According to the Energy Saving Trust, an uninsulated home loses around 25% of its heat through the roof. Insulating the loft is one of the simplest ways to cut that loss, and if installed correctly it should last around 40 years.
This guide covers the main materials, typical costs and savings, whether you can do it yourself, and where to look for help with the cost.
How Much Insulation Do You Need?
The recommended minimum depth for mineral wool loft insulation is 270mm. Other materials reach the same performance at different thicknesses. Many older homes have much less, and topping up still helps — although it saves far less than insulating an empty loft.
Types of Loft Insulation
Mineral Wool
Glass wool or rock wool rolls are the most common choice for loft floors. They are laid between the joists, then a second layer is laid across them. They are widely available and straightforward to fit, but you need gloves, a mask and eye protection, and the material must not be squashed.
Rigid Insulation Boards
Foam boards, such as PIR, give more insulation for a given thickness. They are often used between and over rafters when insulating at roof level or in rooms in the roof. This is a job for a professional.
Natural Fibres
Materials such as sheep’s wool, wood fibre or cork are alternatives to mineral wool. They usually cost more.
Multifoil Insulation
Thin multifoil products, such as SuperFOIL, are used where depth is limited. Their performance depends heavily on correct installation. See our multifoil comparison.
Typical Costs and Savings
The Energy Saving Trust’s figures for insulating a gas-heated home from no insulation to 270mm, based on fuel prices as of July 2026, are:
- Mid-terrace house: around £650 to install, saving around £140 a year
- Semi-detached house: around £750, saving around £190 a year
- Detached house: around £1,100, saving around £360 a year
- Detached bungalow: around £1,200, saving around £350 a year
Topping up from 120mm to 270mm costs around £500–£800 professionally and saves around £13–£35 a year, depending on the house. Costs are average unsubsidised prices; DIY is cheaper.
Dividing cost by saving gives a rough simple payback of around 3 years for a detached house, 4 years for a semi-detached house and about 4½ years for a mid-terrace — before any change in energy prices.
DIY or Professional?
The Energy Saving Trust says you can insulate the loft yourself if it is easy to access, free of damp and not a flat roof. Lay the first layer between the joists and the second at right angles across them to reach the full depth, and make sure you do not cover vents or block eaves ventilation.
Use a professional if:
- the loft is damp or has condensation problems
- it is hard to access
- you use the loft as a living space, or want to insulate at rafter level
- you have a flat roof
If you hire someone, get more than one quote, ask what guarantee they offer and how they will handle ventilation, and check them yourself — for example on the TrustMark register. The Energy Saving Trust does not recommend spraying foam onto the underside of a roof to fix a damaged or leaking roof.
Help With the Cost
- Warm Homes: Local Grant (England): council-run funding for insulation in lower-income households in homes rated EPC D–G.
- Scotland and Wales: Home Energy Scotland and Nest have their own grants and support.
- 0% VAT: professionally installed insulation is zero-rated for VAT until 31 March 2027.
- ECO4 ends on 31 December 2026, and the Great British Insulation Scheme closed on 31 March 2026. See our ECO4 explainer.
Common Mistakes to Avoid
- Blocking ventilation: lofts need airflow to prevent condensation. Keep eaves vents clear.
- Covering downlights: check whether recessed lights can be covered, and use suitable covers or insulation-contact rated fittings.
- Squashing insulation: storing things on it makes it less effective. Raise any boarding above the insulation.
- Forgetting the hatch: fit an insulated hatch and draught-strip its edges.
- Leaving pipes and tanks exposed: the loft gets colder once the floor is insulated, so lag pipes and fit a tank jacket to prevent freezing.
Source: Energy Saving Trust, “Roof and loft insulation” (figures based on fuel prices as of July 2026). Last reviewed September 2026.