Envelope · Insulation
Insulation thickness calculator
Thickness = λ × (1 / U target − Rsi − Rse − R of the rest of the build-up). A wall that must reach U 0.25 W/m²K, with R 0.40 m²K/W in its other layers and mineral wool at λ 0.035: 1/0.25 − 0.13 − 0.04 − 0.40 = 3.43 m²K/W, × 0.035 = 120 mm, so 130 mm of board, which gives U 0.233. Pick the element and the product, type the U you need, and the page gives the thickness.
Everything runs in your browser. Nothing is uploaded and nothing is stored.
How this is calculated
The λ values in the list are typical declared values; the one on your product's declaration of performance is the one to use. The R of the other layers is the sum of thickness ÷ λ for each of them (brick, block, plasterboard, timber), without the surface resistances, which the page adds.
What this page does not do
- No thermal bridges or fixings. Studs, rafters and fixings through the insulation raise the U: the full method is in the U-value calculator.
- No ground floors. A floor on the ground has the ground's resistance too (EN ISO 13370); this page treats the floor as a separating floor.
- No condensation. More insulation can move the dew point: check it with the condensation calculator.
Questions people ask
- How do you calculate the insulation thickness for a U-value?
- Thickness = λ × (1/U − Rsi − Rse − R of the other layers). For U 0.25 on a wall with R 0.40 in its other layers and λ 0.035: (4 − 0.13 − 0.04 − 0.40) × 0.035 = 0.120 m, 120 mm.
- How much insulation do I need for U 0.18?
- In a wall with R 0.40 in its other layers: 1/0.18 − 0.57 = 4.99 m²K/W, which is 175 mm of mineral wool at λ 0.035 (180 mm of board) or 110 mm of PIR at λ 0.022.
- Why does PIR need less thickness than mineral wool?
- Its λ is lower, 0.022 against 0.035 W/mK, and the thickness for the same R is proportional to λ: about 37 % less board for the same U.