Definition: Solid wall insulation adds an insulating layer to walls without a cavity — either internally or externally. It’s the biggest single heat-loss fix for pre-1920s homes, typically £8,000-£15,000.

Internal vs external

Internal is cheaper (£8,000-£12,000) but shrinks rooms slightly; external (£10,000-£15,000) keeps interiors untouched and refreshes the facade, but may need planning permission.

Savings

Cutting wall heat loss by up to 40% saves £300-£500 a year in a typical solid-wall semi. Fully funded for eligible households under the Warm Homes: Local Grant.

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