Cavity Wall Insulation UK: Costs, Grants & Is It Worth It?
TLDR: Cavity wall insulation fills the gap between inner and outer walls with insulating material. Costs run £500-£1,500 privately but ECO4 and Great British Insulation Scheme grants cover it for many households. Annual savings of £100-£275 make payback 3-8 years without grants, immediate with funding. It's one of the most cost-effective home improvements available to British homeowners.
What Is Cavity Wall Insulation?
If your home was built between the 1920s and the 1990s, there's a good chance it has cavity walls—two layers of brick or block with a gap (cavity) between them. Originally, builders left this gap to prevent damp from penetrating to the inner wall. But that empty space is also letting heat escape from your home at an alarming rate.
Up to 35% of heat loss in an uninsulated home occurs through the walls. For a cavity wall property, that's essentially money flowing out through a gap that could easily be filled with insulating material.
The process is remarkably straightforward:
- Small holes are drilled in the outer wall (usually 22mm diameter, about the size of a 50p coin)
- Insulating material is blown into the cavity under pressure
- Holes are filled and colour-matched to surrounding brickwork
- The entire job is typically completed in 2-4 hours for a semi-detached house
Once installed, the insulation creates a continuous thermal barrier around your home, dramatically reducing heat loss and keeping warmth where it belongs—inside.
Real Homeowner Experiences
Emma and Richard, Leeds (1970s semi-detached): "We'd lived in our house for fifteen years before finally getting the cavity walls insulated. Looking back, I can't believe we waited so long. The difference was noticeable within hours—the rooms just felt warmer, and the boiler wasn't running constantly anymore. We got it done free through ECO4 because I'm on Universal Credit. The installers were in and out in about three hours. Our gas bills dropped by roughly £180 a year. The only regret is not doing it sooner."
David, Edinburgh (1950s detached bungalow): "Scotland gets properly cold in winter, and our old bungalow was draughty despite having double glazing. A friend suggested checking if we had cavity walls—turns out we did, and they'd never been filled. I paid privately because we didn't qualify for grants. Cost was £1,200 for the whole house. First winter after installation, we saved about £220 on heating. The walls feel warmer to the touch, and we've stopped getting condensation on the inside windows. Best home improvement we've made, honestly."
Sandra and Michael, Bristol (Victorian terrace with 1930s extension): "Our house is a bit unusual—the original Victorian front is solid brick, but the 1930s kitchen extension has cavity walls. We got just the extension done through the Great British Insulation Scheme. Completely free because of our Council Tax band and EPC rating. The kitchen used to be freezing in winter despite the radiator being on full. Now it holds heat properly. The installer said filling cavities in extensions is really common—lots of people don't realise only part of their house might be suitable."
James, Manchester (1960s end-terrace): "I was sceptical about cavity wall insulation because I'd heard horror stories about damp problems. But the surveyor explained our property was ideal—sheltered location, good cavity width, no existing damp issues. They used polystyrene beads with adhesive because we're on a slightly exposed corner. Two years later, no problems whatsoever. House is noticeably warmer, and we're saving about £150 a year on gas. I'd recommend getting a proper survey from a TrustMark installer rather than just going with whoever's cheapest."
Types of Cavity Insulation
Not all cavity insulation is the same. The material used depends on your property's characteristics and exposure level:
| Type | Description | Thermal Performance | Best For | Cost Premium |
|---|---|---|---|---|
| Mineral wool (glass or rock) | Fibrous material blown under pressure | Good | Most homes, fire-resistant, breathable | Standard pricing |
| Polystyrene beads with adhesive | Expanded polystyrene bonded together | Very Good | Irregular cavities, exposed sites, partial fills | +10-20% |
| Polyurethane foam | Expanding foam that sets solid | Excellent | Narrow cavities (50-75mm), maximum performance | +30-50% |
Mineral Wool: The Standard Choice
Mineral wool (either glass fibre or rock wool) is the most commonly used material in the UK. It's fire-resistant, allows walls to breathe, and has been used successfully for decades. For most properties in sheltered or moderately exposed locations, it's the ideal choice.
Polystyrene Beads: For Challenging Properties
Bonded bead insulation uses expanded polystyrene beads coated in adhesive. The beads bond together once inside the cavity, creating a more stable fill that's less prone to slumping or settling. This makes it particularly suitable for exposed locations where wind-driven rain is a concern, or for properties with irregular cavities containing debris or partial blockages.
