Loft Insulation: How Many Rolls Do I Need to Reach 270 mm?
A 35 m² loft with 100 mm of insulation needs 8 rolls of 100 mm insulation to reach 270 mm, allowing 5% for cuts. With none, it takes 13 rolls. The charts below cover other lofts, and the page explains how to lay the layers.
Rolls to top up to 270 mm
For a 35 m² loft, with typical rolls (a 100 mm roll covers about 8.2 m², a 170 mm roll about 4.8 m²) and 5% for cuts:
| Insulation already there | Depth to add | Rolls to buy |
|---|---|---|
| None | 270 mm | 13 |
| 50 mm | 220 mm | 10 |
| 100 mm | 170 mm | 8 |
| 150 mm | 120 mm | 6 |
| 200 mm | 70 mm | 4 |
The number of rolls is the same whether you buy 100 mm or 170 mm rolls, because a thicker roll covers proportionally less floor: it is the same amount of insulation, and thicker rolls simply mean fewer layers to lay.
Rolls by loft size
To reach 270 mm using 100 mm rolls, with either 100 mm already there or none at all:
| Loft area | With 100 mm there | With none |
|---|---|---|
| 20 m² | 5 | 7 |
| 30 m² | 7 | 11 |
| 40 m² | 9 | 14 |
| 50 m² | 11 | 18 |
| 60 m² | 14 | 21 |
The coverage printed on your pack is what counts. Enter it in the loft insulation calculator if it differs from our typical figure.
How to lay it
- Lay the first layer between the joists, up to the top of them, pushing it right into the gaps without squashing it. A typical joist is about 100 mm deep.
- Lay the second layer across the joists, at right angles, which covers the timber and reduces the heat lost through it.
- Keep the eaves clear with a vent tray, so that air can still flow in under the roof.
- Leave a gap around recessed downlights and flues (unless they are fire-rated and covered with a purpose-made hood), and keep insulation off cables that carry a heavy load.
- Insulate the loft hatch, and fit a draught seal. Do not insulate under the cold water tank, but do insulate the tank and the pipes.
Before you buy
- Measure the depth you have. Use a ruler in several places. Old insulation is often thinner than you think and unevenly spread.
- Check the loft for damp, leaks and wiring. Fix a leak, and get an electrician to look at old cabling, before you cover it.
- Think about storage. If you want to store things in the loft, use raised boards on stilts so that they sit above the insulation without squashing it.
- Check for grants and schemes. Government and energy-supplier schemes may help with the cost, depending on your home and income.
Mistakes that cost people money
- Squashing the insulation. It works by trapping air, and compressed wool insulates less.
- Blocking the eaves. That stops the roof from breathing and can lead to condensation and damp.
- Covering up recessed lights that are not rated for it.
- Leaving the loft hatch uninsulated, so that heat escapes through the biggest gap.
Frequently asked questions
How many rolls of loft insulation do I need for a 35 m² loft?
To go from 100 mm to 270 mm you need 8 rolls of 100 mm insulation, allowing 5% for cuts. If the loft has none, 270 mm takes 13 rolls of 100 mm.
How thick should loft insulation be?
The usual recommendation for mineral wool in the UK is 270 mm. Older homes often have 100 mm or less, so topping up is one of the cheapest ways to cut heating bills.
Can I put new loft insulation over old?
Usually yes: lay the new layer across the joists on top of the old, provided it is dry and in good condition. Do not squash the old layer or add a vapour barrier between them.
Do I need to leave gaps in the insulation?
Leave the eaves clear so that the roof can breathe, and keep insulation away from downlights, flues and heavily loaded cables, unless the products are rated for it.
More guides
Tools & materials for this project
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Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and check the Building Regulations and any planning rules for anything structural or close to a boundary.