Insulation fitted under ECO4 or GBIS? Check your repair options

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External Wall Insulation •Reviewed September 2026

How to Spot External Wall Insulation Damp Early: 10 Warning Signs

Recognise these signs early and get them checked before water does more damage.

Do not leave it
Once water gets behind external wall insulation it can stay in the wall and cause hidden damage, so it is worth reporting problems promptly rather than waiting to see if they get worse.

Why External Wall Insulation Fails

External wall insulation (EWI) is fixed to the outside of a wall and finished with render or cladding, which is meant to keep the weather out. When the system or its details are badly designed or installed, rain can get in behind it and moisture can be held against the wall.

The National Audit Office (October 2025) estimated that 98% of homes with external wall insulation fitted under ECO4 and the Great British Insulation Scheme have major issues requiring remediation, between 22,000 and 23,000 homes. Recognising the warning signs helps you get the right inspection sooner.

Fitted under ECO4 or GBIS? There is a free route.

TrustMark offers a free Find & Fix audit for external wall insulation installed under these schemes. If faults are found, TrustMark arranges for them to be fixed and you should not be asked to pay. Call 0333 555 1234 (option 1) or email onsitebookings@trustmark.org.uk. For internal wall insulation, contact Ofgem's ECO contact centre on 0808 169 4447.

The 10 Warning Signs

1. Dark Staining or Streaking on External Render

What to look for:
  • Vertical dark streaks below windows or roof edges
  • Discolouration around joints or seams
  • Heavy green or black algae growth
What it can mean:

Water is running down the surface instead of being shed clear. This can point to missing drip details, poor detailing or the render breaking down.

Urgency:

Medium: mention it at an inspection

2. Cracks in Render or Basecoat

What to look for:
  • Hairline cracks spreading in patterns
  • Cracks that are wide or getting wider
  • Cracks at corners, around windows, or at wall edges
  • Map-like cracking patterns
What it can mean:

Movement of insulation boards, missing reinforcement at openings, or poor adhesion. Cracks can let water in behind the render.

Urgency:

High: report it to the installer in writing

3. Bubbling or Delamination

What to look for:
  • Soft areas when you gently press the render
  • Visible bulging or blistering
  • Hollow sound when tapped gently
  • Render coming away from insulation boards
What it can mean:

The render may be separating from the insulation boards, often because water has got behind it. Freezing weather can make this worse.

Urgency:

Urgent: report it now, and keep people clear if render could fall

4. Internal Damp Patches

What to look for:
  • Dark patches on inside walls, especially in corners
  • Peeling wallpaper or paint
  • Tide marks or salt deposits (white crystalline patches)
  • Walls that feel cold and clammy to touch
What it can mean:

Moisture may be getting through the EWI system to the inside of the wall, for example through failed seals around windows or gaps in the weatherproofing. Condensation can look similar, so an inspection is needed to tell them apart.

Urgency:

Urgent: report it and arrange an inspection

5. Mould Growth (Internal)

What to look for:
  • Black, green, or brown spots or patches
  • Musty smell, especially in corners or behind furniture
  • Mould appearing after EWI installation (previously not present)
  • Condensation on windows increasing since installation
What it can mean:

Cold spots at junctions, reduced ventilation, or moisture trapped within the wall. Government guidance says damp and mould can cause or worsen respiratory problems such as asthma, especially for children, older people and people with lung conditions. Read the guidance.

Urgency:

High: possible health risk, report it promptly

6. Blocked or Covered Airbricks

What to look for:
  • Airbricks (ventilation grilles) covered by insulation
  • Render extending below the damp proof course (DPC)
  • No visible ventilation grilles at floor level where there used to be some
What it can mean:

Under-floor ventilation may be blocked, or the insulation may be bridging the damp proof course. Both can cause damp in floors and walls and, over time, timber decay. These are installation defects to report.

Urgency:

High: report it to the installer or auditor

7. Water Drips or Leaks After Rain

What to look for:
  • Water dripping from render edges or soffits
  • Damp spots appearing during or after rain
  • Water pooling at the base of walls
  • Internal leaks around window reveals
What it can mean:

Water is getting into or around the EWI system, through failed seals, missing drip details or gaps in the render. It may be running behind the insulation.

