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10 Signs of External Wall Insulation Failure

External wall insulation (EWI) problems are often subtle at first. Knowing the early warning signs helps you get them checked before water does more damage. If your EWI was fitted under ECO4 or the Great British Insulation Scheme, TrustMark offers a free audit and you should not be asked to pay for repairs.

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Quick EWI Failure Checklist

Check your property for these signs. Any one of them is worth getting inspected:

Render cracks (especially long straight lines)
Bulging or rippling render
Heavy green or black algae growth
Damp patches appearing after rain
Interior mould (especially corners)
Render pieces falling off
Water staining around windows
Hollow sounds when tapped
Discolouration or streaking
A fairly new system already showing any of these

Detailed Warning Signs Explained

This is general guidance on what these signs can mean. Only an inspection can confirm the cause in your home.

1

Render Cracking

Cracks in the render are one of the most common early signs of a problem. The pattern and location can point to the cause.

What different cracks can indicate:

  • Straight lines in a grid: May follow the joints between insulation boards, which can point to how the boards were laid.
  • Diagonal cracks from window corners: Often stress cracks at openings where extra mesh reinforcement is normally needed.
  • Fine map-like cracking: Can be shrinkage in the render layer. Worth checking before water gets in.
  • Cracks along roof lines or ground level: Can indicate movement or poor detailing at junctions.

Why it matters: Even fine cracks can let rain in behind the render. Report cracking in a relatively new system to the installer in writing.

2

Bulging or Delamination

The render or boards separating from the layer beneath. This can sound hollow when tapped, and rain getting behind it can make things worse.

Possible causes:

  • • Poor bonding between base coat and insulation
  • • Adhesive not suited to the wall surface
  • • Installation in wet or cold conditions
  • • Too few mechanical fixings for the system
  • • Water getting in behind the render

Simple check: Gently tap the render with your knuckles. A hollow sound in patches can suggest delamination. Do not press hard or pull at loose render.

3

Biological Growth (Algae, Moss, Mould)

Green or black growth on the render surface. Some growth is normal on shaded, north-facing walls; heavy or spreading growth suggests the surface is staying wet.

Why it grows:

  • • Render surfaces on insulation stay colder, so dew forms more easily
  • • Poor drainage, so water sits on the surface instead of running off
  • • Shaded or north-facing walls that rarely dry out
  • • Nearby trees, gutters or overflows keeping the wall wet

Significance: Growth itself is not structural damage, but it shows where the render stays wet, which is worth mentioning at any inspection.

4

Water Penetration After Rain

Damp patches appearing on internal walls within hours or days of heavy rain. This points to penetrating damp: water is getting through or around the EWI system.

Common entry points:

  • • Window and door reveals (missing drip details or failed sealant)
  • • Cracks in render
  • • Poor flashing at roof junctions
  • • Behind wall-mounted fixtures (satellite dishes, lights)
  • • The bottom edge of the EWI, where it meets the ground or damp-proof course

Why it matters: Water that gets behind the insulation can stay in the wall and cause hidden damage.

5

Interior Mould Growth

Black mould appearing on internal walls, especially in room corners, behind furniture, or around window reveals.

How EWI can contribute:

  • • Cold spots at poorly insulated junctions such as window reveals
  • • Penetrating damp from failed render or seals
  • • Ventilation not improved when the home was made more airtight
  • • Moisture held within the wall

Health: Government guidance says damp and mould can cause or worsen respiratory problems such as cough, wheeze and asthma, and that children, older people and people with lung conditions or weakened immune systems are at greater risk. Read the guidance. If you are worried about symptoms, speak to your GP or call NHS 111.

6

Render Falling Off

Sections of render detaching and falling away from the wall. This is an advanced problem and usually means a larger area needs investigating.

Possible causes:

  • • Freeze and thaw: water in cracks expands when it freezes
  • • Adhesive or fixing failure
  • • Wind pulling at loose sections
  • • Reinforcing mesh not properly embedded
  • • Movement or decay in the wall behind

Safety: Falling render can injure people. Keep people away from the area below and tell the installer, or TrustMark if the work was under ECO4 or GBIS, straight away. If there is an immediate risk to people, call 999.

7

Staining and Discolouration

Water streaks, rust stains, or dark patches on the render. These can show water running down the face (poor drainage details) or corroding fixings.

Types of staining:

  • Dark streaks from gutters or windows: Poor drip details
  • Rust stains: Fixings or beads corroding
  • White salt deposits (efflorescence): Moisture moving salts to the surface
  • Dark patches in a pattern: Can show board joints or fixings getting wet
8

Poor Detailing at Junctions

Visible gaps, cracks, or poor finishes where EWI meets windows, doors, rooflines, or ground level. These areas are the most vulnerable to water getting in.

Junctions to look at:

  • • Window sills and reveals: look for drip details and intact sealant
  • • Roof and wall junction: the roof overhang or flashing should shed water clear of the EWI
  • • Ground level: the system should stop above the damp-proof course with a proper edge detail
  • • Corners: cracks here can mean reinforcement is missing
9

Cracks Where the Building Moves

Buildings move slightly with temperature and moisture. Depending on the system and the building, the design may need movement joints, for example where the original wall already has them or where different materials meet.

If cracks line up with where the building is likely to move: ask a surveyor, or the Find & Fix auditor, to check whether the design allowed for it.

10

Problems in a Fairly New System

External wall insulation fitted under ECO4 and GBIS should carry a 25-year guarantee. A system that is already cracking, leaking or detaching after a few years is worth reporting now rather than waiting.

Government scheme installs:

The National Audit Office (October 2025) estimated that 98% of homes with external wall insulation under ECO4 and GBIS have major issues requiring remediation, and 6% have health and safety risks needing immediate correction. The figures come from audits carried out between June and August 2025.

What to Do If You Spot These Signs

Step 1: ECO4 or GBIS? Use the Free Route

If your external wall insulation was installed under ECO4 or GBIS, TrustMark's Find & Fix audit is free, and you should not be asked to pay for repairs. Contact TrustMark on 0333 555 1234 (option 1) or onsitebookings@trustmark.org.uk. For internal wall insulation, contact Ofgem's ECO contact centre on 0808 169 4447.

How the free audit works

Step 2: Document Everything

Take dated photos of all defects. Note when symptoms appeared. Keep installation paperwork, the guarantee certificate, your Ofgem letter if you have one, and all correspondence. You will need these for the installer, guarantee provider or any complaint.

Step 3: Contact the Installer in Writing

Describe the defects and ask for an inspection and repair. If the installer has stopped trading or will not fix it, the 25-year guarantee should cover repairs, up to £20,000, or up to £25,000 for Installation Assurance Authority guarantees since June 2026. TrustMark's complaints process runs from the business, to its Scheme Provider, to the Dispute Resolution Ombudsman.

The redress routes that exist

Step 4: Independent Survey If No Free Route Applies

Do not try to open up or repair EWI yourself. An independent surveyor, separate from the installer and from anyone who could be paid to do the repair, can assess the cause and advise whether repair or removal is needed. Independent surveys typically cost several hundred pounds depending on the property and scope, so get a fixed quote first. You can check a surveyor on RICS Find a Surveyor.

What an EWI survey checks

Sources

Last reviewed September 2026.

Spotted EWI Failure Signs?

ECO4 or GBIS external wall insulation: start with the free Find & Fix audit. For anything else, 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 The free audit route