External Wall Insulation Explained: Costs, Benefits and What to Expect

External wall insulation (EWI) fixes insulation boards to the outside of your walls and covers them with a reinforced basecoat and render. It reduces heat loss through the walls and gives the house a fresh exterior at the same time.

How it works

1. Insulation boards, commonly expanded polystyrene (EPS) though mineral wool is also used, are fixed to the cleaned wall.

2. A basecoat with reinforcing mesh is applied over the boards.

3. A primer and silicone render finish complete the system.

Because the insulation sits outside, you don't lose floor space inside, and the walls stay warmer.

Who benefits most

Homes with solid walls, which have no cavity to fill, gain the most. Many older London properties fall into this group. Homes with cavity walls are often insulated by filling the cavity instead, so EWI is less common there.

Benefits

• Lower heat loss, which can reduce heating bills.

• A more comfortable home, with fewer cold walls.

• A refreshed look with a long-lasting finish.

• No loss of internal space and little disruption indoors.

What does it cost?

Published 2026 guides give a wide range, commonly about £90 to £180 per m² fully installed. For a typical three-bedroom semi that means roughly £10,000 to £18,000. The spread comes from the insulation thickness, finish, access and the extra work around windows, roofline and services.

What else to plan for

• Building Regulations. Adding insulation is generally covered by the regulations, so check requirements with your installer and local building control.

• Planning. Conservation areas and listed buildings usually need permission, and some will not allow EWI on the front.

• Details. Window sills, reveals, downpipes, meter boxes and roof overhangs often need adjusting.

• Boundaries. If the thicker wall comes close to a neighbour's land, check for Party Wall implications.

• Wall condition. Damp or defective brickwork should be fixed first.

Funding schemes and grants change often, so check GOV.UK for what is available when you plan the work.

Thinking about insulated render? Send us photos on WhatsApp on 07846 928256 and we'll tell you whether it suits your home and give you a clear quote.

Related reading

• Is insulated render worth it?

• Do you need planning permission to render your house?

• Rendering a house extension

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