Quick answer
For most UK homes with unfilled cavity walls, yes. Cavity wall insulation costs £800 to £1,500 for a typical semi and saves £100 to £200 per year on heating. Payback is 4 to 8 years.
But it's not suitable for every house. Exposed coastal properties, walls with cracks or existing damp, and some solid-wall houses shouldn't have it. Always get a proper survey first.
What cavity wall insulation costs in 2026
Professional installation costs around £800 to £1,500 for a typical semi-detached house. The price depends on the size of your walls, the type of insulation used, and how easy the walls are to access.
| House type | Wall area | Typical cost |
|---|---|---|
| Mid-terrace | 40-50 m² | £600-£900 |
| Semi-detached | 60-80 m² | £800-£1,200 |
| Detached | 120-160 m² | £1,200-£2,000 |
| Bungalow | 50-70 m² | £700-£1,100 |
Installation takes one day for most houses. The installer drills holes in the mortar joints every square metre or so, blows insulation into the cavity, then fills the holes and matches the mortar colour.
This is not a DIY job. You need specialist equipment to blow the insulation in evenly and you need to know where to avoid drilling (around windows, damp-proof courses, and wall ties).
How much you save on heating
A house with unfilled cavity walls loses about 30% of its heat through the walls. Filling the cavity cuts that loss to around 10%.
Annual savings for a typical semi-detached house with gas heating:
- No insulation to filled cavity: £150 to £250 per year
- Partial cavity (pre-2000 builds sometimes): £50 to £100 per year
These figures assume gas at 10p/kWh and a house heated to 19-20°C through winter. If you run electric heating, savings are about three times higher.
Payback period: 4 to 8 years for professional installation. After that, the savings stack up year after year. Over 25 years, you could save £3,000 to £6,000 in total.
Free cavity wall insulation and grants
Free installation is available through ECO4 if you meet the criteria:
- You receive benefits (Pension Credit, Universal Credit, Housing Benefit, etc.), or
- Your household income is under £36,000 and your home has an EPC rating of D or below
Contact your energy supplier or use the gov.uk Simple Energy Advice tool to check eligibility. Energy companies fund ECO4 installs, so there's no direct cost to you if you qualify.
If you don't qualify, you pay the full cost. There are no other mainstream grants for cavity wall insulation in 2026.
When cavity wall insulation isn't suitable
Not every house should have cavity wall insulation. Skip it if:
You don't have cavity walls
Solid-wall houses (most pre-1920 builds) have no cavity to fill. You'd need external or internal solid wall insulation instead, which costs more and is a bigger job.
Check by looking at the brickwork. Cavity walls show a stretcher bond (all bricks running lengthways). Solid walls show alternating long and short bricks (Flemish or English bond). See our guide on what a cavity wall is for photos.
Your walls already have insulation
Houses built after the mid-1990s often have insulation fitted during construction. Check your EPC or ask a surveyor. If the cavity is already filled, there's nothing to add.
The property is in an exposed coastal area
Wind-driven rain can penetrate cavity-filled walls more easily than empty cavities in very exposed locations. If your house is within 500m of the coast and faces prevailing winds, get a specialist survey first. The British Standard (BS 8208) has exposure zones that installers should check.
The walls show cracks, spalling, or existing damp
Insulation won't fix damp and can make it worse by giving moisture a path inward. If your render is cracked, bricks are flaking, or you see damp patches inside, fix those first. Then reassess whether cavity fill is safe.
The cavity is narrow or irregular
Some older cavity walls have gaps as narrow as 40mm. Modern insulation needs at least 50mm to work properly. A surveyor will measure this before installation. If the cavity is too narrow or partially filled with rubble (common in 1920s-1940s builds), the job becomes riskier.
Types of cavity wall insulation
Three main materials are used in the UK:
Polystyrene beads (EPS)
Tiny beads mixed with adhesive, blown into the cavity. Cheap, effective, and the most common choice for retrofits. Good thermal performance and won't settle over time.
Mineral wool
Rock wool or glass wool blown into the cavity. Slightly better fire resistance than beads. More expensive but preferred in some commercial or heritage builds.
Foam (polyurethane or urea-formaldehyde)
Rarely used now. Urea-formaldehyde foam was common in the 1980s but is mostly phased out due to health concerns about formaldehyde off-gassing. If your house already has it and it's in good condition, leave it. Don't add more.
Most installers in 2026 use polystyrene beads unless you specifically ask for mineral wool.
The damp risk explained
Cavity wall insulation can cause damp if installed badly or in the wrong house. Here's why:
The cavity is there to create an air gap. Rain hits the outer brick, water runs down the inside face of the outer leaf, and the gap stops it reaching the inner wall. When you fill the cavity, you risk creating a bridge for moisture to cross.
