Research and analysis

Solid wall insulation installed under ECO4 and GBIS: Statistical audit results

Published 13 October 2025

Applies to England, Scotland and Wales

This report summarises the results of the randomised samples of audits which were commissioned by Ofgem on behalf of DESNZ in order to understand the extent of non‑compliance in Solid Wall Insulation (SWI) measures installed under the fourth iteration of the Energy Company Obligation (ECO4), and the Great British Insulation Scheme (GBIS).

The results showed that a majority (92%) of External Wall Insulation (EWI) installations and a large minority (27%) of Internal Wall Insulation (IWI) installations under these schemes were found to have at least one major technical non‑compliance, which will affect the performance of the system. In addition, a small percentage of installations (6% of EWI and 3% of IWI installations) were found to have health and safety risks to the occupants.

DESNZ is continuing to audit properties under both schemes, with all recipients of EWI under ECO4 and GBIS now being offered an audit in light of the high rates of non‑compliance found.

Background

In October 2024 Trustmark notified DESNZ that an abnormally high rate of non‑compliance had been detected within ECO4 and GBIS SWI installations, which consist of either EWI or IWI. In April 2025, Ofgem commissioned, on behalf of DESNZ, 2 random samples of audits of EWI and IWI installations, respectively, across the household populations of both schemes in order to understand the rate of non‑compliance.

The population of un-audited properties which have received solid wall insulation under ECO4 or GBIS was eligible to be sampled. One sample was drawn for external and hybrid insulation for solid walls (the EWI sample), and a second sample was drawn for internal insulation for solid walls (the IWI sample).

The audits were carried out by chartered surveyors against PAS2035 standards.

Results

Table 1 shows the breakdown of results for each type of measure investigated. Any measure can have multiple non‑compliances associated with it; these results reflect the worst category of non‑compliance found in relation to an SWI installation in the property.

‘Major’ non‑compliances are a type of non‑compliance which will undermine the performance of the system, and may over time result in a risk to the health of the occupants, while ‘Minor’ non‑compliances are issues that could affect the system performance over time. ‘Category 1’ non‑compliances are where the non‑compliance poses a risk to the health and safety of occupants.

For EWI, 92% of audits found at least one major technical non‑compliance, and a further 6% were found to have Category 1 non‑compliances. For IWI, 27% of properties were found to have a major technical non‑compliance, and a further 3% were found to have a Category 1 non‑compliance.

Table 1: Rates of non‑compliance found within each sample of audits

	Category 1	Major	Minor	Pass
EWI sample	6%* (10%)	92%* (88%)	1%	1%
IWI sample	3%* (4%)	27%	35%	35%* (34%)

* The figures in brackets reflect the % of non‑compliances without taking into account remediation of Category 1 findings whereby works weren’t required at site.

The combined rate of Category 1 and Major non‑compliance for EWI is 98%, with a 95% confidence interval of (96%, 99%). The combined rate of Category-1 and Major non‑compliance for IWI is 30%, with a 95% confidence interval of (25%, 35%).

Sampling approach

A random sample was drawn from the population of un-audited properties which have received solid wall insulation under ECO4 or GBIS. Sampled households were contacted individually by phone, email, or in person at the property, up to 6 attempts per property.

378 properties with EWI and 380 properties with IWI were audited in total, from a population of 24,600 EWI installations and 36,100 IWI installations. The sample sizes were chosen to enable construction of relatively narrow 95% confidence intervals for the rates of non‑compliance.

Contact was attempted with approximately 3,800 households in order to achieve these sample sizes. Of the households not included in the sample, around two-thirds did not reply to the multiple (up to 6) contact attempts, and around one third refused an audit. Replacement properties were drawn from the randomised list of projects in scope. The high rate of non‑compliance identified in these audits is unlikely to be due to these levels of non‑response.

Conclusions

Both samples of audits show a high rate of major and Category 1 non‑compliance which exceed any reasonable tolerance level.

Annex: Classification of non-compliance

‘Non‑compliance’ means that a qualified assessor has found that the work carried out does not meet the relevant PAS standard. The scale and seriousness of non‑compliance can vary, as can the amount of work required to put it right. The classification of non‑compliance for EWI and IWI for TrustMark and Certification Bodies has been standardised as follows:

Category	Classification	Timeline
Minor non‑compliance	May potentially affect system performance. Delayed remediation may impact system performance.	Installer required to provide evidence of correction within 8 weeks, if not remediated by this point Certification Bodies should inform the installer of implications of not having done so by 12 weeks from the audit.
Major non‑compliance	Will affect system performance. Remediation is required to avoid system failure.	Installer required to provide evidence of correction within 8 weeks, if not remediated by this point Certification Bodies should inform the installer of implications of not having done so by 12 weeks from the audit.
Major non‑compliance (Category 1)	Category 1 – risk to property or health and safety of occupants identified	Within 24 hours the risk to health or property needs to be mitigated and made safe. This does not necessarily mean fully remediated. Installer is then required to provide evidence of full correction within 8 weeks, if not remediated by this point Certification Bodies should inform the installer of implications of not having done so by 12 weeks from the audit.