How to use the Loft Insulation Calculator
- Measure the loft area in m². Deduct walkways or storage platforms if required.
- Select insulation type, then choose the target thickness. Use 270–300 mm for mineral wool, 120–150 mm for PIR as a planning guide.
- Pick a roll width or board size that matches your supplier range. Add 5–10% wastage for cuts and awkward spaces.
- Review the rolls or boards required and export or copy the result.
Use this for ordering and budgeting. For tenders or whole house measures, request a QS estimate with BoQ, prelims, overheads and profit, and risk.
UK insulation requirements at a glance
Existing homes commonly target a loft U-value around 0.16 W/m²K. This is typically met with mineral wool at about 270–300 mm or PIR at about 120–150 mm. Exact thickness depends on the product lambda value. Confirm with current guidance and manufacturer datasheets.
Typical thickness by material
- Mineral wool about 270–300 mm for a cold loft at joist level
- Cellulose about 270–300 mm for similar performance
- PIR boards about 120–150 mm where depth is limited
These are planning figures. Always check stated thermal conductivity and installation guidance.
Worked examples
Mineral wool example
Area 50 m². Target 300 mm mineral wool. Roll coverage 1200 × 7000 mm = 8.4 m² per roll. Base rolls ≈ 50 ÷ 8.4 = 5.95 → 6 rolls. With 10% wastage: 6.6 → round up to 7 rolls.
PIR example
Area 50 m². Target 140 mm PIR boards at 1200 × 2400 mm = 2.88 m² each. Base boards ≈ 50 ÷ 2.88 = 17.36 → 18 boards. With 10% wastage: 19.1 → round up to 20 boards.
Ventilation, access and storage
- Maintain eaves ventilation paths to avoid condensation and mould.
- For storage, raise the deck above the insulation using loft legs or joist extensions.
- Keep downlights and services clear in line with product guidance and fire hoods where specified.
Tips to avoid under-ordering
- Measure between obstacles and allow for partitions, trimmers and changes in joist direction.
- Use higher wastage where there are many cuts or irregular geometry.
- Match roll width to joist centres to reduce trimming waste.