Problems happen when:
- The insulation is installed unevenly, leaving voids or clumps that channel water inward
- The outer wall has cracks or failed pointing that lets water in faster than it can drain
- Wall ties (the metal rods connecting inner and outer leaves) are corroded and collecting moisture
- The property is in an exposed area where wind drives rain horizontally into the wall
A good installer will survey your walls before drilling. They check for exposure, existing damp, wall condition, and cavity width. If the property isn't suitable, they'll tell you. If they don't survey and just quote a price based on your postcode, walk away.
Guarantees and what to check
Reputable cavity wall installers are registered with CIGA (Cavity Insulation Guarantee Agency). CIGA guarantees cover the installation for 25 years and are backed by insurance.
Before you book an installer, check:
- CIGA registration. Ask for their registration number and verify it on the CIGA website.
- Survey included. A proper survey should assess wall type, cavity width, exposure, and damp risk. If they quote without visiting, that's a red flag.
- Guarantee certificate. You should receive a CIGA certificate after installation. Keep it safe. You'll need it if you sell the house.
- What happens if damp appears. Ask the installer what their process is if damp problems develop. Good installers will investigate and remediate. Bad ones disappear.
How to tell if you already have cavity wall insulation
Check your EPC first. It should list cavity insulation under "walls".
If you don't have an EPC or it's old, look for drill holes in the mortar joints outside. They'll be about 20-25mm diameter, spaced roughly a metre apart, filled with mortar that might not match the original colour.
If you're still unsure, hire a surveyor to check. They can use a borescope (a camera on a flexible cable) to look inside the cavity through a single small hole. Costs around £100 to £150 for a survey.
Cavity wall vs solid wall insulation
If your house has solid walls (no cavity), you'll need external or internal wall insulation instead. That's a much bigger job:
- External wall insulation: £8,000 to £15,000 for a typical house. Cladding or render applied to the outside. Major disruption to the building's appearance.
- Internal wall insulation: £4,000 to £8,000. Insulated plasterboard fitted to internal walls. Loses floor space and requires replastering.
Cavity wall insulation at £800 to £1,500 is far cheaper and simpler. If you have the option, do cavity fill first. We cover external wall insulation separately in our guide to whether external wall insulation is worth it.
Does cavity wall insulation help with a heat pump?
Yes. Heat pumps work best in well-insulated homes because they deliver heat at lower flow temperatures than gas boilers.
If you're planning a heat pump, insulate first. The installer's heat-loss calculation will be lower, so you can fit a smaller, cheaper unit. A well-insulated semi might need a 5kW heat pump instead of an 8kW one. That saves £1,000 to £2,000 on the install.
See our full guide on whether heat pumps are worth it for the detailed sums.
The actual payback calculation
Here's a worked example for a typical case:
Semi-detached house, unfilled cavity walls
- Install cost (professional): £1,000
- Annual heating saving: £180
- Payback period: 5.5 years
- 25-year total saving: £4,500
- Net benefit: £3,500
The insulation lasts 25+ years under most guarantees, so the long-term benefit is significant. If you qualify for free installation through ECO4, your net benefit is the full £4,500 over 25 years.
What to do next
If you think your house has unfilled cavity walls:
- Check eligibility for ECO4. Use the gov.uk Simple Energy Advice tool. If you qualify, apply before paying for it yourself.
- Get a survey. Contact a CIGA-registered installer and ask for a full wall survey. Make sure they check exposure, damp risk, and cavity width.
- Get at least two quotes. Prices vary by region and installer. Compare what's included (survey, guarantee, aftercare).
- Check the guarantee. Make sure it's CIGA-backed for 25 years and transferable to future owners.
- Ask about damp remediation. What happens if damp appears within the guarantee period? Get it in writing.
Bottom line
If your house has unfilled cavity walls and you're in a low-exposure area with no existing damp problems, cavity wall insulation is usually worth it. You'll spend £800 to £1,500 and save £100 to £200 per year on heating.
Payback takes 4 to 8 years, but the insulation lasts 25+ years, so the total benefit is significant. Just make sure you use a proper installer who surveys first and provides a CIGA guarantee.
If you're in an exposed location or your walls show signs of damp, get specialist advice before proceeding. Done right, it's one of the best retrofits you can do. Done wrong, it can create problems that cost more to fix than you'd ever save.
Sources and further reading
- Cavity Insulation Guarantee Agency (2025), "CIGA guarantee standards and installer registration", ciga.co.uk
- British Standards Institution (2020), "BS 8208-1: Guide to assessment of suitability of external cavity walls for filling with thermal insulants", BSI Group
- Department for Energy Security and Net Zero (2024), "Energy Company Obligation (ECO4) guidance", gov.uk
- Building Research Establishment (2018), "Cavity wall insulation in existing housing: performance and risk assessment", BRE Trust
- Energy Saving Trust (2025), "Cavity wall insulation", energysavingtrust.org.uk