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How to insulate an attic or loft
Heat rises, and an uninsulated roof is one of the biggest sources of heat loss in a house. Insulating a loft is often one of the most cost-effective energy improvements you can make.
1. Cold attic or warm roof?
If the attic is only used for storage, insulate the floor of the attic and keep the roof space above cold and ventilated. If you want to use the space as a room, insulate between and under the rafters instead, which is more complex. See Insulation materials compared: wool, EPS, XPS, PIR for materials.
2. How thick?
Recommended attic insulation levels are high: in many climates 25–30 cm of mineral wool or more. Lay one layer between the joists and a second layer across them to cover the joists, which are thermal bridges. The Insulation calculator works out thickness and R-value.
3. Ventilation and vapour control
Keep the eaves vents clear so the cold roof space stays dry; use eaves trays or baffles. In a warm roof, a vapour control layer on the warm side stops moist indoor air reaching the cold parts of the roof. Seal gaps around the loft hatch, pipes and light fittings, which let warm moist air through.
4. Work safely
Wear gloves, long sleeves, eye protection and a dust mask when handling mineral wool. Walk only on the joists or on boards laid across them. Do not cover recessed lights unless they are rated for it, and keep insulation clear of chimneys and flues.
5. Storage over insulation
Do not compress insulation with boards laid directly on top: compressed insulation loses much of its value. Use raised loft legs or platforms to keep storage boards above the full insulation depth.
Common mistakes
- Blocking the eaves ventilation.
- Compressing insulation under storage boards.
- Leaving gaps around the loft hatch and pipes.
- Covering recessed lights or flues.
- Only one thin layer between the joists.
Frequently asked questions
How thick should loft insulation be?
Often about 25–30 cm of mineral wool, depending on climate and local rules.
Should I insulate the attic floor or the roof?
The floor if the attic stays cold storage; the roof if you want a warm room.
Do I need a vapour barrier in the loft?
For a cold loft, sealing air leaks is usually more important; warm roofs need a vapour control layer.
Can I board over loft insulation?
Yes, if you raise the boards above the insulation so it is not compressed.
Is loft insulation worth it?
It is usually one of the most cost-effective energy savings in a house.