The ECO4 and GBIS Insulation Failures: Faulty Installs and Remediation

The audits of solid wall insulation installed under the Energy Company Obligation 4 (ECO4) and the Great British Insulation Scheme (GBIS) found that 30,000 homes had defects following external and internal wall insulation installations¹. The randomised sample of audits was commissioned by Ofgem to understand the extent of non-compliance in solid wall insulation measures installed under these schemes¹.

The headline rates are stark. Results show that 92% of external wall insulation installations and 27% of internal wall insulation installations of those audited had at least one major technical non-compliance². A later consultation document puts the remediation figures higher still, at 98% of homes with external wall insulation and 29% of homes with internal wall insulation³. The two documents disagree, and both figures are set out below.

The schemes themselves have now closed. GBIS ran from 25 July 2023 to 31 March 2026, and the service to check eligibility has closed⁴. What remains live is the remediation route: the guarantee register, the installer reinstatement process and the contact channels that a household can verify before responding to any approach about its insulation.

What the audits found, and why the numbers differ

The audit was a randomised sample commissioned by Ofgem, and its stated purpose was to understand the extent of non-compliance in solid wall insulation measures installed under these schemes¹. It was not a census. That distinction matters when reading the percentages, because a sample rate of 92% is an estimate of the population, not a count of every installation.

The figures also come from documents with different jobs. The statistical audit results, published in October 2025, report non-compliance found in the sample¹. The consultation on reforming consumer protection for home upgrade schemes, published in June 2026, reports homes requiring remediation: 98% of homes with external wall insulation and 29% of homes with internal wall insulation installed under ECO4 and GBIS had major issues requiring remediation³. A non-compliance finding and a remediation requirement are not the same test, which is one reason the two sets of numbers sit apart.

TrustMark had already flagged the problem. In 2024 TrustMark reported quality issues with some solid wall insulation installed through government schemes¹. The audit followed that signal rather than discovering it.

The practical consequence for a household is that the audit rates describe a population, not an individual home. A property with external wall insulation installed under either scheme sits inside a group where the great majority of audited installations had at least one major technical non-compliance². Whether that particular installation is defective is established by inspection, not by the rate.

What drives the failure rate

The failures are concentrated in solid wall insulation, and specifically in external wall insulation. That is the most complex of the common retrofit measures: it involves a substrate of unknown condition, a fixing regime, a weatherproofing detail at every opening and a finish that must shed water for decades. The audit found major technical non-compliance in 92% of audited external wall insulation installations².

Internal wall insulation performed better on the same test, at 27%². The measure is simpler to inspect and less exposed to weather, which is consistent with the gap, though the documents do not attribute cause.

The scheme design itself created pressure. GBIS was built to deliver the rapid installation of value-for-money insulation measures and, in some cases, heating controls⁸. It was designed to deliver improvements to the least energy-efficient homes in Great Britain to help tackle fuel poverty and reduce carbon emissions, and it targeted around 300,000 households⁹. Speed and volume at that scale, across a supply chain that had to be TrustMark-registered, is the background against which the quality failures occurred.

The government's response was to suspend installers. The Welsh Government's written statement records the UK Government taking immediate action to suspend installers and protect consumers, and identifies poor quality workmanship by companies installing solid wall insulation as the issue¹¹.

How the remediation route works

Remediation runs through the installer, the obligated supplier and the guarantee register, in that order of practical contact.

All installers under GBIS had to be TrustMark-accredited and carry a registration number¹². All installers delivering measures under the scheme had to be a TrustMark-registered business⁸. That registration is the thread a household pulls first, because it identifies who did the work and which guarantee applies.

The guarantee position is set out in TrustMark guidance: a minimum two-year warranty for work, and a minimum 25-year guarantee for certain measures installed under the ECO¹³. ECO4 guidance refers to a minimum 25-year insurance backed guarantee for certain insulation measures⁷. The current list of appropriate guarantees sits on the TrustMark website⁸.

Ofgem's role has limits that are worth stating plainly. Ofgem sets targets for obligated suppliers, ensures compliance and monitors progress, but has no oversight of contractual arrangements and states that its ability to help resolve complaints is limited¹². Ofgem does not administer appropriate guarantees or financial protection for measures installed under the Great British Insulation Scheme⁸. Complaints about an installation are handled under the published complaints process¹⁴.

The installer reinstatement process has produced results. 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 identified⁶. That figure describes installers, not homes.

Verifying a genuine contact about your insulation

Approaches about insulation remediation are exactly the kind of contact that fraudsters imitate, so the verification route matters more than the message.

TrustMark was nominated by the Department for Energy Security and Net Zero to directly support customers of the scheme that failed, and it publishes three contact routes: a telephone number, 0333 555 1234, an email address, disputes@trustmark.org.uk, and an online support form¹⁵. Ofgem publishes a scheme enquiry email address, GBIS.enquiry@ofgem.gov.uk, for questions about the scheme¹⁴.

A household that receives a letter, call or visit can check the claim against those published channels rather than the details in the message. Suspected fraud can be reported through Ofgem's counter fraud section, which also sets out Ofgem's role in preventing fraud¹⁴.

