Simple U-Value Layer Estimate

How to use the Simple U-Value Layer Estimate

1. Select the element type — wall, roof, or floor.
2. Choose the relevant Part L limiting-value benchmark for the project route.
3. Build the idealised construction layer by layer and replace generic λ assumptions with declared values for the exact products.
4. View the calculated U-value updating live as you add or adjust layers.
5. Treat the comparison as an early estimate and obtain the correct project calculation before specification or submission.

Why use this tool

• Provides a transparent idealised layer-sum estimate for early option comparison.
• Layer-by-layer breakdown shows which materials contribute most to thermal performance.
• Uses the currently effective 2021 edition incorporating 2023 amendments and distinguishes limiting values from compliance.

U-values measure heat flow through a building element, but a simple homogeneous layer sum cannot represent repeating timber or metal paths, fixings, thermal bridges, junctions, cavities, moisture or the perimeter-to-area dependency of ground floors. The displayed benchmarks are the limiting values from Approved Document L Volume 1, 2021 edition incorporating 2023 amendments: Table 4.1 for new dwellings and Table 4.2 for new fabric elements in existing dwellings. They are not universal design targets and being below one does not establish Part L compliance. Generic conductivity values are only starting assumptions; use declared design values for the exact products and obtain the calculation required by the competent assessor and Building Control. The published 2026 edition generally takes effect on 24 March 2027, subject to transitional arrangements.

Part of Builder Tools Calculators — construction tools for UK tradespeople.