Why external wall insulation fails, and how to avoid it
The short answer
Most EWI failures start at the details: short roof overhangs, unextended sills, systems taken down to ground level, gaps around services and no attention to ventilation. In 2025 the National Audit Office and Ofgem-commissioned audits found widespread non-compliance in solid wall insulation installed under ECO4 and the Great British Insulation Scheme.
What the audits found
TrustMark flagged high levels of non-compliant external wall insulation under ECO4 and GBIS in October 2024. The NAO reported in October 2025 on around 28,000 EWI installations under the two schemes, and the government says poor-quality work under them will be put right at no cost to households. Non-compliance ranged from serious safety risks to missing paperwork.
Questions to ask before you sign
- Which certified system will you use, and can I see the certificate?
- How will the roofline, sills and the base of the wall be detailed? Can I see drawings?
- What happens to downpipes, gas meters, vents and cables?
- Will you check ventilation inside?
- Will I get photographs of the hidden work and the building control certificate?
- What are the guarantee terms and who backs them, in writing?
If your existing EWI is failing
If it was installed under ECO4 or GBIS, contact the installer and TrustMark first: remediation should be at no cost to you. Otherwise, a condition survey identifies the cause before any money is spent on repairs.
Related services
Questions people ask
Can failed EWI be fixed?
Usually. Some failures need only the details rebuilt; others need sections stripped and reinstated.
Who regulates EWI installers?
There is no single licence. Funded work needs scheme-specific certification; for private work, check the system, the details and the paperwork you'll receive.