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Internal · rolls · depth · R-value
How many rolls of loft insulation do I need?
Rolls for a loft in two layers - one between the joists, one across them - and the depth it reaches against the depth you are aiming for. The coverage of a roll falls as it gets thicker, so it is read off the pack, not guessed.
Measure it
Every box has its own unit next to it.
Change the unit beside any box and the number converts. The (i) beside each question says what the box wants and where the figure usually comes from. Anything marked per cent is a percentage of the quantity above it.
Check the rates
Rates and assumptions used
These are the figures the calculator works to. Change any of them and the order list updates — your merchant’s pack sizes and your own waste allowances beat the defaults every time.
Order list
The orange figure is the one you order to. Highlighted rows are the other numbers you order to. The small print under each row is the sum that produced it, so you can check it against your own take-off. Amber rows are warnings, not quantities.
How this one works
- Rolls per layer =
area × (1 + waste %) ÷ coverage of one roll, rounded up - Depth when done =
already there + between the joists + across the joists - R-value of what you lay =
thickness ÷ conductivity
The figures the calculator works to. Every one can be changed in step 2, and your merchant’s pack sizes and your own waste allowances beat these defaults every time.
| Rate | Default | Why |
|---|---|---|
| Conductivity | 0.044 W/mK | Mineral wool loft roll is about 0.040 to 0.044. Check the pack. |
A worked example
With the defaults the page opens on:
- How long is the loft? 8 metres
- How wide is it? 5.5 metres
- How much is there already? 0 millimetres
- What depth are you aiming for? 270 millimetres
- Between the joists 100 mm
- What does one roll of that cover? 8.3 square metres
- Across the joists 170 mm
- What does one roll of that cover? 5.6 square metres
- How much spare? 5 per cent
- What does a roll between the joists cost? £20.00
- What does a roll across the joists cost? £25.00
Loft roll
- Loft area: 44 m² — L × W
- Rolls between the joists: 6 no. — area +5% ÷ 8.3 m² a roll (100 mm)
- Rolls across the joists: 9 no. — area +5% ÷ 5.6 m² a roll (170 mm)
- Depth when done: 270 mm — 0 + 100 + 170 mm
- Against your target: reaches 270 mm — what is there already + the two layers
- R-value of what you lay: 6.1 m²K/W — (100 + 170 mm) ÷ 0.044 W/mK
Budget
- Rolls cost: £345 — excludes boarding, delivery and VAT
Change any figure above and every line recalculates; the small print under each row is the sum that produced it.
Questions people ask
How many rolls of loft insulation do I need?
Area plus a little spare, ÷ what one roll covers, for each layer. An 8 m × 5.5 m loft (44 m²) at 5% spare takes 6 rolls of 100 mm at 8.3 m² a roll between the joists and 9 rolls of 170 mm at 5.6 m² a roll across them.
Why two layers?
The first fills between the joists; the second lies across them at right angles, so the joists are covered too and the gaps close. 100 mm + 170 mm gives the 270 mm the calculator starts at.
How deep should loft insulation be?
That is your target. The calculator starts at 270 mm, the depth usually quoted for mineral wool; the scheme or regulations you are working to set the one that applies. Give what is there already and it tells you how far short or over the layers leave you.
Why is the roll coverage typed in?
A thicker roll covers less, and it differs from maker to maker. It is printed on the pack, so the calculator asks for it rather than guessing.
Calculators for the same job
Doing this for a whole job rather than one wall? SiteLedger keeps the valuations, supplier statements, invoices and variations against the project they belong to.
Read before you order. These work out quantities, not structural design — spans, lintels, rebar and foundation depths need an engineer and Building Control sign-off. Waste allowances are defaults, not promises: awkward cuts, long runs and bad weather all eat more than 5%. Where a merchant sells whole packs, the figure is rounded up to whole packs.