What it is, how it works, what it costs, what you could save, and the damp and suitability questions worth asking before you buy.
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Around a third of all the heat lost in an uninsulated home escapes through the walls, according to the Energy Saving Trust. If your home has unfilled cavity walls, filling them is one of the most cost-effective upgrades available to a UK homeowner. This guide covers how cavity wall insulation works, what it costs, what it saves, when it is not suitable, and the damp questions that come up again and again.
Last verified: 13 August 2026. Cost and savings figures are typical estimates from the Energy Saving Trust and Checkatrade, not a guarantee. Your own quote will vary.
A cavity wall is made up of two masonry walls with a gap, or cavity, between them. Cavity wall insulation fills that gap with an insulating material so that heat stays inside the house rather than crossing the cavity and escaping through the outer wall. The work is done from outside: an installer drills small holes in the external wall, injects the insulation, then fills the holes back in. There is no redecorating and no mess indoors (Energy Saving Trust, verified August 2026).
If your house was built after the 1920s, its external walls are probably cavity walls. Homes built before then usually have solid walls, which need a different and much more expensive approach (see our solid wall insulation cost guide).
Three quick checks, per the Energy Saving Trust:
If you are unsure, a surveyor or an accredited installer will confirm it, usually as part of a pre-installation survey.
An empty cavity lets heat move across the gap and out through the outer leaf of brickwork. Filling the cavity with an insulating material traps pockets of still air, which slows that heat transfer down. The result is a warmer home for the same amount of heating, which is where the bill saving comes from. The same insulation also slows heat coming in during hot weather, so a filled cavity helps a little in summer too.
Three materials cover most UK installations. There is no single "best" for every home, and your installer will recommend based on cavity width, exposure and the condition of the walls.
| Material | Typical cost per m² | Notes |
|---|---|---|
| Mineral or glass wool | £25 to £30 | The most common blown fill. Well proven, good value, breathable. |
| Polystyrene beads | Similar to wool | Bonded beads flow well into awkward cavities and cope with slightly exposed walls. |
| Polyurethane foam | £30 to £65 | The best insulator of the common types and it bonds the wall, but the most expensive and hardest to remove later. |
Per m² material costs: Checkatrade cost guide, updated July 2026, verified August 2026. Most homes are quoted per property rather than per m².
For a typical home in Great Britain the Energy Saving Trust puts the cost at around £2,200, and around £1,000 in Northern Ireland. Costs and savings both scale with how much external wall you have, so a detached house saves the most, around £380 a year.
| House type | Typical installation cost | Typical annual bill saving |
|---|---|---|
| Detached house | £3,900 | £380 |
| Semi-detached house | £2,200 | £200 |
| Detached bungalow | £1,700 | £160 |
| Mid-terrace house | £1,100 | £110 |
| Mid-floor flat | £950 | £110 |
Cost and saving figures: Energy Saving Trust (England, Scotland and Wales), verified August 2026.
On those figures, the insulation pays for itself over roughly a decade, and considerably faster if a grant covers part of the cost. For the full breakdown, including what moves the price up or down, see our cavity wall insulation cost guide.
For a suitable home with an unfilled cavity, yes. It is one of the few upgrades that is quick to fit, needs no redecoration, cuts a genuinely large share of your heat loss and comes with a long guarantee. The warmth and comfort benefit shows up immediately, and the bill saving continues for decades.
Being straight about it: the payback is measured in years, not months. On Energy Saving Trust figures a semi-detached house saving £200 a year against a £2,200 cost takes about eleven years to break even on the bills alone. That still makes it one of the best-value insulation measures in the UK, and much better value than solid wall insulation, but it is not an overnight win. If you qualify for funding, the picture changes completely.
The Energy Saving Trust sets out the conditions for standard cavity wall insulation. Your home is suitable if:
It may not be suitable if your home is in a very exposed position, is prone to wind-driven rain or flooding, has a narrow cavity, or has brickwork or pointing in poor repair. That is exactly what the pre-installation survey is for, and a good installer will tell you if your home fails the test. Be wary of anyone who quotes without surveying.
Installed correctly in a suitable home, it should not. The Energy Saving Trust is clear that cavity wall insulation should not cause damp when the installer has checked that the building is not exposed to extreme weather or flooding and does not have a narrow cavity where water could bridge across.
