Around a third of UK homes — mostly pre-1930s — have solid walls with no cavity, losing roughly a third of their heat through the walls. Solid wall insulation is the highest-impact, highest-cost fabric measure. Here’s how to get it right — and funded.
External (EWI): insulation boards fixed to the outside, finished with render or cladding. Around £18,000 for a typical three-bed semi (EST, May 2026). No lost room space, deals with thermal bridging best, refreshes the façade — but may need planning consent in conservation areas and changes the building’s appearance.
Internal (IWI): insulated plasterboard or stud systems on inside faces. Around £12,000 for a typical three-bed semi (EST, May 2026). Room-by-room flexibility and no external changes — but loses 5–10cm per wall, needs careful detailing around joists and windows, and rooms are disrupted during works.
For a typical solid-wall semi, properly installed wall insulation can cut heating demand by 20–30% — often £300–£500 a year at current prices, more for larger or draughtier homes. It also lifts EPC scores substantially, which matters for WH:LG eligibility (EPC D–G) and property value.
Solid walls handle moisture differently from modern cavity construction — they need to breathe or be designed with a clear condensation strategy. Poorly specified internal insulation can trap moisture and cause damp or timber decay. Insist on: a pre-install moisture assessment, vapour-control design appropriate to the wall build-up, and PAS 2030/2035 compliant installation. This is not a DIY measure.
ECO4 (to 31 Dec 2026) funds solid wall insulation for eligible benefits/Flex households — it’s one of the scheme’s highest-scoring measures. Warm Homes: Local Grant (England, EPC D–G, income criteria) lists insulation among fundable measures — 24% of all WH:LG installations to end-July 2026 were insulation. Outside grants, EWI/IWI qualifies for 0% VAT as an energy-saving material.
Facts checked 21 August 2026. We link to official sources and show when facts were last verified. If a scheme changes, we update the page.
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