Attic Insulation Roll Count Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/10/2026
The number of rolls is calculated with (surface × overlap margin) ÷ usable area per roll, rounded up. For 60 m² of attic with 5 m² rolls and a 5% margin, you need 13 rolls.
Explanation
Insulating an unconverted attic usually means rolling out mineral wool (glass wool or rock wool) over the floor, often in two crossed layers to eliminate thermal bridges at the joists. This calculator answers the purchasing question: how many rolls to order to cover the surface. It doesn't address the target thermal performance, which is covered by our required insulation thickness calculator and our insulation thermal resistance calculator: these two calculators determine the total wool thickness to lay to reach a target thermal resistance (often R ≥ 7 m²·K/W for renovated attics), a choice that in turn affects each roll's usable area (a thick roll covers fewer m² than a thin one for the same volume of wool). The usable area of a roll, shown on the packaging, is therefore left as an input: it typically ranges from 3 to 7 m² depending on thickness and width. The overlap margin covers the second crossed layer (never perfectly aligned with the first), the cutouts around hatches, ducts, and chimneys, and the slight overlap between adjacent strips recommended to avoid gaps.
Example: 60 m² of attic, 5 m² rolls
Inputs
Attic surface: 60 m². Usable area per roll: 5 m². Margin: 5%.
Calculation
Surface with margin = 60 × 1.05 = 63 m². Number of rolls = 63 ÷ 5 = 12.6, rounded up to 13.
Result
You need to buy 13 rolls to insulate these 60 m² of attic with a 5% margin.
Frequently asked questions
Should I count the surface for one or two layers of insulation?
This calculation covers a single layer covering the surface. If you lay two crossed layers (recommended to eliminate thermal bridges at the joists), you need to multiply the result by two, or directly enter double the actual attic surface. The second layer, laid perpendicular to the first, covers the same area.
Why does the usable area per roll vary so much?
For an equal amount of wool, a thicker roll (for better thermal resistance) covers a smaller area than a thin one. The roll's width also matters: attic rolls are generally 1.20 m wide to fit a standard joist spacing, but some formats are 0.60 m. The actual usable area is always shown on the packaging.
Should the attic access hatch also be insulated?
Yes, the access hatch is a significant thermal bridge if left uninsulated: it's treated separately with an insulated box or a custom-cut panel, and its small area doesn't need to be counted in this roll calculation, but it shouldn't be forgotten during installation.