Loft insulation calculator
The calculator needs JavaScript. The worked example, formula and figures below show exactly how it works, so you can do the sum yourself. A guide, not advice. Check your supplier's or maker's figures before ordering.
Short answer
Worked example
| You enter | Value |
|---|---|
| Loft floor area | rectangle 8 m × 5 m |
| Joist depth | 100 mm |
| Insulation already there | 0 mm |
| Target total depth | 270 mm |
| Layer 1 | 100 mm |
| Layer 1 roll coverage | 8.3 m² |
| Layer 2 | 170 mm |
| Layer 2 roll coverage | 5.6 m² |
Order: 5 rolls for layer 1 and 8 rolls for layer 2 (270 mm total)
| Result | Value |
|---|---|
| Layer 1 rolls (between the joists) | 5 |
| Layer 2 rolls (across the joists) | 8 |
| Total depth after | 270 mm |
| Loft area | 40 m² |
Show the working
- Loft area: 40 m²
- Layer 1 (100 mm): 40 m² ÷ 8.3 m² per roll, rounded up = 5
- Layer 2 (170 mm): 40 m² ÷ 5.6 m² per roll, rounded up = 8
- Total depth: 0 + 100 + 170 = 270 mm
- Target 270 mm: reached
How it works
- Rolls per layer = loft area ÷ roll coverage (m² on the pack), rounded up
- Total depth = insulation already there + layer 1 + layer 2
- Lay layer 1 between the joists and layer 2 at right angles across them, so the joists are covered
Our 270 mm target is a commonly quoted figure for mineral wool that we're still checking; a different material needs a different depth for the same U-value.
Don't squash insulation: if it ends up deeper than the joists and you want to board the loft, raise the boards on loft legs.
Our default (adjustable)
These are typical planning figures and our own allowances, not facts about your product. Change any of them in the calculator to match your bag, pack or tin.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Target total depth | 270 mm | Our assumption Not yet checked against the source | Our choice: change it to suit your job Brief v2 I3: target 270 mm mineral wool, to be sourced to the Energy Saving Trust (Researcher). Stored as an assumption until an https source is cited. |
Regulations and standards
- Approved Document L 2021 (roof U-values, England)
Explained: Approved Document L (conservation of fuel and power).
Our checks use the guidance for England unless a badge says otherwise. Wales, Scotland and Northern Ireland have their own rules: check the source for where you live.