Calculator
Insulation calculator: loft rolls and PIR boards
For loft rolls divide the area by the roll coverage for each layer (around 8 m² for 100 mm and 5 m² for 170 mm as a rough guide); for PIR boards divide the area by 2.88 m² per board and add 5 to 10 percent.
Enter your sizes, see the materials and a diagram, then send the list for a delivered price.
Quantities are estimates using standard UK rules of thumb and are shown with their working. We check them against your job before we price anything.
How the calculation works
- Loft: rolls per layer = area ÷ coverage (check the pack)
- Common loft target: 270 mm total (100 + 170 mm)
- PIR: boards = area ÷ 2.88, + 7%
Worked example
A 45 m² loft insulated from scratch to 270 mm: 45 ÷ 8.3 = 5.4, so 6 rolls of 100 mm, and 45 ÷ 4.9 = 9.2, so 10 rolls of 170 mm (check the coverage on your chosen roll).
What to order with it
- Mineral Wool Insulation: Loft rolls, cavity batts and acoustic slabs.
- PIR Insulation Boards: Foil-faced rigid boards for floors, walls and roofs.
- Tools & Site Safety: Hand tools, cutting discs, consumables and PPE.
Quantities are estimates. Pack sizes, bag weights and coverage vary by manufacturer; we confirm them on your quote.
Questions
How thick should loft insulation be?
270 mm in total is the common recommendation, usually as 100 mm between the joists and 170 mm across them.
How thick should PIR insulation be?
It depends on where it goes and the U-value required. Pitched roofs often need 100 mm or more between and under rafters; floors and walls vary. Use the maker's U-value tables or ask your designer.
Send your list, get a delivered price
Tell us what you need and where it is going. We check the quantities and come back with a delivered price before you commit.