Cavity Wall Insulation Calculator — Area, Cost & Savings

In 2026 cavity wall insulation costs roughly £35–£55 per m² installed. The Energy Saving Trust quotes a typical installed cost of around £2,700 in Great Britain. By house type, expect roughly £1,500 for a mid-terrace, £2,000–£2,750 for a semi-detached house (EST ~£2,700) and around £4,600 for a detached house. The price depends on property size, access, cavity width and insulation type — blown mineral wool is usually cheapest, while injected foam costs more but can give better thermal performance.

Energy Saving Trust figures (gas-heated home, GB) put the annual heating-bill saving at about £410 for a detached house, £240 for a semi-detached house, £180 for a detached bungalow, £140 for a mid-terrace and £110 for a mid-floor flat. Savings depend on how much exposed external wall the property has, not on which fill material is used — mineral wool, EPS bead and PU foam all reach a similar in-situ wall U-value in the same cavity. Try our Heat Loss Calculator to see how insulation affects room-level heating demand.

Most UK houses built after 1920 have cavity walls (two layers of brick or block with a gap between). Houses built before 1920 typically have solid walls. A surveyor can confirm by drilling a small hole or using a borescope. Cavities in exposed locations or with existing damp may not be suitable for all insulation types.

The three main types are: blown mineral wool fibre (most common and cheapest), blown polystyrene beads (EPS, good for damp areas), and injected polyurethane foam (best thermal performance but most expensive). All are installed by drilling small holes in the outside wall and injecting the material under pressure.

When installed correctly in suitable properties, cavity wall insulation should not cause damp. However, in properties with existing damp issues, severely exposed locations, or damaged brickwork, cavity fill can sometimes create a moisture bridge. A proper survey before installation is essential to identify any risks.