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Cavity Wall Insulation Damp Problems: Causes & Solutions

The Energy Saving Trust says cavity wall insulation shouldn't cause damp if it is installed correctly in a suitable home. If you've developed damp, mould or cold patches since it went in, the insulation may be part of the problem, or something else may be. Here's how to tell, and how to get it put right, starting with the guarantee.

Check the Guarantee Route Diagnose Your Problem

Cavity Wall Insulation Guarantees: Key Figures

25 Years
Length of a CIGA or IAA guarantee
41 Days
Working days before you escalate from the installer to CIGA
£20,000
Maximum rectification work under a CIGA guarantee
£21-£22
Per m² typical removal cost if you pay yourself (Checkatrade, July 2026)

Sources are listed at the end of this page. Last reviewed September 2026.

How Cavity Wall Insulation Can Lead to Damp

1. Rain Crossing the Filled Cavity

In a cavity wall, the gap between the outer and inner leaves helps stop rain that soaks into the outer wall from reaching the inside. Once the cavity is filled, water that gets through a very wet outer wall can cross to the inner wall. This is why the Energy Saving Trust does not recommend cavity wall insulation for homes on exposed sites, at risk of flooding, or with narrow or uneven cavities.

When this is more likely:

  • • Walls exposed to driving rain
  • • Damaged brickwork or pointing letting water in
  • • Leaking or overflowing gutters and downpipes
  • • Rubble or mortar debris left in the cavity before it was filled

2. Wet Insulation Holding Moisture

If a lot of water gets into the cavity, wet insulation can hold moisture against the inner wall and dry out slowly. Water penetration was the reason for about a third of the claims CIGA handled in the second quarter of 2026.

Common ways water gets in:

  • • Porous, cracked or damaged bricks
  • • Missing or failed mortar joints
  • • Failed seals or sills around windows and doors
  • • Cracked or damaged render

3. Cold Spots and Condensation

Properly installed cavity insulation usually reduces the risk of condensation on walls. Where the fill has gaps or voids, those areas stay colder, and moisture from cooking, bathing and drying clothes can condense there and lead to mould. Condensation can also have nothing to do with the insulation.

Condensation hotspots:

  • • Behind wardrobes and beds (restricted airflow)
  • • External wall corners
  • • North-facing walls
  • • Kitchens, bathrooms and bedrooms

4. Installation Defects

An installer should survey the home and advise against cavity wall insulation where it isn't suitable. Problems often trace back to that survey or to the work itself.

Typical installation failures:

  • • No proper check of wall condition, cavity width or exposure
  • • Insulating a home that was unsuitable (exposed, cavities under 50mm, poor brickwork, flood risk)
  • • Insulating walls with existing damp before the cause was fixed
  • • Not checking the cavity was clear of rubble
  • • Blocking airbricks, underfloor grilles or wall vents
  • • No cavity barrier where the wall joins a neighbouring home

Signs Your Cavity Wall Insulation May Be Causing Damp

Visual Signs

  • Black mould on walls (especially corners)
  • Damp patches appearing after rain
  • Peeling wallpaper or paint
  • Discolouration or tide marks on walls
  • Excessive condensation on windows
  • White salt deposits (efflorescence) on walls

Physical & Environmental Signs

  • Walls feel cold or damp to touch
  • Musty, damp smell in rooms
  • Problem worse in winter or after heavy rain
  • Damp only on external walls (not internal walls)
  • Problem started after CWI installation
  • Rooms feel harder to heat

Key Diagnostic Question:

Did the damp start after the cavity wall insulation was installed? Timing is a useful clue, but not proof. Damp can also come from poor maintenance, and neither an installer nor a guarantor will pay to fix damp caused that way. If you have a guarantee, report the problem to your installer first, because the investigation is part of the guarantee. If you have no guarantee, an independent damp survey typically costs £150 - £300 (HomeOwners Alliance).

Solutions for CWI Damp Problems

1

Cavity Wall Insulation Extraction (Full Removal)

Where the insulation is confirmed as the cause, removing it restores the empty cavity so the walls can dry. If a guarantee covers a confirmed defect, extraction is arranged for you at no cost.

