From £750–£900 for a standard loft top-up (EST, May 2026) to £3,000–£7,500 for room-in-roof and flat-roof systems. Every roof type priced below — and the funding routes that can remove the cost.
Loft (270mm quilt): ~£900 installed from bare joists; ~£750 for a 120→270mm top-up. Room-in-roof: £3,000–£7,500 depending on slope area and boarding. Flat roof (warm-deck): £80–£120/m², typically £2,500–£5,000 — best value when re-covering anyway. Pitched (between/under rafters): £45–£75/m² with PIR.
Loft quilt: £18–£28/m². Rafter-level PIR: £45–£75/m². Warm flat roof: £80–£120/m². All at 0% VAT to 31 March 2027; all EST-anchored (May 2026) with labour included. Access, storage-boarding and ventilation detailing move real quotes within these ranges.
Loft and room-in-roof insulation are core measures in ECO4 (to Dec 2026) and the Warm Homes: Local Grant (to Mar 2028) — insulation was 24% of all WH:LG installs to end-July 2026. Could be fully funded for some eligible households; eligibility does not guarantee installation or zero contribution.
If the loft is under 270mm, the top-up beats everything in payback (1–3 years). Room-in-roof matters most in converted spaces that roast in summer and freeze in winter. Flat roofs: insulate when the covering is renewed. Run the 60-second check to see your order.
AC-coupled (battery has its own inverter) is the retrofit default: no rewiring of your array, works with any brand or age of system, £3,500–£6,000 installed for 5–10kWh usable. DC-coupled only makes sense if your inverter is due replacement anyway — a hybrid inverter + battery swap can be cheaper combined.
Feed-in Tariff payments are unaffected by adding storage — generation payments continue and deemed export isn’t metered down. On SEG, a battery lets you shift export into peak-price windows instead.
Match usable capacity to your evening consumption and summer surplus — the sizing guide has the tables. Most 4kWp homes land at 5–8kWh usable.
A standard loft costs ~£900 (270mm from bare), a top-up ~£750; room-in-roof £3,000–£7,500; flat roofs £80–£120/m² (EST-anchored, May 2026).
Via ECO4 (to Dec 2026) or WH:LG (to Mar 2028) it could be fully funded for some eligible households — eligibility doesn't guarantee installation or zero contribution.
270mm mineral-wool loft quilt — lowest cost, fastest payback of any home-energy measure.
Warm-deck (insulation above the deck) is the modern standard — it avoids condensation risk in the structure.
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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