Roofing & buildingFree calculator
Work out how many packs of insulation you need for a loft, attic floor or stud walls. Enter the area, layers and the coverage printed on the pack.
Your shopping list
How to measure
- Measure the length and width of the loft floor or wall area.
- Check the coverage printed on the pack. It depends on the thickness and brand.
- Choose one layer between joists, or two layers with the second laid across.
The formula
Area = length × width Packs = area × layers × (1 + waste %) ÷ coverage per pack (round up)
Worked example
A 30 × 20 ft attic is 600 sq ft. With one layer, 5% waste and 40 sq ft per bag you need 16 bags.
Tips before you buy
- Current UK guidance is 270 mm of mineral wool in lofts; many US attics need R-38 to R-60.
- Do not block eaves ventilation or cover recessed lights that are not rated for it.
- Wear gloves, a mask and goggles when handling mineral wool.
Frequently asked questions
How thick should loft insulation be?
Around 270 mm (about 11 in) of mineral wool in the UK, usually laid as two layers.
What R-value do I need in my attic?
Most US and Canadian climates need R-38 to R-60 in the attic. Check your local energy code.
Do I need a vapour barrier?
In cold climates a vapour barrier usually goes on the warm side of wall insulation. Follow local building rules.
Results are estimates for planning only. Always check quantities with your supplier before you buy.