Insulation Calculator

Insulation is one of the most cost-effective energy efficiency improvements for UK homes. Loft insulation to the recommended 270mm depth costs £300-£600 and saves approximately £180-£250 per year. Cavity wall insulation costs £500-£1,500 and saves £150-£300 per year. These short payback periods make insulation an excellent investment.

Solid wall insulation (internal or external) is more expensive at £4,000-£14,000 per property but saves £200-£400 per year. Floor insulation costs £500-£1,000 for a timber floor and saves £40-£70 per year. Draught-proofing windows and doors costs £100-£300 and saves £30-£60 per year.

Government schemes may help cover the cost. The Great British Insulation Scheme provides insulation measures free or at reduced cost to eligible households. This calculator estimates the materials needed, installation cost and annual savings for each insulation type based on your property's characteristics.

UK Building Regulations U-values 2026. Loft: 0.16 W/m²K (270mm mineral wool or equivalent). Cavity walls: 0.30 W/m²K (full-fill cavity insulation). Solid walls: 0.30 W/m²K (internal or external solid wall insulation). Floor: 0.18 W/m²K. Roof (pitched): 0.16 W/m²K. Windows: 1.4 W/m²K (modern double glazing). Higher numbers = worse insulation. Existing buildings: 'reasonable provision' applies — typically retrofit to current standards when major refurbishment occurs.

Loft insulation — biggest impact for least money. Loft is responsible for 25% of heat loss in uninsulated homes. 270mm mineral wool (standard recommendation): £400-£900 typical UK detached install. Saves £200-£500/year heating. Payback: 1-3 years. DIY install: £150-£300 materials only. Apply over existing 100mm: easier and cheaper. Don't compress (loses insulation value). Access boards needed if using loft for storage. Vapour control: most lofts ventilated (no membrane needed); rooms-in-roof require careful detailing.

Cavity wall insulation — when to fit. 90% of UK homes built since 1920 have cavity walls. Insulation: blown beads or mineral wool fibre into gap. Cost £400-£1,500 typical UK house. Saves £300-£500/year heating. Payback 2-5 years. ECO4 grants available for low-income households (free in some cases). Caution: exposed locations may suffer damp problems (rain-driven cavity fills). Pre-1920 solid walls: NOT cavity — need solid wall insulation (internal or external — more expensive £8,000-£15,000+).

Solid wall insulation — Victorian/Georgian properties. Pre-1920 brick homes: solid 9-inch walls, no cavity. Internal wall insulation (IWI): £40-£100/m² installed. External wall insulation (EWI): £80-£200/m² installed. EWI better thermally but expensive, requires planning permission in conservation areas. Both significantly improve U-value (from ~2.0 to ~0.3 W/m²K). Typical 3-bed Victorian terrace: £8,000-£20,000 full insulation upgrade. Saves £500-£1,200/year heating. Payback 12-30 years depending on energy prices.

Heat pumps and insulation — order of operations. Heat pumps need well-insulated home to be cost-effective. Run efficiently at 35-45°C flow temp (vs gas 70-80°C). Poorly insulated home + heat pump = high electricity bills, system can't keep up. Recommended order: (1) loft + cavity wall insulation; (2) windows/doors (draught-proof); (3) heat pump install. ECO4 and Boiler Upgrade Scheme £7,500 grant require minimum insulation standards. Calculate heat loss formally before installing heat pump — oversized = expensive; undersized = uncomfortable.