In 40 seconds
External wall insulation (EWI) on a typical UK house usually costs roughly £90–£150 per square metre installed with a render finish, which works out at about £8,000–£15,000 for a three-bed semi before extras. Grants can cut that sharply: EWI is a qualifying measure under ECO4 (extended to 31 December 2026, for households on means-tested benefits with a lower EPC) and was covered by the Great British Insulation Scheme (GBIS), with the Warm Homes Plan succeeding them. For solid-wall homes built before the 1920s, EWI can cut heat loss by up to about 35% and save roughly £200–£700 a year, with payback often around 10–15 years (less with a grant). The big risks are poor installation — render cracking and damp — which is why a PAS2035/TrustMark installer and retrofit coordinator matter. The honest answer is always a range, because it depends on your wall area, material and access.
Most EWI guidance is published by firms fitting or selling it, so the numbers tend to be optimistic and the rules and risks glossed over. The pages below give typical cost ranges, explain which grants apply, weigh whether EWI is worth it for your home, set out the common problems to avoid, and cover solid wall insulation costs — before you take a single quote.