External Wall Insulation Problems: Damp, Mould and What to Do
External wall insulation (EWI) is meant to make solid-walled homes warmer and cheaper to heat. Where it is badly designed or installed, it can let water in, cause damp and mould, and damage the wall behind it. If yours was fitted under ECO4 or the Great British Insulation Scheme, there is a free route to get it checked and fixed.
Check the free route first
If your external wall insulation was installed under ECO4 or the Great British Insulation Scheme (GBIS), TrustMark runs the government's free Find & Fix audit. A qualified professional checks the work and, if problems are found, TrustMark arranges for the installer or retrofit coordinator to fix them. You should not be asked to pay. TrustMark says it may take several months to contact every household, and the audit may not cover purely cosmetic issues.
Contact TrustMark on 0333 555 1234 (option 1) or onsitebookings@trustmark.org.uk. Anyone contacting you about the audit should be able to quote the Unique Measure Reference Number from your Ofgem letter. We are not part of TrustMark or the scheme.
What is External Wall Insulation (EWI)?
External wall insulation involves fixing insulation boards to the outside of a property, then covering them with render or cladding. It was funded under government schemes such as ECO4 and GBIS as a way to improve energy efficiency in solid-walled homes.
Why Can EWI Cause Damp Problems?
The National Audit Office found that 92% of homes with scheme-funded external wall insulation had major issues that will affect the insulation's performance, often creating the risk of water ingress and mould. The common ways EWI goes wrong are set out below. This is general guidance; only an inspection can say what is happening in your home.
1. Moisture Getting Trapped
Older solid walls often rely on moisture being able to dry out through the masonry. If the system, render or detailing is not suited to the wall, or water gets in behind the insulation, moisture can be held in the wall, leading to:
- Damp showing on interior walls
- Mould growth on internal surfaces
- Deterioration of the underlying wall
2. Installation Defects
Common workmanship problems include:
- Inadequate surface preparation
- Poor sealing around windows and doors
- Render or insulation bridging the damp-proof course
- Inadequate ventilation provisions
- Cold spots (thermal bridging) at junctions
3. Unsuitable Properties
Problems are more likely where EWI was fitted without a proper assessment of:
- Older homes built with breathable materials such as lime mortar
- Properties with existing damp issues
- Exposed locations with heavy driving rain
- Homes without adequate ventilation
Common EWI Problems
Damp and Mould
- Black mould on interior walls (especially corners and behind furniture)
- Damp patches that worsen in winter
- Musty smells
- Wallpaper peeling
- Paint discolouration and bubbling
Condensation
- Excessive moisture on windows
- Water running down walls
- Wet window sills and frames
- Mould growth around windows
Damage to the Wall and Finish
- Cracked or crumbling render
- Cracks in the insulation system
- Water ingress behind the insulation
- Damage to original brickwork
- Timber rot in window frames
Health
Government guidance from MHCLG, DHSC and UKHSA says damp and mould can cause or worsen respiratory symptoms such as cough, wheeze and shortness of breath, asthma and rhinitis, respiratory infections, eye irritation and skin rashes, and can affect mental health. Children, older people, and people with lung conditions or weakened immune systems are at greater risk. If you are worried about symptoms, speak to your GP or call NHS 111. Read the government guidance on the health risks of damp and mould or our page on EWI health risks.
Warning Signs to Watch For
If you have EWI, watch for these early warning signs:
- Musty smell when entering rooms
- Black spots appearing on walls or ceilings
- More condensation since installation
- Walls feeling cold or damp to touch
- Peeling wallpaper or paint
- Visible cracks in external render
- Staining or discolouration of the external finish
Solutions and Next Steps
Immediate Actions
- ECO4 or GBIS install:
- Contact TrustMark about the free Find & Fix audit (see above)
- For internal wall insulation, contact Ofgem's ECO contact centre on 0808 169 4447 or ECOhelp@ofgem.gov.uk
- Document everything:
- Take dated photos of all problems
- Keep records of when issues started
- Save all correspondence and your guarantee certificate
- Manage moisture in the meantime:
- Open windows regularly
- Use extractor fans
- Consider a dehumidifier (a temporary measure, not a fix)
- Contact the installer:
- Report problems in writing
- Ask for an inspection
- Ask which guarantee covers the work
Independent Assessment
If no free route applies, an independent survey can assess:
- Extent of moisture in the wall
- Quality of installation
- Underlying wall condition
- Recommended remediation approach
- Likely scope of work
Remediation Options
Depending on what the inspection finds, options include:
- Targeted repairs: Fix specific defects such as seals, cracks or junction details
- Improvements: Better ventilation or changes to details that trap moisture
- Removal: Taking off part or all of the system where repair is not enough
Costs
If your EWI was fitted under ECO4 or GBIS, you should not be paying for repairs. For other work, these are typical published figures, not quotes:
- Repairing scheme EWI (NAO estimate): £5,000 to £18,000 per property if done before major damage
- Removing EWI: around £30 to £40 per m², excluding scaffolding and making good (Checkatrade, July 2026)
- Damp survey: typically £150 to £300 (HomeOwners Alliance, August 2026)
- Installing EWI: around £15,000 for a typical 3-bed semi (Energy Saving Trust, June 2026)
Get fixed written quotes. See our EWI removal cost guide for more detail. Last reviewed September 2026.
Who Pays, and What Is Not Covered
There is no government compensation scheme. For ECO4 and GBIS external and internal wall insulation, responsibility for repairs runs in this order:
- The original installer, who is liable for the cost of putting it right
- The 25-year guarantee, if the installer has stopped trading or will not fix it: up to £20,000, or up to £25,000 for Installation Assurance Authority guarantees since 17 June 2026
- The National Energy Foundation, for some homes where the installer has gone out of business and the guarantee was cancelled, fraudulent or missing (ask TrustMark or Ofgem's ECO helpline)
The guarantees do not cover ill health, damaged belongings or loss of earnings. For complaints, TrustMark's process runs from the installer, to its Scheme Provider, to the Dispute Resolution Ombudsman, which cannot deal with compensation claims or help if the business has stopped trading. For anything beyond repairs, get independent advice, for example from Citizens Advice on 0808 223 1133. Read more on the redress routes that exist.
Sources
- National Audit Office: Energy efficiency installations under the Energy Company Obligation (HC 1334, 14 October 2025)
- TrustMark: free Find & Fix audit offer
- TrustMark: complaints process
- DESNZ: Greater protections to restore families' trust in home upgrades (17 June 2026)
- Public Accounts Committee: Faulty energy efficiency installations (HC 1229, 23 January 2026)
- MHCLG, DHSC and UKHSA: Understanding and addressing the health risks of damp and mould in the home
- Checkatrade: External wall insulation cost
- HomeOwners Alliance: How much does a house survey cost?
- Energy Saving Trust: Solid wall insulation
Last reviewed September 2026.
If No Free Route Covers Your Home
ECO4 or GBIS external wall insulation? Start with the free Find & Fix audit. For private work or anything the schemes do not cover, we can introduce you to a specialist through Better Home Hub. Sending an enquiry is free; any survey or work is agreed and paid for between you and the specialist, who may pay Better Home Hub a fee. Check any specialist's registration, insurance and guarantee yourself.