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U-values, free in your browser — no sign-up, no nonsense · BS EN ISO 6946 · BR 443
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What are you calculating?

Wall

Start point
Type of building work — sets which Part L limiting U-value the pass/fail is judged against
Existing wall construction
Floor geometry — a ground-floor U-value depends on the exposed perimeter and area (ISO 13370), so it changes with the plot.
Defaults per BR 443 §9.2 / RdSAP: void height 225mm, void wall U 1.5, wind 5 m/s (UK heating-season mean). Insulation between joists is a bridged layer — use ⋯ → Bridge (joists ≈ 10.8%).
Add the floor build-up below (insulation, screed, covering). The dense slab and hardcore are excluded per BR 443. B′ = A ÷ (½P).
Build the flat roof below without the tapered course (deck, VCL, waterproofing, any flat overlay board) — define the tapered course here. The U-value is the area-weighted mean of 1/R (log-mean) across the fall; averaging thickness overstates performance.
Max = min + fall × longest run (e.g. 1:60 over 6 m ≈ +100 mm). Rectangular-field method. BS 6229: no point may exceed 0.35 W/m²K.
Comparing build-ups — tap Load to bring one back into the editor
Live cross-section · outside → inside
Fitting quality (air-gap correction) — Typical by default
How well will the insulation be fitted? Adjusts the ΔU air-gap correction (BS EN ISO 6946 Annex F).
–W/m²K
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Show the working
Building Control-ready PDF — full layer breakdown, method references and a unique report ID. Free, no sign-up.
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A build-up can pass its U-value target and still fail on interstitial condensation — the two are separate checks, and Building Control increasingly want the second one evidenced.
The screening check above is free. The £25 +VAT report is the submittable evidence: verdict, fRsi, interface results, vapour data with sources, method statement, report ID. See an example report.
Work backwards to a target Pick the U-value you need — U-Monkey solves the insulation thickness that gets you there, and applies it.
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Design-stage U-values by the combined method of BS EN ISO 6946:2017, BR 443 conventions, with a built-in condensation risk check to BS EN ISO 13788 (free screening; submittable report available). Part of the Energycount Monkey suite — the acronyms between your drawings and a build, handled. © Energycount Limited.