Insulation is counted the same way as any sheet material: area divided by what one board or roll covers. The harder question, how thick it needs to be, is a separate one.
Boards
The Celotex guide lists boards at 1200 × 2400 mm, which is 2.88 m² a board. 20 m² with 5% waste is 21 m², which is 7.3 boards, so 8.
Rolls
Rolls print the area they cover, and it changes with thickness: a thicker roll is shorter. Baglist uses 8 m² as an example. 21 m² at 8 m² a roll is 2.6, so 3 rolls.
Read the area off the pack, not the length. Two rolls that look the same size can cover very different areas.
| Product | Covers | Count |
|---|---|---|
| Board 2400 × 1200 mm | 2.88 m² | 8 boards |
| Roll, example | 8 m² | 3 rolls |
Thickness comes first
Nothing here says whether the layer is warm enough. Building regulations set the target for new work and some renovation, and the maker's tables say which thickness reaches it. Pick the product first, then count it.
Loft insulation is often laid in two layers, one between the joists and one across them. Count each layer separately and add the two together.
Figures are estimates for a shopping list. Check the pack or datasheet before you buy.