External Wall Insulation — Insulating Solid Walls From the Outside
Homes with solid walls (mostly built before the 1920s) have no cavity to fill. External wall insulation fixes insulation boards to the outside and covers them with render or cladding.
Around a third of the heat lost from an uninsulated home escapes through the walls. External wall insulation is a big job and a big investment, so it is worth doing carefully: the Energy Saving Trust recommends fitting it alongside other outside work, fixing any damp first and using an installer who will guarantee the work for 25 years. In October 2025 the National Audit Office reported quality failures in wall insulation fitted under government schemes.
- Typical yearly saving (solid wall insulation)
- £140–£500
- External wall insulation, 3-bed semi
- ~£15,000
- Guarantee a TrustMark installer should offer
- 25 years
- Heat lost through uninsulated walls
- ~33%
What Is External Wall?
External wall insulation (EWI) is a layer of insulation boards fixed to the outside walls, covered with a reinforcing layer and a finish such as render, brick slips or cladding. Because it changes how moisture moves through older walls, the design, materials and detailing around windows, roofs and pipes all matter.
How It Works
Survey and Design
The walls are checked for damp and structural problems, and the system, thickness and details around windows, eaves and pipes are designed.
Preparation
Scaffolding goes up. Gutters, downpipes and fixtures are removed or extended, and any repairs are made first.
Insulation and Finish
Insulation boards are fixed to the walls, then covered with a reinforcing mesh and the chosen finish.
Sign-Off
Fixtures are refitted and you should receive guarantee documents. Keep photos and paperwork of the work.
Is It Right for My Home?
External wall insulation is mainly for solid-walled homes, and some non-traditional construction types. You need good access to the outside walls, and the walls must be structurally sound and free of damp before work starts. Check with your council whether you need planning permission — especially in a conservation area or for a listed building.
Key Benefits
Lower Heating Bills
Solid wall insulation saves around £150 a year in a mid-terrace house and £500 in a detached house (Energy Saving Trust).
Protects the Brickwork
A well-installed external system can protect the outer walls from the weather.
No Loss of Room Space
Unlike internal wall insulation, rooms keep their size.
Warmer Walls
Warmer inside wall surfaces can reduce condensation, once any existing damp has been fixed.
Work Done Outside
Most of the work happens outside, so disruption indoors is limited.
Lower Carbon Emissions
Less heating demand means lower emissions from your home.
Ask for a Free External Wall Quote
Typical Costs and Savings
Typical figures to help you compare, not a quote for your home. Your costs and savings depend on the property, how you heat it and how you use energy.
| Example | Typical Cost | Typical Saving a Year |
|---|---|---|
| Detached house | — | £500 |
| Semi-detached house (3-bed) | ~£15,000 external / ~£12,000 internal | £280 |
| Mid-terrace house | — | £150 |
| Detached bungalow | — | £220 |
| Mid-floor flat | — | £140 |
Energy Saving Trust savings for insulating the solid walls of a gas-heated home, based on fuel prices as of July 2026. The Energy Saving Trust gives typical costs only for a three-bedroom semi-detached house; costs vary a lot with wall area and the work needed. Source: Energy Saving Trust — solid wall insulation. Last reviewed September 2026.
Help With the Cost
Funding depends on where you live, your income and your home, and schemes change. We are not a government body — check the official scheme before relying on it.
Warm Homes: Local Grant (England)
Council-run funding for wall, loft and floor insulation in lower-income households living in homes rated EPC D–G.
Read more on our grants guide →0% VAT until 31 March 2027
Professionally installed insulation in a home is zero-rated for VAT until 31 March 2027.
Read more on our grants guide →Frequently Asked Questions
Your next step
Thinking about External Wall Insulation?
It is free to ask and there is no obligation.