External wall insulation wraps your house in a thick, rendered coat. Done well it cuts heat demand by 25–35%, transforms comfort, and jumps the EPC by two bands. Done badly it traps moisture and rots the house from the inside.
The pros
- Keeps the thermal mass of the wall warm — rooms feel warmer at the same air temperature.
- No internal disruption (no loss of floor area, no stripping plaster).
- Opportunity to refresh the elevation — colour, texture, detailing.
The cons
- Changes how the house looks — often a planning issue in conservation areas.
- Needs breathable systems on solid wall / lime construction. Cement-based systems trap moisture.
- Scaffolding, weather-dependent. A month on site is typical.
Typical cost
On a three-bed semi (ca. 120m² wall area), £16,000–£22,000 before grants. ECO4 can cover most or all of it for qualifying households.