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Internal · sheets · packs
How many insulation boards do I need?
Rigid board by application. Floor sheets, rafter sheets and cavity boards come in different sizes — the coverage per sheet is what changes the order, not the thickness.
1
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.
2
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.
How this one works
Sheets = area × (1 + waste %) ÷ sheet coverage
R-value of the layer = thickness ÷ 0.022 for PIR. The build-up total will be higher once you add everything else
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
Foil tape roll
50 m
Sheets per pack
6 no.
Varies with thickness — thicker boards, fewer per pack.
Conductivity
0.022 W/mK
PIR board. Mineral wool is about 0.035, EPS 0.038.
A worked example
With the defaults the page opens on:
How much area? 60 square metres
Where is it going? Floor — 2400 × 900
How thick? 25 mm
Spare for cuts 7.5 per cent
Boards
Area: 60 m²
Coverage per sheet: 2.16 m²
Waste allowance: 7.5 %
Sheets: 30 no.
Packs: 5 packs — at 6 a pack
Foil tape: 2 rolls — joints taped both ways
R-value of this layer: 1.1 m²K/W— thickness ÷ 0.022 W/mK
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 insulation boards do I need?
Area × waste ÷ coverage per sheet. The coverage changes with the application - 2.16 m² for a 2400 × 900 floor sheet, 2.88 m² for a rafter sheet, 1.08 m² for a cavity board - which is what changes the order, not the thickness.
What is the R-value of the layer?
Thickness ÷ conductivity. For PIR at 0.022 W/mK, 100 mm gives 4.5 m²K/W. It is the R-value of this layer only; the build-up total is higher once everything else is added, and what Part L or your SAP calculation asks for is a separate question.
How much foil tape?
Joints taped both ways: about 2.4 m per sheet, in 50 m rolls.
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.
