U-value calculator
Build up a wall, roof or floor layer by layer, from outside to inside. You get the U-value, whether it meets the Part L target, and how thick the insulation needs to be to pass.
Free, and nothing you type leaves your device. The figures in brackets are typical λ values in W/mK: use your product's declared value.
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Worked example
A full-fill cavity wall: brick outer leaf, 150 mm glass mineral wool cavity batts, 100 mm aircrete block and 13 mm plaster:
U-value 0.18 W/m²K, against a target of 0.18 for a new wall in an extension in England
- Brick, outer leaf (exposed), 102.5 mm
- Glass mineral wool batt 0.032 (cavity), 150 mm
- Aircrete block, 600 kg/m³, 100 mm
- Gypsum plaster, 13 mm
How the sum works
Each layer's thermal resistance is its thickness in metres divided by its conductivity (λ, in W/mK). The calculator adds the surface resistances inside and out (which depend on whether heat flows up, sideways or down), unventilated air gaps from their thickness, and stops counting at a ventilated cavity or a roof space, as BS EN ISO 6946 sets out. The U-value is 1 divided by the total resistance.
Where timber studs, rafters or joists run through the insulation, the calculator uses the combined method: the average of the upper and lower limits, with the timber fraction you give. Ground floors are worked out by BS EN ISO 13370, from the exposed perimeter and the floor area.
The targets
For an extension in England, Approved Document L Volume 1 (2021 edition) sets limits of 0.18 W/m²K for a new wall, 0.15 for a roof and 0.18 for a floor. The calculator also has new-home limiting values, the notional dwelling, renovation of an existing element, and Wales and Scotland. Choose "My own target" for anything else.
Before you rely on it
The λ values are typical figures. Use the maker's declared value for the product you're buying, and check corrections such as wall ties and fixings through the insulation. Building control may ask for a U-value calculation from the insulation maker or a competent person: this is a quick check for pricing and design, not a substitute.