Build up a loft, roof, wall or floor layer by layer and get its U-value in seconds. Compare it to UK Building Regs targets, and see what it means for warmth and condensation. Free to embed on your own site with a credit.
Work out the U-value of a loft, roof, wall or floor build-up. Lower is better.
Typical thermal conductivity (lambda) values are shown for guidance; enter your product's declared value for accuracy. Surface resistances follow BR 443. Target U-values are typical current UK guidance (Approved Document L); verify against current Building Regulations. This is a guide, not a substitute for a professional assessment. Last verified: 17 August 2026.
A U-value measures how much heat passes through a building element. It is measured in watts per square metre per degree (W/m2K), and lower is better: a lower U-value means less heat escapes and lower bills. The calculator adds up the thermal resistance of each layer you enter, plus the standard inside and outside surface resistances (BR 443), and gives U = 1 divided by the total resistance. For the full explanation of U-values, lambda and thermal bridging, see our U-values explained guide.
As a rough guide to typical current UK targets (Approved Document L, verify against current Building Regulations): a loft or roof around 0.16, an external wall around 0.18 for new work, and a floor around 0.18 to 0.25. Existing homes are often well above these, which is where topping up insulation pays off.
The condensation angle: a lower U-value keeps the inside surfaces of your walls and ceilings warmer, which reduces surface condensation and mould. Loft condensation itself, though, is usually a ventilation problem rather than a U-value one, so read our loft condensation guide and the 2026 condensation report alongside this.
A note on multifoil: multifoil insulation is deliberately not in the material list. Multifoils do not have a simple lambda value; their performance depends on adjacent air gaps and manufacturer test data, so we would rather point you to our multifoil guide than give a figure that misrepresents them.
Sources: typical lambda values from published manufacturer and CIBSE figures (typical, check your product's declared value); surface resistances from BR 443; target U-values from Approved Document L. Last verified: 17 August 2026.
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The short answers.
A U-value measures how much heat passes through a building element, in watts per square metre per degree (W/m2K). Lower is better: a lower U-value means less heat escapes. It is the reciprocal of the total thermal resistance of the build-up, including the inside and outside surface resistances.
As typical current UK guidance (Approved Document L, verify against current regs): a loft or roof around 0.16, an external wall around 0.18 for new work, and a floor around 0.18 to 0.25. Many existing homes are well above these, which is exactly where adding insulation pays off.
It uses the standard U-value method and BR 443 surface resistances, with typical published lambda values you can override with your product's declared figure. It is a reliable guide for planning, but it is not a substitute for a professional assessment, and it does not model thermal bridging or moisture in detail.
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