Cavity wall insulation was sold to millions of UK homes, and for most of them it quietly works. But a minority of installs went wrong — wet fill, slumped fill, missed voids — and those homeowners are often told by the original industry that nothing is wrong. We see the aftermath on surveys across London and the South-East: damp and mould remediation appears on 32 of our real quotes, and a failed cavity is one of the recurring causes. This guide explains how blown fill fails, how to tell if yours has, and when the honest fix is extraction — sometimes followed by insulating the wall properly from the outside.
How blown cavity fill fails
A cavity wall is two leaves of brick with an air gap between them, and the gap has a job: rain that drives through the outer leaf runs down the inside face of it and drains away, never reaching the inner leaf your plaster sits on. Blown insulation — mineral fibre, beads or old urea-formaldehyde foam — fills that gap. When it goes wrong, it goes wrong in three main ways:
- Wet fill on exposed walls. Where an elevation faces prevailing wind-driven rain, or the outer leaf is in poor condition — tired pointing, cracked render, blown bricks — moisture penetrates the outer leaf faster than the wall can dry. Fibre fill in that cavity acts like a sponge bridging the gap, wicking water to the inner leaf. Wet fill also insulates poorly, so the wall gets damp and cold at once.
- Slump. Loosely blown fibre can settle over the years, compacting into the bottom of the cavity. The result is a wall insulated at the ground floor and bare near the top — cold bedrooms over warm living rooms is the classic pattern.
- Voids and cold spots. Fill blown from a grid of drill holes has to flow around wall ties, mortar snots and debris. Rushed installs leave unfilled pockets — under windows and above lintels are the usual suspects — and each void is a cold patch where condensation and mould concentrate first, exactly as described in our cold bridging guide.
An installer’s detail worth knowing: the give-away for a filled cavity is the drill pattern — mortar-coloured dots roughly a metre apart in the joints, usually neatly re-pointed. We find them on surveys of houses whose owners had no idea the walls were ever filled. If the previous owner never mentioned it and the paperwork is long gone, the wall still keeps the record.
The symptoms of a failed fill
- Damp patches that track the weather. Internal damp that worsens days after wind-driven rain on one elevation — typically the south-west face — points at moisture crossing the cavity.
- Patchy cold. Rooms or wall areas that never warm up, mould behind furniture on external walls, or a blotchy pattern on a thermal image: slump or voids.
- Recurring mould in the same spots. Cleaned, repainted, back by February — a cold, damp substrate underneath.
- Musty smell on external walls, plaster that feels cool and clammy, wallpaper lifting at skirting level.
None of these is proof on its own — solid walls, leaking gutters and plain condensation from poor ventilation can mimic all of them. Which is why diagnosis comes before any talk of extraction.
Getting an honest diagnosis
The first question is the one most homeowners can’t answer from the kerb: what are these walls, actually? Brick bond, wall thickness and age give strong clues — our cavity wall vs solid wall guide walks through them — but rendered walls and part-rebuilt houses blur every rule. We check wall type as part of our free survey, along with the things a cavity diagnosis needs: the drill-pattern evidence of previous fill, the condition of pointing and render on the wet elevation, gutter and downpipe defects that soak walls from outside (gutter and downpipe works appear on 80 of our quotes for a reason), and whether the damp pattern actually matches a cavity failure or something more mundane.
Be wary of anyone who diagnoses a failed cavity from your doorstep and quotes extraction the same afternoon. Equally, be wary of the reverse — the insistence that certified fill cannot fail. Both are sales positions, not surveys.
Extraction: what it involves
Where fill has genuinely failed — wet, slumped or mouldy — it has to come out before anything else is decided. Leaving saturated fibre in a cavity and insulating over the top seals a damp sponge inside your wall. Extraction is specialist work: contractors open access points around the base and face of each wall, break up the old fill with compressed-air lances and vacuum it out, then verify with a borescope that the cavity is clear and the wall ties are clean. Done properly it is methodical, noisy and surprisingly effective; the wall then needs a drying period before its true condition can be judged.
