What is the recommended depth for loft insulation?
The Energy Saving Trust recommends at least 270 mm of mineral wool. In England, building regulations set a target U-value of 0.16 W/m²K when a loft is insulated, which LABC says is typically about 250 mm of mineral wool roll. In practice most installers lay 270 to 300 mm, the depth the Energy Saving Trust recommends.
How do I measure my loft insulation?
- Kneel on a board across the joists, never on the plasterboard between them
- Push a ruler or tape measure straight down through the insulation until it touches the ceiling
- Check in at least five places, including near the eaves and the hatch
- Write down the smallest reading as well as the typical one
What if my loft has 100 mm?
100 mm was common in lofts insulated in the 1980s and 1990s. Topping up to 270 mm is worth doing if the existing layer is dry and even. The Energy Saving Trust's figures put the saving from topping up 120 mm to 270 mm at about £17 a year for a semi-detached house, so the case is best made with comfort and the EPC in mind as well as the bill.
Can loft insulation be too deep?
Adding more beyond about 300 mm gives smaller and smaller returns. The bigger risks are blocking the eaves or burying cables and downlights, which matter more than the last few millimetres.
Related services
Sources: Energy Saving Trust: roof and loft insulation; Which?: loft insulation costs and savings; LABC: building regulation requirements for loft insulation; GOV.UK: extending the ECO4 end date, government response; GOV.UK: VAT on energy-saving materials (notice 708/6); Property Care Association: spray foam guidance.
