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, so it pays for itself within about 1-3 years. Cavity wall insulation costs £500-£1,500 and saves £150-£300 per year. These short payback periods make insulation an excellent investment, and the two measures together are the usual starting point for an older house. The loft alone accounts for 25% of heat loss in an uninsulated home.

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 closed on 31 March 2026 and ECO4 is winding down, so the main route in England is now the Warm Homes: Local Grant from your council, for privately owned and privately rented homes rated D to G where household income is usually £36,000 or less. This calculator estimates the materials needed, installation cost and annual savings for each insulation type based on your property's characteristics.

Building Regulations set the targets the work must meet, and Approved Document L Volume 1 has two sets of U-values, so check which applies. Upgrading an element you already have (the 'improved' column of Table 4.3) means 0.16 W/m²K for a loft or pitched roof, which 270mm of mineral wool achieves, 0.55 for a full-fill cavity wall, 0.30 for a solid wall insulated internally or externally, and 0.25 for a floor. A new element such as an extension (Table 4.2) is stricter: roof 0.15, wall 0.18 and floor 0.18 W/m²K. Replacement windows need 1.4 W/m²K or a Window Energy Rating of Band B. Higher numbers mean worse insulation. A retained element only has to be upgraded where it is technically feasible and pays back within 15 years, and the 2026 edition of Approved Document L keeps all these figures when it takes effect on 24 March 2027.

Loft insulation gives the biggest impact for the least money. The standard recommendation is 270mm of mineral wool, and if you already have 100mm it is easier and cheaper to lay the new layer on top. Doing it yourself costs £150-£300 in materials. Do not compress the insulation, because squashing it loses insulation value, and if the loft is used for storage fit raised access boards above it. Most lofts are ventilated, so no vapour control membrane is needed, but rooms-in-roof require careful detailing.

Around 90% of UK homes built since 1920 have cavity walls, which are insulated by blowing beads or mineral wool fibre into the gap. In exposed locations a filled cavity can suffer damp problems from rain driven into the wall, so ask the installer about the site. Houses from before 1920 usually have solid 9-inch brick walls with no cavity and need a different approach. Internal wall insulation (IWI) costs £40-£100/m² installed and external wall insulation (EWI) £80-£200/m². EWI performs better thermally but costs more and requires planning permission in conservation areas. Either route takes the wall from a U-value of about 2.0 to about 0.3 W/m²K, and the payback is measured in decades rather than years.

Heat pumps make insulation more important, not less. They run efficiently at a flow temperature of 35-45°C, compared with 70-80°C for gas, so in a poorly insulated home the system cannot keep up and electricity bills climb. The sensible order is loft and cavity wall insulation first, then draught-proofing of windows and doors, then the heat pump. The £7,500 Boiler Upgrade Scheme grant no longer requires loft or cavity insulation (that condition was dropped in 2024), but insulating first still lets you fit a smaller, cheaper heat pump. Have the heat loss calculated formally before the heat pump is specified, because an oversized unit is expensive and an undersized one leaves the house uncomfortable.