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 about £12,000 (internal) to £15,000 (external) for a three-bedroom semi, according to the Energy Saving Trust.
Internal vs external
Internal is cheaper (about £12,000 for a typical semi) but shrinks rooms slightly; external (about £15,000) keeps interiors untouched and refreshes the facade, but may need planning permission.
Savings
Cutting wall heat loss by up to 40% saves about £280 a year in a typical solid-wall semi (Energy Saving Trust). Fully funded for eligible households under the Warm Homes: Local Grant.
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Prices for 160 towns, with the grant position for your nation.