How many insulation rolls do I need?

How it works

Net square metres are increased by the entered allowance for cuts, then divided by the pack coverage at the chosen thickness and rounded up. Existing and added depths are summed for reference, but no U-value or regulatory pass is claimed from thickness alone.

• Knauf shows that thermal resistance depends on both conductivity and thickness, while a complete U-value needs every layer and surface resistance.
• Only existing depth defaults to zero for a bare loft; coverage, new thickness, and cutting allowance come from the product and layout.

Worked example

											Area m2: 20
Existing depth mm: 0
Added depth mm: 270
Roll coverage m2: 5.5
Waste percent: 10
										

												Rolls exact: 4
Rolls needed: 4
Total depth mm: 270
											

When to use this

Cold-loft top-ups use thickness-specific pack coverage, timber-frame walls order net bay area, and ceiling-joist layouts allow for pipes, hatches, and perimeter cuts.

Edge cases

• A pack’s 5.5 m² coverage at one installed thickness cannot be reused at another thickness unless its technical data gives the same figure.
• Any fractional pack requirement rounds up because mineral-wool rolls and slab packs are bought whole.
• Total millimetres are reported without a U-value or Building Regulations verdict because joists, air layers, other materials, and thermal bridges are absent.

References