Spray Foam: Maximum Performance
Polyurethane spray foam offers the best thermal performance per millimetre, making it ideal for narrow cavities where other materials wouldn't provide sufficient insulation. However, it's more expensive and creates a rigid fill that's harder to extract if problems occur. Some mortgage lenders have concerns about spray foam in cavities, so check before proceeding if you might sell soon.
Costs and Savings in 2025
Cavity wall insulation costs depend primarily on house size and type:
Installation Costs (Private Installation)
| Property Type | Typical Cost | Price Range |
|---|---|---|
| Mid-terrace (2-3 bed) | £500 | £400-£650 |
| End-terrace (3 bed) | £650 | £550-£800 |
| Semi-detached (3 bed) | £700 | £550-£900 |
| Detached (3-4 bed) | £1,100 | £850-£1,500 |
| Large detached (5+ bed) | £1,400 | £1,200-£1,800 |
| Bungalow (detached) | £800 | £600-£1,000 |
Prices vary by region—expect to pay 10-15% more in London and the South East, and 10-15% less in Wales, the North East, and Scotland.
Annual Savings
According to the Energy Saving Trust's latest figures (based on current gas prices):
| Property Type | Typical Annual Saving | Range |
|---|---|---|
| Mid-terrace | £95 | £75-£120 |
| End-terrace | £140 | £110-£170 |
| Semi-detached | £160 | £130-£200 |
| Detached | £230 | £185-£285 |
| Bungalow | £180 | £140-£220 |
Savings depend on your heating habits, thermostat settings, and how draughty your home was before installation. Some households report savings significantly higher than these estimates, particularly if they were previously heating a very leaky home.
Payback Period
For private installations, payback typically runs 4-7 years. With current high energy prices, many homeowners are seeing payback in under 5 years. If you qualify for a grant (see below), the payback is immediate—you're saving from day one with no upfront cost.
UK Government Grants and Schemes
ECO4 (Energy Company Obligation)
ECO4 is the main route to free cavity wall insulation for eligible households. Major energy suppliers are required by law to help improve home energy efficiency, and cavity wall insulation is one of their primary measures.
Eligibility criteria for ECO4:
- Property has an EPC rating of D, E, F, or G
- You receive qualifying benefits: Universal Credit, Pension Credit, Child Tax Credit, Working Tax Credit, Income Support, Income-based JSA, Income-related ESA, Housing Benefit, or Child Benefit (with income conditions)
- OR you live in a low-income area and your household income is below £31,000
If you qualify, the insulation is installed completely free. There's no catch—energy companies fund it as part of their legal obligations.
Great British Insulation Scheme
Launched in 2023, this newer scheme specifically targets homes with poor insulation. It has slightly different eligibility criteria that may help households who don't qualify for ECO4:
Group 1 (means-tested): Households receiving qualifying benefits (similar to ECO4)
Group 2 (property-based): Any household where the property has an EPC of D, E, F, or G AND is in Council Tax bands A-D (England) or A-E (Scotland/Wales). No benefits required.
The Great British Insulation Scheme is particularly useful for working households who don't receive benefits but live in poorly insulated homes. Check the Simple Energy Advice website or contact your energy supplier to verify eligibility.
Scottish Home Energy Scotland Loans
Scottish homeowners can access interest-free loans for energy efficiency improvements through Home Energy Scotland. While cavity wall insulation may be available free through ECO4/GBIS, loans can help fund additional measures or cover properties that don't qualify for grants.
Welsh Nest Scheme
Welsh households receiving certain benefits may qualify for free insulation through the Nest scheme. Priority is given to properties with poor energy efficiency ratings and householders who are elderly, disabled, or have young children.
Is Your Home Suitable for Cavity Wall Insulation?
Not every property with cavity walls is suitable for insulation. A qualified surveyor will assess several factors before recommending the work.
Good Candidates
- Cavity width of 50mm or more: Standard materials need adequate space. Narrower cavities may require specialist foam insulation.
- Empty, clear cavity: The gap between walls should be free of debris, rubble, and old insulation.
- Walls in good condition: No cracks, missing mortar, or structural issues that could allow water ingress.
- Sheltered or moderately exposed location: Properties not subject to severe wind-driven rain.
- No existing damp problems: Any damp issues should be resolved before insulating.
Properties That May Not Be Suitable
- Solid walls: Pre-1920s properties typically have solid walls with no cavity to fill.
- Cavity already filled: Check by removing a plug socket on an external wall—if there's insulation visible, it's already done.
- Significant damp or structural issues: These must be addressed before insulating.
