Loft Insulation Calculator — How Many Rolls Do I Need?

This loft insulation calculator works out how many rolls of mineral wool insulation you need to insulate or top up your UK loft (or attic) to the Energy Saving Trust recommended depth of 270mm (or your chosen target depth). Enter your loft length and width, your current insulation depth and the coverage printed on the roll pack, and it returns the number of rolls needed, split by layer, with an optional cutting allowance and cost estimate.

For an L-shaped loft, press Add section and enter each rectangle as its own section. Roll counts are rounded up per section and then added together, so the estimate covers the extra cuts where the sections meet and never under-orders. Use three sections for a T or U shape.

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 and the second layer is laid across the joists at 90° to avoid thermal bridging through the timber. We divide your loft floor area by the coverage of a full roll. UK loft rolls are combi-cut (partially perforated), so a single roll spans several joist bays at once — its coverage is the whole pack area, not one bay. The defaults use Knauf Loft Roll 44 (λ 0.044 W/mK): about 13.89 m² per roll at 100mmand about 8.01 m² per roll at 170mm. You can override both coverages in Advanced Options to match your own product.

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 a cutting allowance (5–10% is typical) to account for cutting around obstacles (pipes, cables, hatch openings, tanks) and irregular loft shapes. Adjust it to suit your loft, and round up to whole rolls when ordering.

Keep the loft ventilated

A cold loft has to stay ventilated so that moisture cannot build up behind the insulation and cause condensation, mould or timber rot. Approved Document C (resistance to moisture) is satisfied for a cold pitched roof by following BS 5250, which calls for cross-ventilation: a continuous eaves opening equivalent to a 25mm gap on shallow pitches (10mm where the pitch is over 15°) plus additional high-level ventilation equivalent to a 5mm continuous gap at the ridge where the pitch is above 35° (or the span is over 10 metres). Never push insulation tight into the eaves — keep a clear air path at the wall plate (eaves trays help), do not cover non-insulation-rated recessed downlights, and leave the gap around the loft hatch and any extract fans.

UK Building Regulations

Approved Document L of the Building Regulations sets a U-value target rather than a fixed depth — for a roof at ceiling level the limiting value is around 0.16 W/m²K. That target is typically met with 270mm of standard mineral wool (λ 0.044 W/mK) or roughly 250mm of higher-performance mineral wool (λ 0.035 W/mK).