⌂ Energycount home GlazeMonkey RenoMonkey WaterMonkey ΨMonkey SoakMonkey NutrientMonkey TreeMonkey GridMonkey All tools → Get a quote
U-values, free in your browser — no sign-up, no nonsense · BS EN ISO 6946 · BR 443
More tools ↗
📁 My build-ups ▾
❓ Guide ✦ Pro

What are you calculating?

Wall

Start point — templates, or a common UK build-up
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).
A party wall is not calculated from its layers: Part L and SAP assign its U-value by construction, because heat is lost by air movement in the cavity, not by conduction to another heated space. Pick the construction (SAP 10.2 Table 3.10 / AD L Table 2.1):
Edge sealing must be effective at the top, bottom and vertical edges and in line with the insulation of the abutting elements, and robust under site conditions. Add the layers below only if you want the cross-section and thermal mass; they do not change the U-value.
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.
Mark which layer is the waterproofing; every insulation layer outside it is corrected for the rain that drains through the joints to the membrane.
p is the October–May mean daily rainfall for the site. Pick the nearest Met Office station (1961–1990 normals, the same set as the condensation check) or type a figure from your own rainfall data; UK values run from about 1.5 (Kent) to 4.5 mm/day (Southern Uplands). f·x per ETAG 031-1 §6.6.1.3, or the product's certified value. ΔUr = p·f·x·(R₁/RT)².
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
–
Judged as — which regulations and limit the pass/fail uses
Existing wall construction
Show the working
Building Control-ready PDF — full layer breakdown, method references and a unique report ID. Free, no sign-up.
Share this result ▾
Issuing these reports? Put your practice on them — Monkey Pro at £10/month

Pro puts your practice on the report instead of ours. Your logo, practice name, address and contact lead the header; your project name, number, client and revision sit beneath it; U-Monkey drops to a small credit line and our marketing page comes off, so the report is one clean page you can issue as your own document.

It also includes:

  • Set up once — your details live on your account, not re-typed per job
  • The same branding on every report from every Energycount tool — Part G, soakaway, thermal bridge, condensation

£10/month +VAT, or £100/year +VAT (two months free). Cancel any time. Pro does not change any report's price; Pro Unlimited (£49/month) includes condensation and thermal bridge reports outright.

Free, always: unlimited U-values, the BS EN ISO 13788 condensation screening, the reverse solver and a Building Control-ready PDF with a verifiable report ID. No sign-up. Same engine, same method, same numbers — Pro changes the wrapper, not the calculation.

See a free and a Pro report side by side →

Work backwards to a target Pick the U-value you need — U-Monkey solves the insulation thickness that gets you there, and applies it.

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). New here? Read the full how-to guide. Part of the Energycount Monkey suite — the acronyms between your drawings and a build, handled. © Energycount Limited.