U-Monkey — the free U-value calculator

Input	Where to get it
λ (lambda)	The manufacturer's declared value — BR 443 §3 requires declared values for insulation. Pick the product from the library and it comes with its source; type your own and it is flagged "user λ" in the working.
Thickness	Your drawing or specification. Where a product only comes in fixed sizes, the tool snaps to them and says so.
Bridging	Stud, joist or rafter width and centres, off the drawing — the tool works the area fraction out. Presets cover the common cases.
Fixings	Wall ties and mechanical fixings that pass through the insulation: λ of the fixing, number per m², cross-section. Ties across an unfilled cavity, and fixings under 1 W/mK (basalt, GRP, plastic), need no correction.
Surface resistances	Nothing to enter. The tool sets R_si and R_se from the element type and heat-flow direction, per BS EN ISO 6946, and shows them in the working.
Ground floor geometry	Exposed perimeter and floor area off the plan. A ground floor's U-value depends on its shape as well as its build-up (ISO 13370), so the same construction gives a different answer on a different plot.

The verdict says	What it means — and what to do
PASS	No condensation in any month, and it stays that way when each assumed vapour value is tested across its plausible range (a third to three times the value used, or the range given with it — brickwork and EPS across BS 5250’s own range), each vapour control layer at a third of its rating, and any metal sheet with its laps unsealed — one at a time, then all together. Nothing in the build-up needs to change for this check.
PASS (seasonal)	Some condensation forms in winter and dries out fully in summer. BS EN ISO 13788 does not count that as accumulation; check the amount and the material it forms against in the interface results.
CONDITIONAL PASS	It passes only while a named value holds, or only while the values do not all sit at the unfavourable end of their range together — usually a membrane or board entered at a typical figure. Enter the manufacturer’s declared vapour resistance (layer ⋯ → Vapour resistance) and it firms up either way. Where it hangs on brick or EPS, whose range BS 5250 itself gives, a vapour control layer on the warm side takes the dependence away — see How to fix it.
NOT A CLEAR PASS	The condensation dries out, but it lands on timber in a quantity that eats into its decay margin, or on a material that cannot absorb it in run-off quantities. Reduce it — see How to fix it.
NOT A PASS	The calculation is not the right test for this construction. BS 5250:2021 settles cold pitched and cold flat roofs by prescriptive rules (sealed ceiling, ventilation) — and BS 6229:2018 notes that cold flat roofs are not now recommended at all — and expects an air and vapour control layer on the warm side of a warm pitched roof — plus a ventilated void under a high-resistance underlay. The panel says which rule applies.
SCREENING ONLY	Internal insulation on a solid wall. Rain and capillary moisture in the masonry are beyond a Glaser calculation, so a BS EN 15026 hygrothermal assessment by a specialist is the right next step. Humidity class 5 (pools, laundries, breweries) reads the same way: BS 5250 wants the building’s own design conditions, which the class curve at 20 °C inside is not.
FAIL	Condensate builds up year on year, or the inside surface is cold enough for mould (f_Rsi). Redesign — see How to fix it.

What are you calculating?

First time here? Pick the element, stack the layers outside to inside, set each thickness. Every box has an i beside it telling you what the number is and where to get it.

Which element should I pick — and what will I need?

The element sets two things you never type in: the surface resistances (they differ with heat-flow direction, so a roof is not a wall) and the target your result is marked against.

Have these to hand before you start: the build-up layer by layer with thicknesses, and a λ for each material — from the product library here, or the manufacturer’s declared value. For a ground floor you also need the exposed perimeter and floor area off the plan, because a ground-floor U-value depends on the shape of the plot as well as the construction.

Not sure where to start? See a worked example builds a full cavity wall for you. Know the U-value you need instead? Know your target? solves backwards to the insulation thickness.

Wall

Start point — templates, or a common UK build-up

Live cross-section · outside → inside

What goes in a layer, and where do λ and thickness come from?

