U-value calculator for walls and roofs
Build up a wall or roof layer by layer and check it against Part L.
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A U-value measures how much heat passes through a wall, roof or floor. The lower the number, the better the insulation. Building control will check that new walls and roofs in an extension meet the Part L targets.
Add each layer from outside to inside, choose the material and thickness, and the calculator works out the U-value and compares it with the target for an extension.
Questions people ask
What U-value do I need for an extension wall?
Part L sets a limiting U-value of 0.18 W/m²K for new walls in an extension to a home in England.
What U-value does a new roof need?
A pitched roof insulated at rafter level needs to reach 0.15 W/m²K.
Why does building control ask for a full calculation?
This quick method ignores thermal bridging through studs, ties and fixings. A full calculation to BR 443 accounts for these and is usually required.
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