London & South-East England
Cavity Wall Insulation Extraction
Wet, slumped or bridged cavity fill taken out, the cavity checked clear, and a plan for what goes in its place.
Is cavity wall extraction right for your home?
- Homes with damp patches on inside walls that appear or get worse after rain
- Cavity walls filled with mineral wool or foam that a borescope shows is wet or slumped
- Exposed walls where the fill has bridged the cavity and should not have gone in
- Owners who want external wall insulation but need the cavity cleared and dried first
- Buyers and sellers where a survey has flagged failed cavity insulation
Cavity Wall Insulation Extraction: key facts
- Cavity wall extraction removes wet, slumped or bridged insulation from the cavity by taking out bricks at the base of the wall and vacuuming or blowing the material out.
- Signs of failed fill: damp patches on inside walls after rain, cold spots, black mould in corners, and wet fibre or crumbling foam seen through a borescope.
- Typical cost £20 to £40 per m² of wall; a 3-bed semi £1,500 to £4,000, plus a refill or external wall insulation if chosen.
- After extraction the cavity can be left empty, refilled with bead once it is dry, or the wall insulated externally, which also solves the exposure problem.
- Extraction is charged at 20% VAT; the refill or external wall insulation afterwards is 0%.
- Arranged and project-managed by Stay Warm, carried out by a PAS 2030 certified, TrustMark registered accredited installer.
What is cavity wall extraction?
Cavity wall insulation extraction takes failed insulation out of the wall. The installer removes bricks at the base of each wall and at intervals higher up, breaks the material loose with air lances and vacuums or blows it out through the openings, then checks the empty cavity with a borescope and reinstates the bricks. Mineral wool, blown fibre, urea formaldehyde foam and bonded bead can all be removed, though old foam takes longer.
A lot of cavities across London, Surrey, Kent and Hertfordshire were filled between the 1980s and the 2010s, and some of those installs are now failing. Walls that face driving rain, cavities that were too narrow or full of mortar droppings, and fills that were injected too thinly all let water track across to the inner leaf. The result is damp patches that appear after rain, cold rooms, mould and an inner wall that never dries. Painting over it or running a dehumidifier does not fix the cause.
Stay Warm surveys the house first, with a borescope in several places, to confirm that the fill is the problem and not a gutter, a pointing fault or condensation. If it is, we arrange and project-manage the extraction through an accredited, PAS 2030 certified installer. Once the cavity is clear and dry, we advise on what comes next: leave it empty, refill with bead, or have our own team fit external wall insulation, which keeps off the rain that caused the failure in the first place.
Who does the work
Cavity wall extraction is arranged and project-managed by Stay Warm and carried out by an accredited installer who holds PAS 2030 for cavity wall insulation and is TrustMark registered.
Cavity Wall Insulation Extraction cost guide
Indicative 2026 market ranges for London and the South-East. Your fixed quote follows a free borescope survey and depends on wall area, fill type and access.
| Job type | Indicative cost |
|---|---|
| Extraction per m² of wall | £20 – £40 / m² |
| 3-bed semi-detached | £1,500 – £4,000 |
| Making good the brickwork | Included |
| Refill with bonded bead once dry | £18 – £30 / m² |
| External wall insulation instead | Quoted separately |
VAT: 20%. Removing failed insulation is not an energy-saving installation, so extraction is charged at the standard 20% VAT rate. A refill with bead or external wall insulation afterwards is 0% VAT until 31 March 2027 and is quoted separately.
What cavity wall extraction includes
How cavity wall extraction works
Free survey
We look at where the damp shows inside, check gutters, pointing and the damp proof course outside, and put a borescope into the cavity to see the state of the fill.
Fixed quote
A written quote for the extraction and making good, with our advice on what to do with the cavity afterwards and a separate price for a refill or external wall insulation.
Extraction
The accredited installer removes bricks, clears the cavity wall by wall, checks it with a borescope and reinstates the brickwork. One to three days for a semi.
Drying and next step
The wall is left to dry, usually a few weeks. Then the cavity is refilled with bead, the wall is insulated externally by our own team, or it is left empty if that is the right answer.
Free survey & fixed quote
Book a free, no-obligation survey and we will send a clear, fixed written quote. One point of contact from survey to sign-off, and a written guarantee on the work.
Get a free quoteCavity Wall Insulation Extraction FAQs
How do I know my cavity wall insulation has failed?
Why does cavity wall insulation fail?
How much does extraction cost?
What happens to the cavity afterwards?
Can I claim on a CIGA guarantee?
Who does the work?
Areas we cover
We arrange cavity wall extraction across Greater London, Surrey, Kent and Hertfordshire. See all areas we cover →
Related guides
In-depth, independent guidance to help you decide.
Last reviewed September 2026 by Stay Warm Insulation.
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