The scheme's own application history is a useful check on any claim that new funding is available. Applications for the Great British Insulation Scheme have closed, and installations had to be complete by 31 March 2026⁴. Some energy suppliers were still accepting applications at the point the scheme closed, and the government advises checking with your supplier⁴. Questions about an existing application go to the energy supplier it was made to⁴.

What the rules require of an installation

The rules that governed these installations are the standard against which a defective one is judged.

All measures available under GBIS had to be delivered according to PAS2030/2035⁸. There was no minimum requirement under the scheme for a property's Standard Assessment Procedure rating to be increased through the measure⁸, which is a notable gap: a measure could be installed to standard without the scheme requiring a measurable improvement in the home's rating.

ECO4 works differently. It sets a main obligation called the Home Heating Cost Reduction Obligation¹⁶. Homes receiving any heating measure must first have all the exterior facing cavity walls and loft, including rafters, or roof, including flat, pitched, roof and room-in-roof, insulated, and park homes must first have the floor, walls and ceiling insulated⁵. Homes receiving any heating measure other than first time central heating or district heating connection must first have at least one insulation measure installed of flat roof, pitched roof, room-in-roof, cavity or solid wall, floor insulation, or the floor, walls and ceiling of a park home⁵.

The eligible measures under ECO4 Flex include solid wall insulation, cavity wall insulation, room in roof insulation, loft insulation, flat roof insulation, electric storage heater upgrade, first time central heating to air source heat pump, boiler upgrade from oil, LPG and solid fuel to air source heat pump, and solar photovoltaic panels¹⁷.

GBIS was narrower. It delivered one insulation measure per home¹², and the eligible measures for both the general and low-income groups were cavity wall including party wall, loft, solid wall, pitched roof, flat roof, under-floor, solid floor, park home and room-in-roof¹². Certain heating controls, such as room thermostats, were allowed as a secondary measure in owner-occupied premises in the low-income group¹².

How it differs across England, Scotland, Wales and Northern Ireland

Both schemes operate across Great Britain, not the United Kingdom. The Welsh Government's written statement describes ECO4 and GBIS as operating across Great Britain¹¹. GBIS statistics apply to England, Scotland and Wales¹⁹. ECO4 guidance describes delivering energy efficiency and heating measures to homes in Great Britain⁷. Northern Ireland sits outside both.

Within Great Britain the eligibility bands differ. GBIS eligibility extended to households in lower council tax band properties, bands A to D in England and A to E in Scotland and Wales, so long as their energy efficiency rating was band D or lower⁹. The general group was for households with properties that have an Energy Performance Certificate rating of D to G within Council Tax bands A to D in England and A to E in Scotland and Wales¹².

The Welsh Government has taken a distinct position on the fallout. It issued a written statement responding to UK Government action on GBIS and ECO4¹¹, and a further written statement on support for householders impacted by the closure of the Energy Company Obligation scheme¹⁵. That is a devolved administration treating the remediation question as one it must answer for Welsh households, even though the schemes themselves are reserved.

The delivery guidance is also administered at GB level. GBIS is a government energy-efficiency scheme administered by Ofgem in Great Britain¹⁰. The scheme works by placing an obligation on medium and large energy companies to deliver measures that result in reduced energy usage¹⁰.

For a household, the practical difference is which council tax band and EPC band made it eligible, and which administration's support statement applies. The remediation route itself, through the installer, the supplier and TrustMark, does not change at the border.

What it means for a household's energy independence

The purpose of these schemes is straightforward: ECO4 obligated energy suppliers are required to reduce heating costs for low income and vulnerable households by funding energy efficiency, insulation and heating measures¹⁷. The main objective of ECO4 is to reduce fuel poverty by improving the least energy efficient housing stock occupied by low income, vulnerable and fuel poor households¹⁸. ECO funding pays for energy efficiency measures such as heating improvements and insulation to be installed²⁰.

A solid wall insulation measure that is installed correctly reduces the heat a home loses through its walls. That is a permanent reduction in the energy the household must buy, and it is the closest thing to independence that a fabric measure delivers: less exposure to price, less exposure to the grid's carbon intensity, less exposure to the supplier's tariff.

A defective installation delivers little of that. The household has paid, in disruption and in some cases in contribution, for a measure that does not perform, while remaining dependent on the same supplier and the same grid. The audit found that 92% of audited external wall insulation installations had at least one major technical non-compliance². Where that is the case, the intended saving is not being realised.

The dependence that remains is structural. These are obligation schemes: the energy company funds the measure because it is required to, and the household's relationship with that company continues afterwards. Ofgem does not administer guarantees or financial protection for GBIS measures⁸, so the long-term protection sits with the guarantee register and the installer, not with the regulator. Remediation restores the intended performance of the measure. It does not change the fact that the home still buys its energy.

The wider context is that the schemes have closed. GBIS ran from April 2023 to March 2026, when funding ended, as with ECO4⁹. The measures installed under them remain in the housing stock, and so do the defects. For households weighing further fabric work, the relevant questions are the ones this episode raises: who is registered, what guarantee applies, and what standard the work must meet. Those are covered in finding an insulation installer and insulation standards and certification.