Process:

  • • Holes drilled in the outer wall
  • • Vacuum equipment extracts the insulation
  • • Cavity inspected with a borescope
  • • Holes filled and made good
  • • Inside walls left to dry before redecorating

Cost if you pay yourself:

  • • £21 - £22 per m²
  • • Whole house: typically £1,680 - £2,520
  • • Urea formaldehyde foam: adds about £500 - £1,000
  • • Usually completed in 1 - 3 days

Typical ranges from Checkatrade (July 2026), not quotes. Last reviewed September 2026.

2

Partial Extraction (Targeted Removal)

Removing insulation only from the worst-affected walls, usually the most exposed. It may cost less, but if damp later appears on other walls you may need further work.

Best for:

  • • Damp confined to one or two walls
  • • Other walls that are sheltered and dry

Before you agree:

Ask for a written quote showing the area to be extracted and why partial removal is enough. Altering or disturbing the insulation can invalidate a CIGA guarantee, so speak to the installer or guarantee provider first.

3

Improved Ventilation & Dehumidification

For condensation (not rain getting in). Better ventilation and lower humidity can manage symptoms without extraction.

Options:

  • • Positive input ventilation (PIV)
  • • Extractor fans in kitchens and bathrooms
  • • Dehumidifiers
  • • Keeping trickle vents and airbricks open

Limitations:

  • • Only helps with condensation, not penetrating damp
  • • Ongoing running costs (electricity)
  • • Treats symptoms, not the cause
  • • May not fully clear mould

Fixes That Don't Address the Cause

  • Chemical damp-proofing: Won't stop rain crossing a filled cavity
  • Replastering or tanking internal walls: Hides the symptom without stopping water getting in
  • Painting over mould: The mould is likely to come back if the moisture remains
  • Leaving it: Damp from water crossing the cavity is unlikely to clear on its own and can damage plaster and decoration

Getting It Put Right: Start With the Guarantee

Cavity wall insulation from a registered installer should come with a 25-year independent guarantee. Older guarantees were issued by CIGA (the Cavity Insulation Guarantee Agency); newer ones are issued by The IAA (Installation Assurance Authority), which CIGA launched in 2020. Some installers used an independent insurance-backed guarantee instead. Having cavity wall insulation does not automatically mean you have a guarantee.

CIGA or IAA Guarantee

The guarantee covers defects in materials or workmanship, transfers when a home is sold, and claims must be made within 25 years of installation. It does not pay compensation, and it does not cover damp from poor maintenance or insulation that has been altered or disturbed.

  • • Report the problem to the installer first
  • • Not resolved within 41 working days? Register a concern with CIGA (concerns@ciga.co.uk, 01525 853300) or The IAA (concerns@theiaa.co.uk, 03333 239045)
  • • If a defect is confirmed, remedial work, including extraction, is free of charge up to £20,000
  • • CIGA aims to resolve simple cases within 50 working days and complex ones, such as full extraction and drying out, within 90
  • • Disagree with the final decision? You have 12 months to go to independent arbitration run by CEDR (£100 plus VAT)

There is no online guarantee lookup. Email guarantees@ciga.co.uk with two proof-of-ownership documents, one of which must be a current utility bill.

Read Our CIGA Guarantee Guide

The Installer

If the installer is still trading, contact them first. They should revisit, check whether their survey or work is part of the problem, and fix it or remove the insulation if required. For contracts from 1 October 2015, the Consumer Rights Act 2015 requires a service to be performed with reasonable care and skill.

If Neither Helps

Use the installer's or guarantee provider's complaints procedure. If the installer was TrustMark-registered, TrustMark's complaints process may apply. The small claims limit is £10,000 in England and Wales and £5,000 in Scotland and Northern Ireland. For advice, call the Citizens Advice consumer helpline on 0808 223 1133, or get independent legal advice. There is no government compensation scheme.

See the Redress Routes That Exist

Get Help with Cavity Wall Insulation Damp

No guarantee, or want an independent view? Tell us about your home and Better Home Hub may introduce a specialist. Surveys and removal are usually paid for unless a guarantee covers them, so check the specialist's registration, insurance and guarantee, and ask for a written quote.

Tell Us About Your Home About Damp Surveys

Sources

Last reviewed September 2026.