Definition: Cavity wall insulation fills the gap between a home’s two brick walls with insulating material, cutting heat loss through walls by around a third. Typical cost: £500-£1,500.

Suitable homes

Most UK homes built after the 1920s have cavity walls. A specialist checks with a borescope; installation takes half a day with small drill holes refilled afterwards.

Payback and grants

Annual savings of £150-£300 give a 3-6 year payback — among the best of any upgrade. Eligible households can get it free under the Warm Homes: Local Grant.

Estimate your numbers: try the free savings calculator or browse all upgrade guides.