How Much Cavity Wall Insulation Do I Need?

How We Calculated This

For a typical 1930s–1980s 3-bed semi-detached house:

• Front wall: 7m × 5m = 35m²
• Rear wall: 7m × 5m = 35m²
• Side wall: 9m × 5m = 45m²
• Gross wall area: 115m²
• Minus windows and doors (~15–20%): ~92–98m²

Is Your House Suitable?

Cavity wall insulation is suitable if:

• Your home was built after 1920 (cavity walls became standard)
• The cavity is at least 50mm wide
• The walls are in good condition (no cracks, damp, or structural issues)
• The property is not severely exposed to driving rain

Types of Cavity Fill

• Mineral wool (blown): Most common, ~£5–8/m² installed
• EPS beads (bonded): Good for damp-prone areas, ~£8–12/m²
• Polyurethane foam: Premium option, ~£15–25/m²

Typical Costs (2026)

• Mid-terrace: £400–600
• Semi-detached: £600–900
• Detached: £800–1,400

Grants may be available through ECO4 or the Great British Insulation Scheme — check with your energy supplier or local authority.

Last updated: April 2026