Independent information
This is an independent information service. Cost estimates and savings figures are compiled from Energy Saving Trust publications and government sources. Actual costs and savings depend on your property's specific characteristics. Always get a professional survey before proceeding.
What Is Cavity Wall Insulation?
According to the Energy Saving Trust, around a third of heat in an uninsulated home escapes through the walls. Cavity wall insulation fills the gap between the inner and outer layers of your external walls.
How Cavity Walls Work
Most homes built from the 1920s onwards have external walls made of two layers of brick or block with a gap (cavity) between them. Originally, this cavity was left empty to prevent moisture passing through to the inner wall.
Unfortunately, the empty cavity also allows warm air to circulate and escape. Filling this gap with insulation material dramatically reduces heat loss whilst still allowing the wall to breathe.
Is My Home Suitable?
Properties built between the 1920s and 1990s typically have cavity walls. You can check by examining the exterior brickwork — if the bricks are all laid lengthways in a uniform pattern, your walls are likely cavity construction. Your EPC report will also confirm your wall type.
Key facts
At a glance
- Cost: £500–£1,500 according to Energy Saving Trust — but often free through GBIS
- Duration: Typically completed in a few hours
- Disruption: Minimal — work is done from outside the property
- EPC impact: Can raise your rating by 1–2 bands
- Guarantee: CIGA-backed 25-year guarantee standard
Installation Process & Suitability
How it's done
The process
An installer drills small holes (roughly 22mm) at regular intervals in the outer wall. Insulation material — usually mineral wool or polystyrene beads — is injected through these holes to fill the cavity.
The holes are then sealed and colour-matched to the existing mortar. The whole process is typically completed in a few hours.
Who qualifies
Suitable properties
- Built 1920s to 1990s (cavity construction era)
- Unfilled cavity walls confirmed by survey
- Walls in good structural condition
- No significant damp or moisture issues
- Not in a severely exposed location with persistent driving rain
Exclusions
Not suitable for
- Properties with existing damp problems
- Unusual or non-standard construction
- Some properties in conservation areas (check with your local authority)
- Very exposed coastal or hilltop locations
- Walls with damaged or blocked cavities
If cavity wall insulation is not suitable, solid wall insulation may be an alternative.
Check If You Qualify for Free Cavity Wall Insulation
Many households can get cavity wall insulation installed completely free through the Great British Insulation Scheme (GBIS). Check your eligibility in minutes.
Key eligibility factors:
- ECO4 closed March 2026
- Insulation routes now via GBIS
- Heat pump routes now via BUS
Check your property against the live MHCLG EPC register to see your current band and every grant you qualify for — free, in under a minute.
Check your propertyGet a free cavity wall survey
A professional surveyor will check whether your property is suitable for cavity wall insulation and confirm your eligibility for free installation through GBIS.
No obligation. We'll connect you with certified installers in your area.
How to get cavity wall insulation
Check eligibility
Use our eligibility checker or contact us. If you receive qualifying benefits or have a low EPC rating, you may qualify for free installation.
Property survey
A qualified surveyor inspects your walls to confirm cavity construction, check for damp, and assess suitability for insulation.
Installation day
Installers drill, inject, and seal — typically completed within a few hours. Minimal disruption, all work done from outside.
CIGA guarantee
You receive a 25-year CIGA-backed guarantee. Your EPC can be updated to reflect the improvement.
Related guides
Grant scheme
Warm Homes: Local Grant
Whole-home upgrades including insulation for lower-income households. Up to £30,000 across multiple measures.
Read guide →Grant scheme
GBIS Scheme
Another route to free cavity wall insulation through the Great British Insulation Scheme.
Read guide →EPC guide
Improve your EPC rating
See how cavity wall insulation boosts your EPC and what other improvements to consider.
Read guide →Ready to insulate your cavity walls?
Check if you qualify for free installation through the Great British Insulation Scheme (GBIS) — many homeowners pay nothing.