Damp problems happen when insulation goes into a home that should never have had it, or when the work is poor. This is a real issue and worth taking seriously: the National Audit Office reported in October 2025 that some wall insulation installed under government schemes had quality failures, and the government is offering no cost TrustMark audits to affected households. The practical lesson is simple. Use an accredited installer, insist on a proper survey, and make sure the work is guaranteed.
If you already have insulation and suspect a problem, contact the installer and the guarantee provider first. Installations are normally guaranteed for 25 years by the Cavity Insulation Guarantee Agency (CIGA) or by an independent insurance-backed guarantee.
It is designed to last the life of the building, and it is not something you replace on a schedule. Work is normally covered by a 25-year CIGA or insurance-backed guarantee, which is a reasonable guide to expected life. Keep the guarantee paperwork with your house documents: it also matters when you sell.
A registered installer surveys the property, then drills a pattern of small holes in the outside wall, injects the insulation through them and seals the holes with mortar. For a typical house the work takes around two hours and is done entirely from outside, so there is no disruption indoors. It is not a DIY job: the equipment, the materials and the guarantee all require a registered installer.
Some households can, but it is not universal. Funding is targeted at low-income and vulnerable households through ECO4 and, in England, the Warm Homes: Local Grant, and eligibility rules are strict. The Great British Insulation Scheme, which had a broader route in, closed on 31 March 2026. Our UK insulation grants guide for 2026 sets out which schemes are open, who qualifies and how to apply, with the official sources and a verification date. Always confirm live scheme details before relying on them.
Use an installer registered with the relevant bodies, typically CIGA, the National Insulation Association or TrustMark, and check the guarantee is in place before work starts. Get more than one quote: prices for the same job vary a lot between firms, and comparing is the simplest way to avoid overpaying. Our guide to choosing insulation installers covers what to ask, and you can see the areas we cover on our areas page.
Cavity wall insulation fills the gap between the two masonry leaves of a cavity wall with an insulating material, so heat stays inside the home instead of escaping through the outer wall. It is injected through small holes drilled in the external wall, which are filled back in afterwards, so there is no mess or redecorating indoors.
An empty cavity lets heat cross the gap and escape through the outer brickwork. Filling it traps still air, which slows heat transfer down. Since around a third of the heat lost in an uninsulated home escapes through the walls, according to the Energy Saving Trust, filling the cavity makes a noticeable difference to warmth and to heating bills.
For a suitable home with an unfilled cavity, yes. It is quick to install, needs no redecoration and cuts a large share of your heat loss. On Energy Saving Trust figures it pays for itself over roughly a decade through lower bills, and much faster if a grant covers part of the cost. It is not suitable for very exposed, flood-prone or narrow-cavity walls, which is why the survey matters.
Installed correctly in a suitable home it should not. The Energy Saving Trust states that cavity wall insulation should not cause damp provided the installer has checked the building is not exposed to extreme weather or flooding and does not have a narrow cavity where water could cross. Problems arise when unsuitable homes are insulated or the work is poor, so use an accredited installer and insist on a proper survey.
It is designed to last the life of the building and is not replaced on a schedule. Installations are normally guaranteed for 25 years by the Cavity Insulation Guarantee Agency or by an independent insurance-backed guarantee, which is a fair guide to expected life.
Homes built after the 1920s usually have cavity walls. Look at the brickwork: if the bricks all face sideways in an even pattern, the walls are probably cavity walls. Filled drill holes in the outside wall, often under the windows, suggest the cavity has already been insulated, and your EPC may state whether the walls are insulated.
There is no single best type for every home. Mineral or glass wool is the most common and best value, at roughly £25 to £30 per m² for materials according to Checkatrade. Polystyrene beads suit awkward or slightly exposed cavities. Polyurethane foam insulates best, at roughly £30 to £65 per m², but costs more and is harder to remove later. A surveyor should match the material to your walls.
Some households can, but it is not universal. Funding is targeted at low-income and vulnerable households through ECO4 and, in England, the Warm Homes: Local Grant. The Great British Insulation Scheme closed on 31 March 2026. See our UK insulation grants guide for who qualifies now, and confirm live scheme details before relying on them.
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