external
External wall insulation
External wall insulation costs £100 to £160 per square metre including scaffold, so a typical semi is £10,000 to £18,000. Grant funding through ECO4 can cover most or all of it, and any grant-funded work has to be done by a PAS 2030 certified installer registered with TrustMark.
What this covers
What it costs. External wall insulation costs £100 to £160 per square metre including scaffold. Full cost breakdown.
What the job covers
- Fixing EPS or mineral wool board to a solid wall and rendering over it
- Extending sills, soffits and downpipes to clear the new wall line
- Detailing around windows and at the damp proof course
- Basecoat, mesh and a silicone topcoat over the insulation
- Work certified under PAS 2030 where a grant is funding it
What is involved
EWI is insulation board fixed to the outside of a solid wall and rendered over, and it is the only practical way to insulate a wall that has no cavity. Ninety or a hundred millimetres of EPS or mineral wool goes on with adhesive and mechanical fixings, then a basecoat with mesh, then a topcoat, which is the same thin-coat system used on a bare wall.
The complication is everything the wall is attached to. The wall gets a hundred millimetres thicker, so window sills have to be extended, soffits and verges have to be packed out, downpipes and flues have to be moved, and the detail at the damp proof course has to stop water tracking behind the boards. Most of the failures in this trade are those details rather than the insulation.
This is also the corner of plastering with a real lead-generation problem. A freephone number, the words free insulation and no address is a broker selling your enquiry, not an installer.
Fixing EPS or mineral wool board to a solid wall and rendering over it, one of the jobs a external wall insulation specialist is hired for.
Choosing someone for the job
Ask for the installer's PAS 2030 certificate and TrustMark registration if a grant is paying for any of it. Ask what happens to the sills, the soffits, the downpipes and the flue, and get that in the quote rather than agreed on site. Ask how the detail works at the damp proof course. And be wary of anything that arrives as a cold call with a freephone number and no address.
Common questions
Can I get external wall insulation for free?
Sometimes. ECO4 and the Great British Insulation Scheme fund solid wall insulation for eligible households, usually on income or benefit grounds and often tied to the property's EPC band. The funding is real; the cold callers offering it are frequently brokers rather than installers.
How much thicker does it make the wall?
Around a hundred millimetres including the render. That is why the sills, soffits and downpipes have to be extended, and it is worth checking what it does to a shared boundary or a narrow passage before committing.