About this calculator
Rolls of mineral wool to top up a loft to the recommended 270 mm, with an approximate U-value.
How it’s calculated
Layers = (target − existing) ÷ roll thickness, rounded up. Rolls = layers × area ÷ coverage per roll. U-value uses λ = 0.044 W/mK.
Worked example
With these inputs:
- Area length
- 8 m
- Area width
- 6 m
- Target total depth (mm)
- 270
- Existing depth (mm)
- 100
- Roll thickness
- 170 mm
- Coverage per roll (m²)
- 6
- Waste allowance
- 5%
the result is:
- Rolls needed
- 9 rolls of 170 mm
- Depth to add
- 170 mm
- Finished depth
- 270 mm
- Approx. U-value of insulation layer
- 0.16 W/m²K
Tips from the trade
- 270 mm of mineral wool is the usual loft target.
- Keep insulation clear of eaves vents and recessed light fittings unless they are rated for contact.
- Board out over the top layer only if you raise the boards on legs. Compressed insulation loses performance.
Results are estimates for planning and pricing. Check quantities against your drawings, the manufacturer’s data and the Building Regulations before ordering.