Loft insulation calculator
The Energy Saving Trust recommends at least 270 mm of loft insulation. That is usually a 100 mm layer between the joists and a 170 mm layer across them.
- 100 mm rolls (between joists)
- 3
- Top layer thickness
- 170 mm
- Top layer rolls
- 5
- Depth when finished
- 270 mm
- Loft area
- 40.0 m²
3 rolls of 100 mm between the joists, then 5 rolls of 170 mm laid across the joists, taking you to about 270 mm.
Roll sizes vary between brands and shops, so check the coverage on the pack. Don’t squash insulation, and keep it away from downlights and wiring.
Rolls for common loft sizes
Starting from an empty loft, with 100 mm between the joists and 170 mm across.
| Loft | Area | 100 mm rolls | 170 mm rolls |
|---|---|---|---|
| 6 × 4 m | 24 m² | 2 | 3 |
| 8 × 5 m | 40 m² | 3 | 5 |
| 9 × 6 m | 54 m² | 4 | 7 |
| 10 × 7 m | 70 m² | 6 | 9 |
| 12 × 8 m | 96 m² | 7 | 12 |
Coverage per roll
Knauf Loft Roll 44, as listed by one supplier. Check yours.
| Thickness | Covers per roll |
|---|---|
| 100 mm | 13.89 m² |
| 150 mm | 9.18 m² |
| 170 mm | 8.01 m² |
| 200 mm | 5.93 m² |
How we calculate this
If you have less than 100 mm, we start with a 100 mm layer between the joists. Then we pick the thinnest roll that takes you to at least 270 mm, laid across the joists.
We divide your loft area by the coverage of each roll and round up.
Questions people ask
How thick should loft insulation be?
The Energy Saving Trust recommends a minimum of 270 mm. If you already have some in good condition, you can add a layer on top to reach that.
Why two layers?
The first layer fills the space between the joists. The second goes across the joists, covering them so heat can’t escape through the timber.
Can I still store things in my loft?
Yes, but don’t squash the insulation, as that makes it work less well. Raised loft boards or legs let you board over it while keeping the full depth.
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