Insulation calculator
Insulating an attic, wall or floor? Enter the area and thickness and the calculator returns the number of packs, the volume and an approximate U-value – waste included. Save the result as a material list to take to the merchant.
An estimate. How many m² a pack covers depends on the thickness – the value is on the pack. The U-value applies only to the insulation layer; a correct U-value for the whole wall also accounts for studs, boards and thermal bridges. Boards 560 mm wide fit studs at c/c 600 mm.
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An estimate, not a finished calculation – check against drawings, regulations and supplier data before pricing or ordering.
What the calculator works out
- Need: the area plus waste (m²).
- Packs: the need divided by m² per pack.
- Volume: area × thickness (m³).
- U-value (approx.): lambda ÷ thickness – for the insulation layer only.
Thickness, lambda and U-value
The same pack covers fewer m² the thicker the insulation you choose. Thicker layers and a lower lambda give a lower U-value (better). Roughly: 10 cm of mineral wool ≈ U 0.4, 20 cm ≈ 0.2 and 40 cm ≈ 0.1 W/m²K. Check what thickness your structure (attic, wall, floor) requires before ordering.
Studs, board width and thermal bridges
Insulation boards of 560 mm fit studs at c/c 600 mm. Consider laying the insulation in two crossing layers so the studs don’t form continuous thermal bridges – in practice this gives a noticeably better wall than the U-value of the insulation layer alone suggests.
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Frequently asked questions
- How much insulation is needed?
- Enter the area and the calculator adds your waste allowance (5% by default), then divides the total by how many m² one pack covers. You get the volume in m³ and the number of packs to order.
- What do lambda and U-value mean?
- Lambda (W/mK) is the material’s thermal conductivity – mineral wool is about 0.033–0.037. The U-value describes how much heat the construction lets through, and lower is better.
- Does the calculator’s U-value cover the whole wall?
- No, it applies to the insulation layer only (U ≈ lambda ÷ thickness). A correct whole-wall value also counts the studs, boards and thermal bridges, which let through more heat than the insulation itself.
- Which c/c should the studs have?
- Studs usually sit at c/c 600 mm, and 560 mm boards then wedge in by friction with no fixings. At a tighter c/c, cut the boards narrower so they fit just as snugly.
- Does it cost anything?
- No, the calculator is free and requires no account.