Urgency:

Urgent: report it and arrange an inspection

8. Increased Condensation on Windows

What to look for:
  • Heavier condensation since EWI installation
  • Water pooling on window sills
  • Mould around window frames and reveals
What it can mean:

Changes to how the home is ventilated, or cold spots at window reveals where the insulation does not meet the frame properly.

Urgency:

Medium: improve ventilation and mention it at an inspection

9. Sagging or Movement of Boards

What to look for:
  • Visible "steps" or unevenness in the wall surface
  • Boards appearing to slump or sag
  • Gaps opening between boards
What it can mean:

The insulation boards may not be properly fixed to the wall, for example too little adhesive or too few mechanical fixings for the system. This is a serious defect.

Urgency:

Urgent: possible safety risk, keep people clear and report it now

10. Musty Smells

What to look for:
  • Damp, musty smell that was not there before EWI
  • Smell worse in certain rooms or corners
  • Smell stronger after rain or in humid weather
What it can mean:

Hidden moisture or mould, for example behind furniture, under floors or within the wall, before visible signs appear.

Urgency:

High: arrange an inspection

What to Do If You Spot Warning Signs

Immediate Actions

  1. ECO4 or GBIS install: Contact TrustMark about the free Find & Fix audit (external wall insulation) or Ofgem's ECO contact centre (internal wall insulation).
  2. Document everything: Take dated photos of all defects. Include close-ups and wide shots showing location.
  3. Note when it started: Record when you first noticed issues and whether they are getting worse.
  4. Check your paperwork: Find the guarantee certificate, installer details and any Ofgem letter with your Unique Measure Reference Number.
  5. Contact your installer: Report issues in writing and keep copies of all correspondence.

Independent Assessment If No Free Route Applies

For private work, or if you want your own evidence, an independent inspection can confirm the cause. Typical published costs, not quotes:

  • Damp survey: typically £150 to £300 (HomeOwners Alliance, August 2026)
  • RICS Home Survey Level 3: typically £630 to £1,500 depending on property value (HomeOwners Alliance, August 2026)
  • Specialist insulation inspection: no reliable published figure; expect several hundred pounds depending on the property and scope, and get a fixed quote first

Where to Raise It

  • TrustMark: free Find & Fix audit for ECO4/GBIS external wall insulation, and a complaints process if the installer was TrustMark-registered (business, then its Scheme Provider, then the Dispute Resolution Ombudsman)
  • Ofgem's ECO contact centre: concerns about ECO4/GBIS internal wall insulation, on 0808 169 4447. Ofgem does not handle individual complaints about installers
  • Guarantee provider: if the installer has stopped trading or will not fix the work
  • Citizens Advice consumer helpline: 0808 223 1133, which can pass complaints about mis-selling or fraud to Trading Standards
  • Your council: environmental health can inspect if you rent your home

Prevention: What Good EWI Looks Like

Signs of a Careful Installation

  • Boards fixed as the system manufacturer specifies, often adhesive plus mechanical fixings
  • Continuous insulation with staggered board joints
  • The system stops above the damp proof course rather than bridging it
  • Proper sealing around all window and door reveals
  • Drip details at roof line, sills and above openings
  • Ventilation kept open (airbricks extended, not covered)
  • Render applied in suitable weather conditions
  • A 25-year guarantee certificate for scheme-funded work

Regular Maintenance

Even well-installed EWI benefits from simple checks:

  • A visual check of walls, sills and junctions once a year
  • Keep gutters and downpipes clear so water does not run down the render
  • Check sealant around windows and doors
  • Clean algae following the render manufacturer's advice
  • Report any cracks to the installer promptly

Repair Costs and Who Pays

If your external wall insulation was fitted under ECO4 or GBIS, you should not be asked to pay for repairs: the installer is liable, then the 25-year guarantee (up to £20,000, or £25,000 for Installation Assurance Authority guarantees since June 2026). The National Audit Office estimated that repairing scheme EWI normally costs £5,000 to £18,000 per property if done before major damage. There is no government compensation scheme, and the guarantees do not cover ill health or damaged belongings. For private work, costs depend on what an inspection finds; see our EWI removal cost guide.

Spotted Warning Signs?

ECO4 or GBIS external wall insulation: start with the free Find & Fix audit. If no free route covers your home, 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.

Ask for a specialist

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Sources

Last reviewed September 2026.