Insulation and U-value calculator
Work out the U-value of a wall, roof or floor build-up and how much insulation it needs. Pick the element type, enter each layer from inside to outside with its thickness and thermal conductivity (λ), then add the area to cover and your insulation price per m³.
The calculator checks the result against Approved Document L 2021 figures and gives the insulation volume and cost. It's free, works in your browser with no sign-up, and there's a free Windows app too.
Worked example: U-value of a cavity wall with 120mm PIR board
A 40 m² external cavity wall extension, built up inside to outside. Each layer's resistance is R = thickness (m) / λ; the air gap uses a fixed R of 0.18.
| Layer | Thickness | λ (W/mK) | R (m²K/W) |
|---|
| Plasterboard | 12.5mm | 0.21 | 0.060 |
| Medium density block | 100mm | 0.51 | 0.196 |
| PIR board | 120mm | 0.024 | 5.000 |
| Residual cavity | 25mm | - | 0.180 |
| Brick outer leaf | 102.5mm | 0.68 | 0.151 |
- Add the wall surface resistances: Rsi 0.13 + Rso 0.04 = 0.17.
- Total R = 0.060 + 0.196 + 5.000 + 0.180 + 0.151 + 0.17 = 5.757 m²K/W.
- U = 1 / 5.757 = 0.17 W/m²K, a pass against the 0.18 extension figure. With 75mm of PIR the same wall comes out at 0.26, which fails for an extension.
- Insulation volume = 40 m² x 0.12m = 4.80 m³; at £62 per m³ that's £297.60.
Questions
How is a U-value calculated?
Each layer's thermal resistance is its thickness in metres divided by its thermal conductivity (R = d / lambda), and those resistances are added together with the inside and outside surface resistances. The U-value is 1 divided by that total R, in W/m2K - this is exactly how the calculator works it out, layer by layer.
What U-value does Part L require for walls?
Approved Document L 2021 (England) gives 0.18 W/m2K for walls in an extension or new element in an existing home (Table 4.2), with 0.15 for roofs and 0.18 for floors, and limiting values of 0.26 for walls, 0.16 for roofs and 0.18 for floors in a new home (Table 4.1). The calculator checks against either set - it's a quick check, not compliance sign-off, and building control or a SAP assessor has the final say.
How much insulation do I need?
Add insulation until the calculated U-value drops to or below the target for the element - the calculator shows Pass or Fail as you change the thickness. For example, a block and brick cavity wall needs around 120mm of PIR board (typical lambda 0.024) to reach the 0.18 extension figure, and more if you use mineral wool (typical 0.038).
How many packs or boards of insulation do I need for an area?
The calculator gives the insulation volume in m3 (area x total insulation thickness). For rolls or boards, divide the area by the coverage printed on each pack at your chosen thickness and round up, allowing a little extra for cutting waste.
What surface resistances does the U-value calculator use?
It uses fixed values per element: Rsi 0.13 and Rso 0.04 for walls, 0.10 and 0.04 for roofs, and 0.17 internal only for ground floors. Floors are a simplified calculation that does not model the ground beneath the slab.
Which lambda values should I use?
The presets are typical figures, for example 0.21 W/mK for plasterboard and 0.68 for brickwork, and you can overwrite any of them. For materials you actually specify or order, use the manufacturer's declared lambda value.
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