How much insulation do I need?
Enter the area and we'll work out how many PIR insulation boards (Celotex/Kingspan-type) you need and the cost — with the calculation shown. A standard board is 2.4 m × 1.2 m, so 2.88 m² each.
1 m² per m².
| PIR insulation board (50mm) | 8 × 2.4×1.2 board | £224 |
- 1Area: 20 m²
- 2Usage rate: 1 m² per m²
- 3Allowing for waste: 20 × 1 × 1.1 waste = 22 m²
- 4Packs: 22 m² ÷ 2.88 per pack = 8 × 2.4×1.2 board
- Material cost only — labour is estimated separately in the schedule.
- Indicative trade-counter price; confirm with a current supplier quote.
How to work out insulation boards
- 1Measure the area
Length × width of the wall, floor or roof you're insulating, in metres.
- 2Divide by board size
A standard PIR board is 2.4 m × 1.2 m = 2.88 m². So a 20 m² area needs about 20 ÷ 2.88 ≈ 7 boards.
- 3Choose the thickness
Thickness (e.g. 50, 75, 100 mm) is set by where it goes and the U-value target — floors and walls differ. The calculator prices your chosen board.
- 4Add ~10% for cuts
Allow for cutting around joists, studs and services — boards notch and offcut more than flat sheets.
Rigid PIR board (Celotex, Kingspan and similar) is the go-to for floors, warm roofs and insulated dry-lining because it packs a low U-value into a thin board. Working from area ÷ board size gives you the sheet count; just remember the thickness is a separate decision set by building regs, not by how many boards you need.
PIR insulation coverage (indicative)
| Board | Covers | Notes |
|---|---|---|
| 2.4 m × 1.2 m board | 2.88 m² | Standard sheet size |
| 20 m² area | ~7 boards | Before waste |
| Common thicknesses | 50–100 mm | By U-value target |
| Waste allowance | ~10% | Cuts & notches |
Thickness depends on the application and target U-value. Add ~10% waste. Indicative — confirm with quotes.
Frequently asked questions
How much insulation do I need?+
Measure the area and divide by the board size (2.88 m² for a standard 2.4 × 1.2 m PIR board). A 20 m² area needs about 7 boards, plus ~10% for cuts. The thickness depends on your U-value target.
What thickness of insulation do I need?+
It depends on the location and the U-value you're targeting — floors, walls and roofs differ. As a rough guide, 100 mm+ PIR is common in floors and warm roofs; check current building-regs U-values for your build.
How many PIR boards in a pack?+
Boards are usually sold individually or in packs that vary by thickness (thinner boards come more per pack). Work out the number of m² first, then the boards, then buy the packs that cover it.
Related tools & guides
Want to know how these figures are calculated? See our cost methodology.
Cost figures shown are indicative estimates, not quotations. You are responsible for verifying all costs (obtain contractor quotes) and any figures submitted to a lender. ScopeWise is a documentation tool, not financial, tax, structural or planning advice. HMO compliance prompts are guidance only — confirm requirements with your local council, as standards and licensing vary by authority.