In short
- External wall insulation (EWI) fixes insulation boards to the outside wall, then finishes them with a meshed base coat and render.
- It suits solid-walled homes, which lose far more heat than insulated cavity walls, and hard-to-treat cavities.
- Typical 2026 installed ranges are about £90–£150 per m² or more; a whole house often runs to £12,000–£22,000.
- The National Audit Office found widespread defects in grant-funded EWI. Detailing at windows, eaves and the ground is everything.
How external wall insulation works
EWI wraps the house in a continuous layer of insulation. Boards of expanded polystyrene, mineral wool or phenolic foam, commonly 50 to 150 mm thick, are bonded and mechanically fixed to the wall. A base coat reinforced with glass-fibre mesh goes over the boards, then a render finish, most often silicone. From the street it looks like a newly rendered house.
Because the insulation is outside the structure, you keep your internal floor space and avoid disruption indoors. It also covers the thermal bridges at floor edges that internal insulation struggles with.
Which Surrey homes benefit most
- Solid brick houses, typically built before the 1920s, with no cavity to fill.
- Non-traditional and system-built houses, including some post-war council estates.
- Homes with cavities that cannot be filled, for example because of exposure or previous failed fill.
- Houses being re-rendered anyway, where the extra cost of insulation is partly offset by work you were paying for already.
Regulations you cannot skip
Adding EWI is a notifiable renovation of a thermal element under Part L of the Building Regulations, so building control must be involved, either your council or an approved inspector. When a wall is renovated in this way, Approved Document L sets a target U-value of 0.30 W/m²K. See Building Regulations for rendering.
Planning permission may also be needed, particularly in conservation areas and in the Surrey Hills National Landscape, where cladding a house in render is not permitted development. See planning permission for rendering.
On residential buildings over 18 metres, Building Regulations prohibit combustible materials in external walls, and Approved Document B carries guidance for buildings over 11 metres. Blocks of flats need a designer's specification, not a domestic quote.
Getting the details right
Most EWI problems begin at junctions: window reveals and sills, the eaves, pipes and flues, and the base of the wall. Water that gets behind the system at any of these points has nowhere to go. At survey we will show you exactly how we handle:
- window sills, which usually need extending;
- the starter track at the base and its height above ground;
- downpipes, gas meters, satellite dishes and boiler flues, which need moving out;
- the roof verge and eaves, which may need extending to cover the extra thickness.
Funding in 2026
The Great British Insulation Scheme closed on 31 March 2026 and ECO4 is due to run until 31 December 2026. The government's Warm Homes Plan, published in January 2026, is the successor framework, with support including the Warm Homes: Local Grant delivered through councils for eligible low-income households. Rules change often, so read our EWI grants guide and check eligibility on GOV.UK before assuming funding.
Questions people ask
Does external wall insulation need planning permission in Guildford?
Often not outside designated areas, but it can. In conservation areas, the Surrey Hills National Landscape and on listed buildings you should expect to need consent. Check with the planning authority or apply for a lawful development certificate.
How much heat does EWI save?
Savings depend on the wall type and how the house is heated. Solid walls lose much more heat than insulated cavity walls, so the gains are greatest there. Ask for an energy assessment rather than relying on a headline figure.
Can EWI cause damp?
Poorly detailed EWI can trap water behind the boards, particularly at sills and the base. Correctly detailed systems with vapour-permeable finishes and mineral wool where appropriate should not. On historic solid walls, the choice of materials needs specialist input.
Is there a grant for external wall insulation in 2026?
Grant routes are in transition. ECO4 runs to the end of 2026 and the Warm Homes Plan is the successor. See EWI grants.