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ECO4 solid wall insulation: external vs internal, and what to check

ECO4 Application is an independent information and eligibility-check site. We are not the government, Ofgem, an energy supplier, a council or an installer. Solid wall insulation can transform an older home, but it is also where ECO4 has had real quality problems. ECO4 runs until 31 December 2026 (Ofgem). Here is what GOV.UK and Ofgem say, checked on 31 October 2026.

Know this first: GOV.UK says there are quality issues with some solid wall insulation fitted through government schemes, including ECO4 since April 2022, such as poor installation or failure to meet building regulations, which can lead to damp or mould. Homes with external wall insulation are being offered a home check; for internal wall insulation, an audit found fewer issues.

Sources: GOV.UK, what to do if you have poor quality wall insulation (updated 18 Feb 2025). Checked 31 October 2026.

External and internal wall insulation

External wall insulation (EWI)Internal wall insulation (IWI)
Where it goesFixed to the outside walls and rendered or cladFixed to the inside of outside walls
Room sizeUnchangedRooms lose some floor space
Look of the houseChanges the outside appearanceOutside unchanged
GOV.UK audit findingHome checks offered to all EWI propertiesFewer issues found; no action normally needed

Which your home gets is decided by the retrofit assessment. Either way, Ofgem's guidance says ECO4 work must be done by, or under, a TrustMark-registered business certified to PAS 2030:2023 within PAS 2035:2023, with a Retrofit Coordinator leading the work.

What to check before agreeing

  • Who the Retrofit Coordinator is, and what the risk and dwelling assessments found (especially about damp).
  • That the installer is on the TrustMark register for wall insulation.
  • Which guarantee covers the work; TrustMark lists appropriate 25-year guarantees.
  • How windows, doors, eaves, pipes and vents will be detailed.
  • Whether you will be asked to contribute (Ofgem says this can happen), in the contract.

If you already have ECO4 solid wall insulation

GOV.UK says Ofgem is sending letters to affected properties. If you have had a home check since January 2025, no further action is needed. If a check finds the work faulty, the installer will be asked to fix it and you should not be asked to pay. Organisations contacting you will quote your Unique Measure Reference from the letter; if you are unsure a caller is genuine, use the contact details on the GOV.UK page. For other complaints, GOV.UK points to TrustMark's complaints process.

Where solid wall insulation fits in ECO4

Solid wall insulation counts towards Ofgem's insulation preconditions for heating work (an exterior wall insulation measure). To see if your household qualifies, use the checker on this page or the ECO4 eligibility rules.

Questions

Is ECO4 solid wall insulation safe?

GOV.UK says there are quality issues with some solid wall insulation fitted through government schemes, including ECO4 since April 2022. Ofgem is writing to affected homes and offering home checks for external wall insulation.

What if my ECO4 external wall insulation is faulty?

GOV.UK says homes with external wall insulation are offered a home check; if work is faulty the installer will be asked to fix it and you should not be asked to pay.

External or internal wall insulation: which is better?

It depends on the home; the retrofit assessment decides. GOV.UK notes an audit found fewer issues with internal wall insulation.