TL;DR:
- GBIS and ECO4 both closed to new applications in March 2026 — the Warm Homes Local Grant is now the primary funding route, administered by local councils
- Cavity wall insulation costs around £1,700–£2,700 for a typical semi-detached home; annual bill savings of £150–£525 depending on existing energy use
- Around 6 million UK homes with cavity walls remain unfilled — most cavity walls built before 1996 are candidates for insulation
If your home was built between about 1920 and 1995, it almost certainly has cavity walls — two layers of brick or block with a gap in between. A significant proportion of those cavities are empty, and filling them is one of the most cost-effective energy efficiency improvements you can make to a UK home.
The grant landscape changed significantly in early 2026, so here’s an updated picture of what’s available and what to expect from the installation process.
Why Cavity Walls Matter
An unfilled cavity wall loses heat roughly twice as fast as a filled one. For a typical three-bedroom semi, this translates to heating bills that are £150–£525 per year higher than they need to be, depending on how energy-intensive your current heating setup is. Cavity wall insulation pays back in 3–5 years at self-funded cost, or immediately if you receive a free installation under a grant.
Beyond bills, insulation improves your EPC rating, which matters for property value, mortgage eligibility (green mortgages require an EPC of C or above), and rental compliance as Minimum Energy Efficiency Standards tighten.
The 2026 Grant Landscape
The two main schemes that funded free cavity wall insulation — the Great British Insulation Scheme (GBIS) and ECO4 — both closed to new applications in March 2026. If you applied before March and are awaiting installation, those should proceed.
The replacement routes are:
Warm Homes: Local Grant
Running April 2025 to March 2028, this is a council-administered scheme providing grants for insulation and heating improvements. Eligibility varies by local authority — councils allocate funding to households in their area based on income, benefits receipt, and EPC rating. EPC ratings of D, E, F, or G are typically required to qualify.
To apply: check your council’s website, or search the government’s Find a Local Warm Homes Grant provider tool. Some councils are running this proactively (contacting eligible residents); others require an active application. The process typically involves a survey visit before works are approved.
Warm Homes: Boiler and Insulation Upgrade (formerly Warm Homes Plan)
This broader scheme is funded through the energy suppliers’ obligation mechanism under the Warm Homes Plan. It targets owner-occupiers and private renters in energy-inefficient homes and is means-tested. Energy suppliers administer applications, so the access point is your current energy supplier.
Self-Funded Cavity Wall Insulation
If you don’t qualify for a grant, self-funding is still reasonable. Cavity wall insulation is one of the cheaper home energy improvements — no scaffolding, no structural changes, work typically completed in a few hours.
What Does It Cost to Self-Fund?
Costs depend on property size and the material used:
| Property type | Typical cost |
|---|---|
| Mid-terrace (2–3 bed) | £900–£1,500 |
| Semi-detached (3 bed) | £1,500–£2,000 |
| Detached (4 bed) | £2,000–£2,700 |
Most installers use mineral wool (rock or glass fibre), polyurethane foam, or expanded polystyrene (EPS) beads. Mineral wool is the most common and generally recommended for standard cavity walls. Foam is sometimes used for wider or irregular cavities.
The installation process is non-disruptive: the installer drills holes at intervals in the external wall, injects the insulation material with a hose, and then fills the holes. A typical home takes 3–4 hours. There’s no need to move furniture or vacate.
Is My Home Suitable?
Cavity wall insulation isn’t appropriate for all properties. Unsuitable candidates include:
- Pre-1920 solid brick homes — solid walls have no cavity to fill (they need external or internal wall insulation instead)
- Exposed or coastal locations — driving rain can penetrate unfilled cavities better than filled ones; some areas require specialist assessment
- Previously filled cavities — check your home’s history; some 1980s EPS bead installations have degraded and need assessment before top-up
- Narrow cavities under 50mm — may not be fillable with standard materials
A qualified installer will carry out a survey before any work, checking cavity width with a borescope (a small camera inserted through a drill hole) and assessing the wall’s condition. The survey is free under grant schemes and typically free as part of a quote from reputable self-fund installers.
Damp Concerns
The main risk associated with cavity wall insulation is damp. Poorly installed insulation in exposed walls can allow moisture bridging — water tracking through the fill from the outer to inner leaf. This was a significant problem in some 1980s installations.
Modern mineral wool and EPS bead materials are designed to be moisture-resistant rather than moisture-blocking, and compliant installations should not cause damp in sheltered locations. For semi-exposed or coastal properties, an independent surveyor’s assessment before installation is worthwhile.
If your property has existing damp issues, resolve those first — insulation won’t fix underlying water ingress problems and may mask them.
Finding an Installer
Look for installers certified under the Cavity Insulation Guarantee Agency (CIGA) scheme. CIGA-registered installations come with a 25-year guarantee that covers remediation if problems develop. You can check whether a property already has a CIGA guarantee at the CIGA website — useful if you’re buying a home.
For grant-funded work, your local council or energy supplier will direct you to approved contractors. For self-funded work, get at least three quotes and verify CIGA registration for each.
The Bottom Line
If your home has unfilled cavity walls and you haven’t already looked into insulation, it’s worth starting with your local council’s Warm Homes Local Grant to see whether you qualify for free installation. If you don’t qualify, the self-funded cost is modest relative to the long-term savings and EPC uplift — most households in suitable properties will see payback within five years.
The one situation to avoid: don’t rush into a self-funded installation from a cold-calling installer offering a suspiciously cheap price. CIGA certification and a proper pre-installation survey are non-negotiable.