Government-commissioned audits of external wall insulation fitted under ECO4 and the Great British Insulation Scheme found major technical non-compliance in the large majority of homes checked — and the same programme that found it now pays to put it right. Every affected household is offered a free on-site audit, the original installer carries the repair cost, and the owner's practical job is to accept the check and recognise a sound repair. Renders World supplies the compliant external wall insulation components that remediation contractors specify, and this guide sets out the published figures, the classification a surveyor will use, and the route from an Ofgem letter to a finished wall.
What ECO4 and GBIS Required — and What the Audits Actually Found
An ECO4 and GBIS insulation quality check starts from one published figure: 92% of externally insulated homes audited against PAS 2035 showed at least one major technical non-compliance, which entitles the household to funded repair. The audits were carried out by chartered surveyors on two random samples drawn from properties that had not previously been audited.
The often-quoted 98% is the sum of two separate findings, and the composition matters more than the headline. Reading them apart tells a homeowner which clock applies to their own property.
| Classification | EWI audits | What it means | Remediation clock |
|---|---|---|---|
| Pass | 1% | No non-compliance recorded against the installation | None |
| Minor non-compliance | 1% | May potentially affect system performance over time | Evidence of correction within 8 weeks |
| Major non-compliance | 92% | Will affect system performance; remediation required to avoid system failure | Evidence of correction within 8 weeks |
| Major non-compliance (Category 1) | 6% | Risk to the property or to occupant health and safety identified | Made safe within 24 hours, then corrected within 8 weeks |
Combined, Category 1 and Major reach 98% of the EWI sample, with a 95% confidence interval of 96% to 99%. The sample was 378 EWI properties drawn from a population of 24,600 installations, which is how the National Audit Office arrives at an estimated 22,000 to 23,000 homes needing corrective work. Internally insulated homes fared differently — 27% major and 2% Category 1, a combined 29% — so an IWI property sits in a materially better position than an EWI one.
One detail rewards a second look, because it changes what a cautious reader should assume. The published table gives Category 1 as 6%, alongside a bracketed 10% that excludes remediation of Category 1 findings where works were not required at site. The stricter reading is the bracketed one, so a household planning around the worst case should work from 10% rather than 6%, and both figures come from the same table.
The trigger for all of this was TrustMark notifying the Department for Energy Security and Net Zero in October 2024 that non-compliance rates were abnormally high. Ofgem commissioned the two audit samples in April 2025, and 39 businesses were suspended from installing new solid wall insulation. Read the full statistical audit results for solid wall insulation for the tables in full.
How to Check Whether Your Installation Qualifies for the Programme
Eligibility is settled by paperwork rather than by inspection, which is the reassuring part: an owner does not need to diagnose a defect to access the remediation route. The government's position is that installers fund the repair work themselves, and no household should be asked to pay.
- Look for the Ofgem letter. Ofgem has written to households whose records show external wall insulation fitted under ECO4 or GBIS, and that letter carries a Unique Measure Reference Number specific to the property.
- Check the original paperwork for references to ECO4, the Energy Company Obligation or the Great British Insulation Scheme. Work supplied free or heavily subsidised through a council or energy supplier referral almost always sits in one of the two.
- Verify anyone who makes contact. A legitimate caller can quote the Unique Measure Reference Number from the Ofgem letter. Where a caller cannot, sharing no personal details and ringing TrustMark on 0333 555 1234 settles it safely.
- Contact Ofgem directly if no letter has arrived. The ECO helpline is 0808 169 4447, and arranging an audit through it is what opens the repair route.
A walk round the elevations is still worth thirty minutes, because it tells the surveyor where to look first. Tap the render across each wall and listen for a hollow tone against the solid one, sight along the elevation for bulging, look inside window and door reveals for insulation carried round the return, and check the first course above the damp-proof course for staining or growth. New damp patches or condensation appearing indoors after the work usually point at ventilation, which is one of the defect types government has named — the others being missing or incomplete paperwork, and missing or exposed insulation.
Your Remediation Pathway, Step by Step
The pathway is deliberately short, and the cost sits with the installer at every stage. The National Audit Office records the original installer as liable for the whole cost of repairing a faulty installation, so the sequence below is about access rather than negotiation.
- Accept the audit offer. Following the ministerial statement of 13 October 2025, comprehensive on-site audits are offered to every household with EWI installed under ECO4 or GBIS, at no cost to the consumer.
- Wait for TrustMark to make contact. TrustMark leads the checks and contacts households by email, phone or post; the volume means it may take several months to reach any given property.
- Document what you have seen. Dated photographs of each elevation, the original installation paperwork and a note of any internal symptoms give the surveyor a head start.
- Let the qualified professional carry out the check. The audit assesses the work against technical standards and safety, and the household is told the result. Appearance is not necessarily within its scope.
- TrustMark arranges the fix. Where problems are found, TrustMark arranges with the installer or retrofit coordinator for them to be corrected, and confirms to the household when the findings are closed out.
