Loft Insulation Calculator
Last updated: 12 August 2026
Work out how many rolls of loft insulation you need. The current UK recommendation is 270mm of mineral wool, normally laid as 100mm between the joists and 170mm across them.
How the calculation works
Rolls = loft area ÷ coverage per roll. A standard 100mm mineral wool roll covers about 8 m² and a 170mm roll about 5.5 m², though pack sizes vary by brand — check the roll before ordering.
The recommended 270mm is achieved in two layers: 100mm laid between the joists, then 170mm laid at right angles across them. The cross layer matters, because it covers the joists themselves, which are otherwise a continuous thermal bridge.
Insulation depth and coverage
| Roll depth | Covers per roll | Use |
|---|---|---|
| 100mm | ~8 m² | Between joists (first layer) |
| 150mm | ~6 m² | Top-up layer |
| 170mm | ~5.5 m² | Over joists (second layer) |
| 200mm | ~4.5 m² | Single deep layer |
Important safety points
Never insulate under a cold water tank — leave the area beneath it clear so residual heat stops it freezing, and insulate the sides and top of the tank instead. Keep insulation clear of eaves ventilation gaps so the loft can breathe, and do not cover recessed downlights unless they are fire-rated and specified for covering. Boarding a loft over insulation needs raised loft legs, or you compress the wool and lose most of its value.
Frequently asked questions
How much loft insulation do I need?
The UK recommendation is 270mm of mineral wool. For a bare loft that means one 100mm layer between the joists plus a 170mm layer laid across them. A 48 square metre loft needs about 6 rolls of 100mm and 9 rolls of 170mm.
How many square metres does a roll of loft insulation cover?
Roughly 8 square metres for a 100mm roll and 5.5 square metres for a 170mm roll, though this varies by manufacturer — always check the pack coverage before ordering.
What depth should loft insulation be?
270mm is the current UK recommendation for mineral wool. Older homes often have 100mm or less, so topping up with a 170mm cross layer is one of the cheapest energy efficiency improvements available.
Should loft insulation go between or over the joists?
Both. The first 100mm sits between the joists, then a second layer is laid at right angles across the top. The cross layer is what covers the joists themselves, which would otherwise act as thermal bridges through the insulation.
Can I board over loft insulation?
Only with raised loft legs or a proprietary boarding system. Laying boards directly on 270mm of wool compresses it to the joist depth and destroys most of its insulating value, as well as risking condensation.
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Estimates are for guidance only. Material dimensions, densities and coverage vary by supplier — always confirm quantities before ordering, and consult a qualified professional for structural work.