The short answer
External wall insulation (EWI) usually saves a UK household somewhere between roughly £200 and £550 a year on heating, with the figure depending heavily on house type, size and the fuel you use. The Energy Saving Trust's solid-wall insulation estimates suggest a mid-terrace house saves least (often a few hundred pounds) because it has fewer exposed walls, while a detached house saves most because it loses the most heat through its larger external wall area. Solid walls can lose around 45% of a home's heat, so insulating them externally typically cuts wall heat loss substantially and lifts comfort as well as cutting bills. Savings are larger for homes on expensive fuels such as electric, oil or LPG than on mains gas, and larger again where energy prices are high.
The honest answer is a range, because savings depend on your walls, your house shape, your heating fuel and the price you pay per unit. The figures below are typical UK guidance, not a promise for your specific home.
Typical annual savings
- Mid-terrace houselowest savings (fewer exposed walls)
- Semi-detached housemid-range savings
- Detached househighest savings (most wall area)
- Solid-wall heat lossaround 45% of total
- Bigger savings ifyou heat with electric, oil or LPG
Why savings vary so much by house type
The amount you save is driven mainly by how much external wall your home has and how much heat currently escapes through it. A mid-terrace house shares walls with neighbours on both sides, so it has only two exposed walls (front and back) and the lowest savings. A detached house has all four walls exposed to the cold, so insulating them returns the most. Semi-detached and end-terrace homes sit in between.
- Wall construction: solid walls (typically pre-1920s, with bricks laid in a header-and-stretcher pattern) lose far more heat than cavity walls, so they gain most from EWI.
- Heated floor area: a larger home has more wall to lose heat through, so the absolute saving is bigger.
- Existing heating fuel: the same kilowatt-hours saved are worth more if you pay a high unit rate, so electric and oil-heated homes see bigger cash savings.
| House type | Typical relative saving | Why |
|---|---|---|
| Mid-terrace | Lowest | only two exposed walls |
| Semi-detached | Mid-range | three exposed walls |
| End-terrace | Mid-range | three exposed walls |
| Detached | Highest | all four walls exposed |
Indicative ranking for guidance. Source: Energy Saving Trust solid-wall insulation estimates.
How the saving is calculated
Savings come from a lower U-value — the rate at which heat passes through the wall. An uninsulated solid wall might have a U-value of around 2.0 W/m²K, while a wall fitted with around 90–100mm of EWI can reach roughly 0.30 W/m²K or better, which is the kind of improvement modern building standards expect. Cutting the U-value this far means the wall loses a fraction of the heat it used to, which is why solid-wall homes feel noticeably warmer and need the boiler on less.
The cash value depends on your fuel price. The same reduction in heat loss is worth more each winter if you are on electricity or oil than on mains gas, so two identical houses can show very different payback simply because of how they are heated.
Comfort and value beyond the bills
The cash saving is only part of the picture. Because EWI keeps the masonry warm, it removes the cold internal surfaces where condensation and mould tend to form, so many households report a warmer, drier, more even temperature rather than just a smaller bill. It also keeps the heat in for longer after the heating switches off, which is harder to put a precise number on but is one of the most commonly mentioned benefits by people who have had it done.
There can be a value effect too. Improving the wall insulation lifts the home's EPC rating (the A–G energy efficiency band), and a better band can make a property easier to sell or let, particularly as minimum energy efficiency standards for rented homes tighten.
Will the savings ever pay for the work?
EWI is a long-term measure. With installed costs commonly running into five figures for a whole house and annual savings in the low hundreds of pounds, simple payback on the cash saving alone is usually measured in many years to a couple of decades. That is why it most often makes sense when it is combined with work you were going to do anyway — re-rendering a tired façade, a wider retrofit, or a grant-supported scheme that covers part of the cost. Treated as a comfort, EPC and damp-resistance upgrade as well as a bill-cutter, it stacks up far better than on the heating saving in isolation.
Frequently asked questions
How much does external wall insulation save per year?
Typical UK savings are roughly £200–£550 a year, depending on house type, size and heating fuel. Detached homes on expensive fuels such as electric or oil tend to save the most; mid-terrace homes on mains gas save the least.
Does external wall insulation save more than cavity wall insulation?
For a solid-wall home it usually saves more in absolute terms, because solid walls lose far more heat than cavity walls and EWI insulates the whole external surface. Cavity wall insulation is much cheaper per home, though, so the cost-effectiveness is different.
Why do mid-terrace houses save the least?
A mid-terrace house only has two walls exposed to the outside, since its side walls are shared with neighbours. Fewer exposed walls means less heat is escaping through them, so there is less to save by insulating.
Sources & further reading
Figures on this page are typical UK ranges drawn from published sources and depend on your specific property. They are guidance, not a quotation.