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What The Loft Insulation Savings Calculator Works Out
- Estimated saving per year: heat loss through the loft is its area x U-value (2.3 W/m2K bare, about 0.4 at 100mm, 0.16 at 270mm) x the heating degree days in the year x 24 hours, giving kilowatt-hours of gas, then priced at your unit rate (defaults to the current Ofgem price cap gas rate). Topping up from nothing cuts the loft's heat loss by over 90%; topping up from 100mm saves a more modest few tens of pounds, which is why "none to some" matters far more than "some to more".
- Typical install cost: DIY rolls at roughly GBP 10 to 15 a square metre lay in an afternoon (first layer between the joists, second across them); professional installs run two to three times that.
- Payback: install cost divided by the annual saving, shown as a range from DIY to fitted. For an uninsulated loft it is usually one to three years, the best payback of any efficiency measure in the house.
- Top up to (advanced): 270mm is the recommended depth; 300mm or 400mm cuts a little more but with sharply diminishing returns, since the U-value only moves from 0.16 to 0.11.
- Heating degree days (advanced): how cold the year is where you live. Around 2,100 is a UK average; the south coast is nearer 1,800 and Aberdeen or the Pennines 2,500 or more, so the same loft saves a quarter more in Scotland.
- Boiler efficiency (advanced): an inefficient boiler burns more gas for the same heat, so the saving is bigger. The basic figure assumes 100%; try 85% to 90% for a condensing boiler or 70% for an old one.
- Costs per m2 and grant (advanced): edit the DIY and fitted prices and enter the share a grant covers. ECO4 and the Great British Insulation Scheme cover many lower-income and lower-EPC households entirely, and checking eligibility is free.
- Saving over the period (advanced): the annual saving added up over your chosen number of years, optionally with gas prices rising each year, and shown net of the fitted cost. Mineral wool lasts 40 years or more if kept dry and uncompressed.
- Carbon saved (advanced): kilowatt-hours saved x 0.183 kg CO2 per kWh of gas. Keep the saving honest: insulate the hatch, lag pipes and tanks above the new layer, keep eaves ventilation, and never squash insulation flat with boarding (use loft legs).
Source and further reading: Energy Saving Trust: roof and loft insulation.