Build the stack outside to inside. Each layer needs what it is and how thick it is; resistance is thickness ÷ λ.

λ — pick the product from the library and it carries its own source. Typing your own is fine, but use the manufacturer’s declared value (BR 443 §3), not a "typical" one; it is flagged "user λ" in the working either way.
Thickness — off your drawing. Where a product only comes in fixed sizes the tool snaps to them and tells you.
The ⋯ menu on each row holds everything else: repeating bridges, fixings through the insulation, vapour resistance, cavity ventilation, thermal mass.

Do not skip the bridging. Studs, joists and rafters passing through an insulation layer are the most common reason a U-value comes out better than the wall really is — often by 20–30%. Enter the width and centres and the tool does the combined method for you.

You never enter surface resistances: the tool sets R_si and R_se from the element and heat-flow direction, and shows them in Show the working.

Fitting quality (air-gap correction) — Auto (BR 443)

How well will the insulation be fitted? Adjusts the ΔU air-gap correction (BS EN ISO 6946 Annex F).

–W/m²K

–

What is this being judged against — and what if another calculator disagrees?

The traffic light compares your U-value with the limit for the nation, building type and work type set under Judged as just below. Check it before you conclude anything: a retained element judged against the new-build limit will fail a construction that actually complies.

Show the working prints every line — each layer’s resistance, the bridging correction, the air-gap correction, R_si and R_se. You never enter the surface resistances; the tool sets them from the element and the direction of heat flow (BS EN ISO 6946).

Getting a different number elsewhere? It is nearly always one of three things: a bridging fraction one calculator applied and the other did not, a different λ for the same-sounding product, or different surface resistances. The working shows all three, so you can find the difference rather than argue about the answer.

Judged as — which regulations and limit the pass/fail uses

Show the working

Show the pass / fail verdict on the PDF report

A PDF for Building Control — full layer breakdown, method references and a unique report ID. Free, no sign-up.

What does this check actually do — and what does it not?

It is a BS EN ISO 13788 monthly Glaser screening against real climate data, plus a surface (mould) check against the BRE IP 1/06 f_Rsi criterion (0.75 for dwellings). It tells you whether interstitial condensation is predicted, where, and whether it dries out again over the summer.

Three inputs matter: the weather station (pick the nearest), the humidity class — class 3, the default, covers dwellings with high or unknown occupancy and is worth leaving alone — and the design year: 1-in-10 suits most buildings, 1-in-20 or 1-in-50 sensitive ones. Roofs are also checked with the surface 2 K below the air, for night-sky cooling.

Where BS 5250:2021 says a calculation is not the right test, it says so instead of calling a pass. Cold pitched and cold flat roofs (BS 5250:2021 says calculation methods are not suitable — its prescriptive guidance applies), warm pitched roofs without an air and vapour control layer, internal insulation on solid masonry are reported as such when the layer list shows them, and so is any building checked at humidity class 5. And where a pass hinges on an assumed vapour value — a generic membrane, a foil facing — it re-runs the check across that value’s plausible range, one value at a time and then all together, and tells you which value the result relies on.

When it is not a pass, it tells you how to fix it: it tries the usual remedies on a copy of your build-up and lists only the ones that pass the whole check, with the value needed. Press ? for what each verdict means.

What it is not: a transient hygrothermal assessment (BS EN 15026). Glaser is a steady-state screening method — it ignores moisture stored in materials, rain and construction water. Where timber sits at a condensing interface or the build-up is high-risk, the tool says so, and that assessment is a separate piece of work.

Condensation risk — BS EN ISO 13788 monthly Glaser (screening)

Weather station (nearest)

Humidity class

Design year

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.

How you get it: after payment you come straight back to this page and the PDF downloads to your device — it is not emailed. Keep this tab open until the confirmation appears; re-download is free from this page for 24 hours.

Included on Monkey Pro Unlimited · See the plans

Issuing these reports? Put your practice on them — Monkey Pro at £10/month +VAT

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 PDF for Building Control 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.