How many rolls of loft insulation do I need for 15.12 square metres?

For one new layer, add the allowance you chose and divide by the square metres printed on one roll or pack, then round up. With 5% extra and 8.01 m² per pack, 15.12 m² needs 2 packs.

Is 270 mm of loft insulation enough?

270 mm is a common UK recommendation for mineral wool, but the right product and build-up depend on the roof, existing material and current guidance. Treat it as a starting figure, not a design for every home.

How much loft insulation do I need?

For an accessible cold loft, mineral-wool rolls are normally laid at ceiling level: between the joists, then across them where another layer is needed. The quantity starts with the horizontal loft-floor area, not the sloping roof.

The five figures

Measure the loft floor length and width. Split a shape that is not rectangular into rectangles and work them out separately.
Measure the uncompressed depth of insulation already there. If it is wet, damaged, compressed or contaminated, stop and decide what must be removed before treating it as useful insulation.
Choose the total depth for the complete build-up. Current UK guidance commonly uses 270 mm for mineral wool, but the material and circumstances matter.
Read the thickness of the exact roll you plan to add.
Read the square metres covered by one roll or pack at that thickness. The same product range can cover a different area at each thickness.

The sum

Subtract the existing depth from the target. Divide that gap by the new roll thickness and round up to a whole number of equal layers. Multiply the loft-floor area by those layers, add the visible allowance for cuts and fitting, divide by the pack coverage and round up to whole packs.

For a 4.2 m × 3.6 m loft floor:

floor area: 4.2 × 3.6 = 15.12 m²;
existing insulation: 100 mm;
chosen target: 270 mm;
chosen new roll: 170 mm thick, 8.01 m² per pack;
one new layer reaches 270 mm;
15.12 m² plus 5% = 15.876 m²;
15.876 ÷ 8.01 = 1.982, so buy 2 whole packs.

These are generic example figures. Replace them with the label for the product you will use.

Checks the area cannot answer

Measure the joist gap before choosing a width. Keep ventilation paths open at the eaves. Check cables, recessed lights and other electrical equipment because insulation can trap heat. Check cold-water tanks and every pipe because the loft will be colder. Do not lay boards directly on deep mineral wool; safe access or storage needs a suitable raised system and consideration of the joists and load.

A room in the roof, insulation at rafter level, a flat roof, spray foam, loose fill and rigid boards need a different design. Damp, leaks, vermin, fragile ceilings or unknown older materials are reasons to stop and get suitable advice.

Do it with BuildBatch

Open the loft insulation calculator, measure the rectangle and replace the example depths and pack coverage with the figures you have chosen.