TrustMark, the government-endorsed quality scheme for energy efficiency retrofits, notified the Department for Energy Security and Net Zero (DESNZ) in October 2024 of high levels of external wall insulation installations that were non-compliant with the relevant quality standard1. The following month it highlighted similar issues with internal wall insulation1. TrustMark is a private not-for-profit company that acts as a government-endorsed quality scheme for energy efficiency retrofits1.
The Energy Company Obligation (ECO) obligates energy suppliers to fund the installation of energy efficiency measures such as insulation in homes, and is intended to tackle fuel poverty and reduce carbon emissions in Great Britain1. ECO4 runs from April 2022 to March 2026, and the Great British Insulation Scheme (GBIS), with broader eligibility, runs from March 2023 to March 20261. Ofgem reports that 243,900 homes have been upgraded under ECO4 to the end of March 2025 and 60,600 under GBIS, with 28,000 installations of external wall insulation (3% of all measures installed) and 45,200 of internal wall insulation (5%) across both schemes combined1.
Ofgem commissioned two random samples of audits on behalf of DESNZ in April 2025 to establish the rate of non-compliance2. Audits were carried out by chartered surveyors against PAS2035 standards, covering 378 properties with external wall insulation and 380 with internal wall insulation, drawn from a population of 24,600 external and 36,100 internal installations2.
| Measure | Category 1 | Major | Minor | Pass |
|---|---|---|---|---|
| External wall insulation | 6% | 92% | 1% | 1% |
| Internal wall insulation | 2% | 27% | 35% | 35% |
Category 1 non-compliance poses a risk to the health and safety of occupants; Major non-compliance will affect system performance and requires remediation to avoid system failure2. The combined rate of Category 1 and Major non-compliance was 98% for external wall insulation, with a 95% confidence interval of (96%, 99%), and 29% for internal wall insulation, with a 95% confidence interval of (25%, 34%)2. The report concluded that both samples "show a high rate of major and Category 1 non‑compliance which exceed any reasonable tolerance level"2.
"in October 2024, TrustMark notified DESNZ of high levels of external wall insulation installations that were non-compliant with the relevant quality standard"
The National Audit Office notes that non-compliance covers a wide range of severity, from major issues that pose immediate risks to the health and safety of the household to minor issues such as missing paperwork1. It also records that in April 2024 TrustMark notified DESNZ of suspected fraud, whereby some retrofit businesses were overclaiming for work undertaken1. The NAO has not inspected the homes affected or gathered evidence directly from affected households or retrofit businesses, and its report does not evaluate the overall value for money of ECO1.
Why it matters for households
Solid wall insulation is fitted to homes that are hard to heat, and its performance depends on the installation being done to standard. Where a major non-compliance is found, the measure will not deliver the expected reduction in heat loss, and the audit report states that defects often create the risk of water ingress, condensation and mould3. Category 1 findings are those where the work poses a risk to the health and safety of occupants, such as inadequate ventilation3. For a household, that affects both the comfort and running costs the insulation was meant to improve, and the condition of the building fabric itself.
The schemes are funded through energy suppliers rather than directly by households, so the cost of putting defects right falls to installers and guarantee providers rather than to the resident in the first instance3. The Department expects all external wall insulation installed under the schemes to have been audited through its find-and-fix programme within 15 months of November 2025, and all recipients of external wall insulation under ECO4 and GBIS are now being offered an audit2.
What happens next
DESNZ is continuing to audit properties under both schemes2. TrustMark and the certification bodies suspended 38 installer businesses, representing 81% of the external wall insulation market, by the end of January 2025, and as of November 2025, 22 installers had been fully reinstated after meeting requirements on TrustMark's "robust six-point plan", including full remediation of the problems identified3. The government has announced that it will end ECO, with no levies on bills from April 2026, while continuing to invest in tackling fuel poverty through its Warm Homes Plan3. It has not been reported whether the period for suppliers to meet existing ECO4 obligations beyond March 2026 will be extended3.
Householders can check how the TrustMark scheme operates, what to look for when finding and checking an insulation or glazing installer, and how solid wall insulation relates to a home's energy independence.