- Very exposed sites: Coastal properties, hilltops, and high-rise flats in exposed locations may need specialist assessment.
- Narrow cavities (under 50mm): Some 1920s-1930s properties have very narrow cavities that standard materials won't adequately fill.
The Importance of Proper Survey
A reputable installer will always conduct a thorough survey before quoting. This should include:
- Measuring cavity width using a borescope camera
- Checking for debris, rubble, or partial fills
- Assessing exposure level using the BRE driving rain index
- Inspecting for existing damp, cracks, or defective mortar
- Confirming property age and construction type
Be wary of installers who quote without surveying. Problems with cavity wall insulation almost always stem from installation in unsuitable properties—a proper survey prevents this.
What About Damp Problems?
You may have heard horror stories about cavity wall insulation causing damp. Let's address this directly.
In the vast majority of installations (over 95%), cavity wall insulation causes no problems whatsoever. Issues occur when:
- Installation was done on unsuitable properties (exposed sites, existing damp)
- Surveyors failed to identify risk factors
- Installation was poor quality (gaps, incomplete fill)
- Properties had hidden problems (blocked cavities, failed mortar)
Choosing a TrustMark-registered, Cavity Insulation Guarantee Agency (CIGA) approved installer provides important protections. CIGA provides a 25-year guarantee, and if problems occur, they'll investigate and arrange remediation—including extraction if necessary.
If extraction is needed (rare), costs run £1,500-£4,000 depending on property size. With a CIGA guarantee, this cost is covered.
Frequently Asked Questions
Q: How do I know if I have cavity walls?
A: Look at the external brickwork. If all bricks are laid lengthways (showing the long face), you likely have cavity walls. If you see alternating long and short faces (header bond), the walls are probably solid. Cavity walls are typically 260-270mm thick; solid walls are around 225mm. Your EPC will also state the wall type.
Q: Will the installation damage my brickwork?
A: The 22mm holes are drilled in the mortar joints, not through bricks. After filling, holes are plugged with mortar matched to your existing pointing. In most cases, it's almost impossible to see where the work was done after a few months of weathering.
Q: How long does installation take?
A: A typical semi-detached house takes 2-4 hours. Larger detached properties may take 4-6 hours. Installation is usually completed in a single visit with no need to access interior rooms.
Q: Will I need to redecorate inside?
A: No. Cavity wall insulation is injected entirely from outside. There's no internal disruption, no dust inside, and no need to move furniture or access internal walls.
Q: Can I get just part of my house done?
A: Yes. Many properties have mixed construction—a Victorian front with a 1930s extension, for example. The cavity section can be insulated even if the solid section cannot. Extensions, particularly flat-roofed kitchen extensions, are commonly done separately.
Q: Will it affect my mortgage or house sale?
A: Properly installed cavity wall insulation with a CIGA guarantee is viewed positively by mortgage lenders and surveyors. It improves your EPC rating, which is increasingly important for property values. Problems only arise with non-standard materials (like some spray foams) or installations without proper guarantees.
Q: How do I find a reliable installer?
A: Use the TrustMark website to find registered installers in your area. Verify they're CIGA-approved. Get at least three quotes, and be wary of anyone who doesn't want to survey before quoting. Check reviews on Trustpilot, Google, and Which? Trusted Traders.
Q: What guarantee should I expect?
A: CIGA provides a 25-year guarantee for approved installations. This covers material defects and installation problems, including extraction costs if required. Always verify the installer is CIGA-registered before proceeding.
The Bottom Line
Cavity wall insulation remains one of the fastest-payback home improvements available to British homeowners. The costs are modest (£500-£1,500), the savings are meaningful (£95-£230 annually), and for many households, grants through ECO4 or the Great British Insulation Scheme cover the entire cost.
If you have unfilled cavity walls and your property is suitable, there's really no good reason not to proceed. Check grant eligibility first—free insulation is obviously better than paid insulation. If you don't qualify for grants, the investment still pays back in 4-7 years, with benefits continuing for the life of your home.
The key is choosing a reputable, TrustMark-registered, CIGA-approved installer who conducts a proper survey. This virtually eliminates the risk of problems and ensures you have a robust 25-year guarantee. Don't be tempted by the cheapest quote if it means cutting corners on survey or installation quality.
With energy prices likely to remain high for the foreseeable future, there's never been a better time to stop your hard-earned money escaping through those empty wall cavities. Get quotes, check grant eligibility, and make 2025 the year you finally plug that heat leak.