Cavity wall insulation under ECO4: questions UK households can ask before work starts (2026)

Guide by ApplyFreeBoiler.uk (Reliant Energy Solutions Ltd), a heating and insulation installer in Bury, Greater Manchester. We are not the government, Ofgem or an energy supplier.

Cavity wall insulation fills the gap between the inner and outer leaves of an external wall to reduce heat loss. It is one of the measures that can be installed under ECO4 where a home is suitable. This checklist is general information from ApplyFreeBoiler.uk, a heating and insulation installer in Bury. It does not replace the installer's own survey and advice, and it does not guarantee funding.

1. Is the wall a cavity wall?

Many homes built from the 1920s onwards have cavity walls, while older homes often have solid walls. Ask the surveyor how they confirmed the wall type for your home.

2. Has the cavity been filled before?

Some homes already have partial or full cavity insulation. Ask whether the surveyor checked through a small inspection hole and what they found.

3. Condition of the outside walls

Cracked render, damaged brickwork, failed pointing or leaking gutters can let water into walls. Ask whether any repairs are needed before insulation goes in.

4. Exposure to wind-driven rain

Homes in very exposed spots can be less suitable for some cavity fills. Ask how the surveyor assessed your home's exposure and the material they recommend.

5. Vents, air bricks and flues

Air bricks, wall vents and flues must stay clear. Ask how these will be protected or sleeved during the work.

6. Damp and condensation afterwards

Keep ventilation habits after insulation, such as using extractor fans and trickle vents. Report any new damp patches to your installer promptly.

7. Guarantees and paperwork

Ask what guarantee comes with the insulation, who provides it, and keep all the paperwork together with your home documents.

8. Check funding and local figures

See ECO measures by council on ApplyFreeBoiler.uk, built from DESNZ Household Energy Efficiency statistics, to see how many ECO measures have been installed in your local authority area. Use the ApplyFreeBoiler.uk eligibility check to see whether an ECO4 heating or insulation pathway may be available for your home in England, Scotland or Wales. ECO4 is due to close on 31 December 2026.