Insulation Calculator - Work Out How Much You Need (UK)
Work out how much loft roll or PIR board insulation you need by area, with a sensible allowance for wastage.
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How to use this calculator
Measure the area you want to insulate in square metres, choose loft roll or PIR board, and pop your figure in. We add a 5% allowance for wastage and show the insulation you need plus a rough cost.
How we calculate this
- Order quantity = area × 1.05 (a 5% wastage allowance).
- Both loft roll and PIR board are priced and supplied by the square metre.
- We round to one decimal place so you've a tidy figure to order against.
Worked example
Insulating a 20 m² loft with roll: add 5% for wastage, giving 20 × 1.05 = 21 m² to order. PIR board for the same area works the same way - 21 m² - though you'll cut boards to fit.
UK notes & gotchas
- PIR boards are rigid and come in standard 2400×1200mm sheets - great for flat roofs, walls and floors where you want a thin, high-performance layer.
- Loft roll comes in widths to suit common joist spacings (typically 370mm and 570mm) so you can lay it between joists.
- For loft insulation, building regs generally look for around 270mm depth of mineral wool, often in two cross-hatched layers.
FAQ
Loft roll or PIR board?
Loft roll (mineral wool) is cheapest and ideal between joists. PIR board is rigid, thinner for the same performance and suits walls, floors and flat roofs.
How much for wastage?
5% as standard for trimming and offcuts; a little more for lots of awkward angles.
How thick should loft insulation be?
Current UK guidance is around 270mm of mineral wool, usually in two cross-hatched layers. This calculator works on area; depth comes from how many layers you lay.