We’ll be straight: extraction itself is not our trade, and a firm that claims to do everything usually does something badly. Where extraction is needed we say so, and the work we quote is what comes after — making the wall perform once it’s dry.
When EWI over a problem cavity is the right fix
After extraction there are two paths. If the fill failed because of a poor install in a fundamentally suitable wall, refilling with a modern bonded-bead system can be a fair option. But if the wall itself caused the failure — an exposed elevation, driving rain, an outer leaf that will keep letting water in — refilling the cavity is repeating the original mistake with newer material.
External wall insulation solves the problem from the correct side. The insulation boards and render system go on the outside of the brickwork, which does three jobs at once: it stops rain reaching the masonry (the render system becomes the weather skin), it puts the whole wall on the warm side so the old damp wall can finally dry inwards and stay dry, and it insulates to a far higher standard than any cavity width allows. The cavity behind it can stay empty and ventilated — doing the drainage job it was built for.
On cost: our EWI projects have a median of £17,300 across 170 priced quotes, with real installed examples — like a 58m² semi in Worcester Park (KT4) at £13,352 — published openly in our 2026 London price report. It is a serious spend, which is exactly why it should only be recommended when the diagnosis supports it.
The honest caveat: most cavity fill is fine
It needs saying clearly, because scare stories sell surveys: the majority of correctly-installed cavity wall insulation in suitable walls performs exactly as intended, decade after decade. Sheltered elevation, sound outer leaf, competent install — that combination describes most filled homes, and those owners are quietly saving money every winter. If your filled walls show none of the symptoms above, the right course of action is nothing at all.
The problems cluster where the preconditions were ignored: exposed sites, poor brickwork, rushed installs. If that sounds like your house — or you simply don’t know what’s in your walls — get a free, fixed, itemised quote. The survey includes checking your wall type and condition, and if the honest answer is “your fill is fine, leave it alone”, that is the answer you’ll get.
Cavity wall insulation problem FAQs
How do I know if my cavity wall insulation has failed?
The classic signs: damp patches on internal walls that get worse after wind-driven rain, cold spots you can feel (or see with a thermal camera) in an otherwise insulated wall, mould returning in the same places after cleaning, and a musty smell on external walls. Failed fill usually announces itself within a few winters of installation — or after render or pointing outside deteriorates and starts letting more rain into the masonry.
Why does cavity wall insulation get wet?
The cavity exists to break the path of wind-driven rain: water that penetrates the outer brick leaf runs down inside the cavity and drains away without touching the inner leaf. Blown fill bridges that gap. In sheltered locations with sound brickwork this rarely matters, but on exposed elevations, or where pointing and render are poor, the fill can wick moisture across to the inner leaf — and wet insulation both stops insulating and feeds damp into your plaster.
What is cavity wall insulation extraction?
Specialist contractors drill out a pattern of bricks or drill holes, then use industrial vacuum and compressed-air lances to pull the old fill out of the cavity, working around the whole house until borescope checks confirm the cavity is clear. It is disruptive and only worth doing when the fill has genuinely failed — but where it has, extraction is the only honest starting point, because leaving wet fill in place means leaving the damp source in the wall.
Should I refill the cavity after extraction or fit external wall insulation?
It depends on why the fill failed. If the cause was a bad install in an otherwise suitable wall, refilling with modern bonded bead can be reasonable. If the wall itself is the problem — exposed elevation, poor outer leaf, persistent driving rain — refilling repeats the experiment. External wall insulation solves it differently: the insulation and its render skin go outside the brickwork, keeping the rain off the wall entirely, and the cavity can stay empty and ventilated as designed.
How do I find out what type of walls I have?
Rough guides: a brick pattern of all-stretchers (long faces only) usually means a cavity wall, mixed headers and stretchers usually solid; cavity walls are typically 250mm+ thick at a door reveal, solid nine-inch walls around 230mm. But extensions, rebuilt sections and rendered walls confuse every rule of thumb, so we check wall type as part of our free survey — including drill-hole evidence of previous fill the owner never knew about.