer

Most cavity wall insulation works without trouble. The problems that do occur come from a handful of causes: walls too exposed to driving rain, cavities too narrow or bridged, installations that left gaps or let the fill slump, and outer walls in poor repair. Each has a different fix, from repointing to full extraction, so identify the cause before paying for anything.

On this page

The problems, cause by cause

Problem	Usual cause	Usual fix
Damp on inside of outside walls	Exposed wall, bridged cavity, wet fill	Survey; repair outer wall; extraction if fill is wet
Cold spots and mould	Voids or slumped fill	Survey; top-up rarely works, extraction and refill
Damp under windows	Failed or blocked cavity tray	Tray repair; local extraction
Damp at the base of walls	Debris at the foot of the cavity, high ground levels	Lower ground; clear the base of the cavity
Cracking brickwork	Wall ties corroding, worse with wet fill	Remedial ties; extraction if fill is wet
Heat loss unchanged	Incomplete fill	Survey; refill the gaps

Walls that should not have been filled

Not every cavity wall suits fill. Research commissioned by the government on non-standard cavity walls estimated that tens of thousands of homes have walls in exposed locations, and hundreds of thousands have narrow cavities, timber or steel frames, or concrete construction. These need different materials or a different approach altogether. The same research defines an exposed location by reference to the BRE guide on avoiding risks with thermal insulation, and includes walls near the coast, at high elevation or facing open land.

If your wall is one of these and the fill has failed, putting the same fill back repeats the problem. See removal and refill for the alternatives.

Installations that went wrong

Common faults found during surveys:

Drilling points too far apart, leaving gaps.
Not enough material blown in.
Bead fill without binder, which settles.
Fill blown into cavities full of mortar droppings and rubble.
Services such as flues and cables not sleeved; Historic England notes that polystyrene can damage cable sheathing.
Air bricks and vents blocked.

The outer wall

Even a perfect fill cannot cope with a wall that takes on too much water. Open mortar joints, cracked render, overflowing gutters, leaking downpipes and paving built up above the damp-proof course all add water. These are cheaper to fix than extraction and should be fixed whatever the survey finds.

Damp that is not the insulation

Condensation in kitchens and bathrooms, rising damp, plumbing leaks and roof leaks can all look like cavity problems. A survey that only looks for one cause will tend to find it. Ask anyone inspecting to look at the outside of the house as well as the cavity.

Cold callers and claims companies

The GOV.UK consumer guide warns that some claims management companies contact people by phone or at the door offering compensation for cavity insulation, and that some of these cases are not genuine. Do not sign anything on the doorstep, get all charges in writing, and check any solicitor with the Solicitors Regulation Authority. You can raise a concern with your installer or guarantee provider yourself, free.

Getting it put right

Have the cavity surveyed with a borescope: see cavity wall insulation survey.
Check for a guarantee before paying: see CIGA guarantee claims.
Fix the outer wall.
If the fill has failed, have it extracted; budget with the removal cost calculator.

If you are seeing one of these problems, tell us about the house and we will find a cavity wall insulation removal company to look at it.

Where our information comes from

Cavity wall insulation problems: what goes wrong and why