How this calculator works
Work out how many millimetres you still need, which is the depth you want less the depth you have. Multiply that by the loft area to get the volume to fill, then divide by the thickness of the roll to find how many square metres of roll you must lay, and by the coverage of one roll to get the number of rolls. We add an allowance for cuts.
Worked example: a 7 × 5 m loft (35 m²) with 100 mm already down, topped up to 270 mm with 100 mm rolls: you need to add 170 mm. That is 35 × 170 ÷ 100 = 59.5 m² of 100 mm roll, 62.5 m² with 5% for cuts. A 100 mm roll covers about 8.2 m², so you need 8 rolls (7.6 rounded up).
Rolls to top up to 270 mm (35 m² loft, 5% for cuts, typical roll coverage)
By the depth already there. A thicker roll covers proportionally less, so the number of rolls is about the same whatever thickness you buy.
| 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 |
Tips before you buy
- About 270 mm of mineral wool is the usual recommendation for a loft in the UK. Deeper is better, but there are diminishing returns, and 300 mm is a sensible limit if you need the space for storage.
- Lay the first layer between the joists and the second across them, which covers the joists and reduces the heat lost through the timber.
- Do not squash the insulation: it works by trapping air, and compressed wool insulates less.
- Leave the eaves clear so that air can flow in, and keep insulation away from recessed downlights unless they are rated for it, and from cables that carry a heavy load, which can overheat.
- Do not insulate under the cold water tank, so that heat from the room below keeps it from freezing, but do insulate the tank itself and the pipes.
- Wear gloves, a mask and long sleeves, and step only on the joists. A grant or a free scheme may be available: check what applies in your area.
Tools & materials for this project
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Frequently asked questions
How thick should loft insulation be?
The usual recommendation for mineral wool in the UK is 270 mm. Many older homes have less than 100 mm, and topping up is one of the cheapest ways to cut heating bills.
How many rolls of loft insulation do I need for a 35 m² loft?
To go from 100 mm to 270 mm you need about 8 rolls of 100 mm insulation, allowing 5% for cuts. If the loft has none, 270 mm takes about 13 rolls of 100 mm.
Can I put new loft insulation on top of old?
Usually yes: lay the new layer across the joists on top of the old, provided the old insulation is dry and in good condition. Do not use a vapour barrier between the layers.
Do I need to leave gaps in loft insulation?
Leave the eaves clear so that the roof can ventilate, and keep insulation away from downlights, cable bundles and flues unless the product and the fitting are rated for it.
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Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.