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Loft Insulation Calculator
How many rolls for each layer, and what depth they add up to.
Part of the 🏠 Homeowners and DIYers toolkit, with more tools made for the job.
The area to insulate.
Layer one, between the joists
On the roll.
On the roll.
Layer two, on top (optional)
Leave empty for 10%.
Type the loft area and the depth and coverage of your rolls, and the rolls to buy appear here.
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Enter the loft area and the depth and coverage of the rolls for each layer. This shows how many rolls to buy for each layer and the depth they build up to.
Loft insulation is usually laid in two layers: a first layer between the joists, and a second on top, crossways, to cover the joists and stop heat escaping through them. Each layer is its own product with its own coverage per roll, so each is worked out separately and rounded up to whole rolls. The depth you need depends on where you live. Some building rules call for around 270 millimetres of mineral wool, and colder places want more. The tool does the quantity. You check what your local rules and advice say about depth.
How it’s worked out
- Area to cover is the loft area plus an allowance for cuts and waste. The tool uses 10 percent unless you change it.
- Rolls for a layer are that area divided by what one roll covers, rounded up.
- The same is done for the second layer, if you add one.
- Total depth is the depths of the layers added together, in millimetres or inches.
A 40 square metre loft with 10 percent extra needs 44 square metres per layer. With 11.08 square metre rolls of 100 mm that is 4 rolls, and with 6.47 square metre rolls of 170 mm it is 7 rolls, which makes 11 rolls and 270 mm in all.
Common mistakes
- Compressing the insulation. Squashed insulation does not insulate as well, so do not press it down or pile stored boxes on it.
- Covering the eaves. Leave a gap for air to flow at the edges, or damp can build up.
- Using the wrong coverage. Each product states its own, so use the one on the roll you are buying.
- Forgetting the safety side. Wear gloves, a mask and eye protection, and do not cover recessed lights or cables without checking the guidance.
Questions
How deep should loft insulation be?
It depends on your country and region, and on what is already there. Some building rules call for about 270 mm of mineral wool, and more in colder places. Check the guidance where you live.
Why two layers?
The first layer goes between the joists and the second crossways on top, so the joists do not leave cold gaps. The second is usually thicker and covers the joists.
Does it work in inches?
Yes. Choose imperial and give depths in inches and roll coverage in square feet.
Does it work for a flat roof or a wall?
The area and roll sums are the same, but flat roofs and walls need different products and care about moisture. Follow the maker's guidance.
Where the method comes from
Estimates only
These are estimates and general information, not building, safety, legal or professional advice. Materials, coverage, waste and rules vary by product, project and place, so check the maker's instructions and your local rules, and use a qualified person for gas, electrical and structural work.
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