Cavity Wall vs Solid Wall Insulation: Costs, Savings and Which Your Home Needs (2026)

Cavity wall insulation costs about £2,200 and saves about £200 per year on energy bills for a typical semi-detached house. Solid wall insulation costs about £12,000 (internal) to £15,000 (external) and saves about £280 per year in the same house (Energy Saving Trust). Cavity wall insulation pays for itself in about 11 years. Solid wall insulation takes about 43 to 54 years. Around 71% of UK cavity wall homes are already insulated, but millions of solid wall properties remain untreated. Both measures can be fully funded under ECO4 for eligible households.

How Do Cavity Wall and Solid Wall Insulation Compare?

Sources: Energy Saving Trust, GOV.UK GBIS Statistics

How Do You Know If You Have Cavity Walls or Solid Walls?

Most UK homes built between the 1920s and 1990s have cavity walls. A cavity wall is two layers of brick with an air gap between them, typically 50-75mm wide. Solid walls are a single layer of brick or stone, usually 225mm or thicker.

The quickest check is measuring your wall thickness at a door or window opening. Walls thicker than 260mm are usually cavity. Walls around 225mm or less are solid. You can also check your home’s age: properties built before 1920 almost always have solid walls, while those built after 1930 almost always have cavities.

An EPC (Energy Performance Certificate) will confirm your wall type and whether insulation has already been installed. You can check your EPC free at GOV.UK.

What Does Cavity Wall Insulation Cost and Save?

Cavity wall insulation is one of the most cost-effective insulation measures available to UK homeowners. It was the single most common installation under the Great British Insulation Scheme, accounting for 51,400 measures — 37% of all 139,200 installations delivered before the scheme closed in March 2026 (GOV.UK).

Costs by property type (Energy Saving Trust):

• Mid-terrace house: about £1,100
• Semi-detached house: about £2,200
• Detached house: about £3,900

An installer drills small holes in the outer wall, injects insulation material (mineral wool, polystyrene beads or polyurethane foam), then seals the holes. The entire job takes 2-4 hours with no interior disruption.

Annual savings range from about £110 for a mid-terrace to £380 for a detached home (Energy Saving Trust).

What Does Solid Wall Insulation Cost and Save?

Solid wall insulation delivers the highest annual savings of any insulation type — about £500 per year for a detached house (Energy Saving Trust). The trade-off is a much higher upfront cost.

There are two methods:

• Internal wall insulation (IWI): about £12,000 for a typical three-bedroom semi (Energy Saving Trust). Insulation boards fixed to interior walls. Reduces room size by 10-15cm per treated wall.
• External wall insulation (EWI): about £15,000 for a typical three-bedroom semi (Energy Saving Trust). Insulation boards fixed to the outside and covered with render. Changes the building’s external appearance.

For a semi-detached home, the Energy Saving Trust puts the saving at about £280 a year, whether the insulation goes inside or outside.

Learn more about the differences in our guide to internal vs external wall insulation.

Which Homes Need Solid Wall Insulation?

Around 7.6 million homes in England have solid walls, and the vast majority remain uninsulated (Department for Energy Security & Net Zero). These include:

• Victorian and Edwardian terraces (pre-1919)
• Georgian townhouses
• Stone-built rural cottages
• Some 1920s properties in certain regions

If your home was built before 1920, it almost certainly has solid walls. Read our guide to the best insulation for Victorian houses for specific recommendations.

Can You Get Cavity Wall or Solid Wall Insulation Free?

The ECO4 scheme runs until December 2026 and can cover the full cost of both cavity and solid wall insulation for eligible households (GOV.UK).

You may qualify if you:

• Receive a qualifying benefit (Universal Credit, Pension Credit, Income Support, JSA, ESA, Housing Benefit, Child Tax Credit or Working Tax Credit)
• Have a household income below your council’s LA Flex threshold (typically £31,000-£38,000)
• Live in a property rated EPC band D, E, F or G

The Great British Insulation Scheme (GBIS) closed in March 2026, but ECO4 remains open. Funding is limited and allocated on a first-come basis, so early application is strongly recommended.

Visit our insulation grants guide for full eligibility details.

Frequently Asked Questions

Is cavity wall insulation worth it in 2026?

Yes. At about £2,200 with annual savings of about £200 for a semi-detached house (Energy Saving Trust), cavity wall insulation pays for itself in about 11 years. It is one of the best-value energy efficiency measures for eligible homes and may be available free under ECO4.

Can you have cavity wall insulation in a 1930s house?

Yes. Most 1930s houses have unfilled cavity walls and are prime candidates for retrofill insulation. The cavity gap in 1930s properties is typically 50-65mm, which is sufficient for injection. A surveyor will check for any issues like rubble in the cavity before installation.

Does solid wall insulation cause damp?

When installed correctly with vapour-permeable materials, solid wall insulation should not cause damp. Problems arise when non-breathable materials trap moisture inside the wall. Always use an MCS or TrustMark-certified installer who understands moisture management in solid walls.

Which insulation should I do first — walls or loft?

Start with loft insulation if yours is missing or below 270mm. It costs less (about £750 for an empty loft or £600 for a top-up, Energy Saving Trust) and, where the loft starts empty, delivers the best return per pound spent. Add wall insulation next to maximise savings.

How long does solid wall insulation last?

Both internal and external solid wall insulation last 30+ years when properly installed and maintained. External systems require periodic render maintenance (typically every 15-20 years).

Ready to find out what insulation your home needs and what it will cost?

Planning this upgrade? Estimate your costs with the free savings calculator or request a free quote from up to three approved installers who cover your area.