- Raise anything urgent immediately. A Category 1 finding must be mitigated and made safe within 24 hours, so an urgent concern goes straight to TrustMark rather than waiting for the queue.
Accepting the check is encouraged even where nothing looks wrong, precisely because several of the recorded defect types are not visible from the ground. TrustMark's free audit offer for affected households sets out what to expect on the day.
What Remediation Means for Materials and Specification
Most corrective work reuses the material families already on the wall, which keeps the finish continuous and the programme short. Renders World supplies remediation contractors from the same shelves as new build, and three points are worth knowing before a repair visit.
- Mechanical fixing is the routine intervention where boards rely too heavily on adhesive. Plug length follows the surveyed board thickness and the substrate behind it, and the insulation fixing accessories range carries the plugs, base tracks, discs and cutters as one shelf.
- Fixing density is a design output, not a rule of thumb. The count comes from the wind-load calculation for the building's height, location and exposure zone, with corners and parapets treated separately — so a remediation specification that quotes a single national number is worth questioning.
- Ventilation is a documented defect category in its own right, and correcting it is usually a trickle vent or extract upgrade rather than anything touching the facade.
A point on scope that a homeowner can hold a contractor to: a reaction-to-fire class declared for an insulation board or slab describes that product on its own declaration of performance and does not describe the finished rendered wall. The wall is assessed as a system, typically under current Approved Document B guidance, by the project's fire engineer. Any remediation quotation that carries a board's class onto the completed facade has confused two different documents.
Where This Sits Against Grants, PAS 2035 and the Warm Homes Plan
The audit programme is one strand of a wider reform, and the neighbouring questions are owned elsewhere on this site. Government has said the consumer protection system will be overhauled through the Warm Homes Plan, having concluded that the existing arrangements no longer command confidence; the announcement of action to protect households with poor-quality insulation records both the suspensions and that commitment.
For the funding side rather than the quality side, the guide to grant funding for external wall insulation covers which schemes reach which households, the Warm Homes Plan grant position tracks the scheme replacing GBIS, and the PAS 2035 retrofit coordinator guide explains the role that signs a compliant project off. The National Audit Office recorded ECO4 as running from April 2022 to March 2026 and GBIS from March 2023 to March 2026, with a consultation on extending ECO4 by six to nine months; this page was reviewed in September 2026 and the audit and repair programme continues beyond the schemes' delivery windows.
Key Takeaway: Government audits found major technical non-compliance in 92% of ECO4 and GBIS external wall insulation checked, with a further 6% classed Category 1 — and every affected household is offered a free on-site audit led by TrustMark, with the original installer liable for the full repair cost. Accepting the audit is the single action that opens the route.
Next Step — Accept the Audit, Then Specify the Repair Properly
Three moves close this out. Find the Ofgem letter and the Unique Measure Reference Number on it, or ring the ECO helpline if none has arrived. Accept the TrustMark audit when it is offered, even where the wall looks sound. Then, when a remediation specification lands, check that every performance figure in it is scoped to the product it was declared against. Where a repair calls for fresh materials, the Renders World EWI component range covers the core, adhesive, fixings, mesh, primer and finish on one account, with the manufacturer's own declaration behind each item.
Written by Mariusz Saja. Technically reviewed by Rafał Wyrzykowski. Last reviewed Sep 2026.
FAQ — ECO4 and GBIS Audits, Costs and Timescales
How do I know a call about an audit is genuine?
A legitimate contact can quote the Unique Measure Reference Number printed on the Ofgem letter for your property, and only legitimate parties hold that number. Where a caller cannot provide it, share no personal details and let nobody into the home; ringing TrustMark on 0333 555 1234 confirms whether the approach is real.
Does the audit or the repair cost me anything?
Neither. The ministerial statement of 13 October 2025 offers on-site audits to every ECO4 and GBIS external wall insulation household at no cost to the consumer, and government has demanded that installers fund repair work themselves under the schemes' protections. The National Audit Office records the original installer as liable for the whole cost of repair.
How long does an installer have to correct a finding?
The standardised timeline gives the installer eight weeks to provide evidence of correction for minor and major findings, after which the certification body informs them of the implications by twelve weeks from the audit. A Category 1 finding is different: the risk to health or property must be mitigated and made safe within 24 hours, with full correction evidenced on the same eight-week clock.
What separates a minor finding from a major one?
A minor non-compliance may potentially affect system performance over time, while a major non-compliance will affect performance and needs remediation to avoid system failure. Category 1 sits above both and describes a risk to the property or to occupant health and safety. The surveyor's report uses these three terms, so recognising them shortens the conversation considerably.
My insulation looks fine — is the audit still worth accepting?
Yes, and government strongly encourages it. Several recorded defect types — incomplete paperwork, insufficient ventilation, insulation missing behind a sound-looking finish — are not visible to an owner from the ground. Only 1% of the EWI sample passed outright, so a clean visual inspection is not the same as a clean audit, and having the check on record is useful when selling or refinancing.