Loft Insulation Calculator \u2014 How Many Rolls Do I Need?

This calculator works out how many rolls of mineral wool insulation you need to insulate or top up your loft to the current UK Building Regulations minimum depth of 270mm (or your chosen target depth).

How we calculate

Top-up depth = Target depth - Current insulation depth

If you have no existing insulation, the first layer (100mm) goes between the joists, with each roll covering approximately 2.0 m² (5m length × 0.4m between standard 400mm joist centres). The second layer is laid across the joists at 90°, with each roll covering approximately 9.6 m² (8m × 1.2m).

If topping up existing insulation, all new insulation is laid across the joists at 90° for full coverage without thermal bridging.

Wastage allowance

We add 10% wastage to account for cutting around obstacles (pipes, cables, hatch openings, tanks) and irregular loft shapes. This is standard practice for loft insulation.

UK Building Regulations

Part L of the Building Regulations requires a maximum U-value of 0.16 W/m²K for roof insulation at ceiling level, which typically requires 270mm of mineral wool (lambda 0.044 W/mK) or 250mm of higher-performance mineral wool (lambda 0.035 W/mK).