Screening free on every build-up. Full written report £25 +VAT (£30 inc VAT).
Build the wall, roof or floor layer by layer; the condensation check runs as you go.
The short answer
Interstitial condensation is moisture forming inside a construction — where warm, moist indoor air working its way outwards meets a layer cold enough to take it below its dew point. The standard way to check for it is the monthly assessment of BS EN ISO 13788 (the Glaser method): plot temperature and vapour pressure through every layer, month by month, and see whether any interface condenses — and whether it dries out again over the year. U-Monkey runs exactly that screening, free, on every build-up you enter.
Why a good U-value isn't the whole story
Insulation moves heat loss — and it moves the cold. Add insulation on the inside of a solid wall and the original masonry gets colder and wetter-facing; get the vapour control layer on the wrong side of a flat roof deck and you build a condensation trap that meets Part L beautifully. The U-value and the condensation profile are two different physics questions about the same build-up, which is why U-Monkey answers both at once rather than selling the second answer as an afterthought.
Where the risk usually hides
| Build-up | The classic risk | What the screening shows you |
|---|---|---|
| Solid wall, internal insulation | Original brickwork turns cold; condensation at the insulation/masonry interface | Which months the interface condenses and whether it evaporates by summer |
| Warm flat roof | Vapour control layer position decides everything | The vapour-pressure profile through deck, VCL and insulation |
| Cold flat roof | Everything rests on a ventilated void no calculation can check | That BS 5250 settles it by prescriptive rules, not a calculation — and, if you model the void as blocked, what happens under the deck |
| Cold pitched roof | Under-ventilated loft meets warm moist air | That BS 5250 settles it by prescriptive rules, not a calculation — plus the ceiling’s own profile, for information |
| Timber frame | Vapour check on the warm side, breather on the cold — in that order | Whether the layer order keeps every interface above dew point |
Ready-made worked examples, each opening in the calculator pre-filled: solid brick wall with internal insulation, timber frame wall, warm flat roof, cold flat roof, inverted flat roof, loft insulation (270mm) and insulation between the rafters.
What the screening actually does
For the build-up you enter, U-Monkey computes the temperature and vapour-pressure profile through every layer for each month of the year, using BS 5250 vapour resistivities (cross-checked against ISO 10456) and monthly UK climate data. Any interface where vapour pressure reaches saturation is flagged month by month, with the annual condensation/evaporation balance. Where published vapour data genuinely varies — membranes, foils, OSB — it says so, and lets you enter the manufacturer's declared value instead. Where a pass hinges on one of those assumed values, it re-runs the check across the value’s plausible range — one value at a time, then all together — and tells you which value the result relies on. And where BS 5250:2021 says a calculation is not the right tool — cold pitched and cold flat roofs, warm pitched roofs without an air and vapour control layer, internal insulation on a solid wall — it says that instead of calling the build-up a pass, whenever the layer list shows one. It is a screening to the standard method, honestly labelled as one, of the build-up exactly as you enter it.
Free screening, £25 report — that's the whole model
The on-screen check is free forever: build-up, U-value, condensation screening, no sign-up. When the design needs the answer in writing — for Building Control, a warranty provider or the project file — the full ISO 13788 report is £25 +VAT (£30 inc VAT): the layer-by-layer working, the monthly interface analysis and a clear conclusion, carrying a report ID anyone can verify in seconds.
Been asked for one by building control? Here is what a condensation risk analysis for building control has to show, when a calculation can't settle the question, and what it costs.
Frequently asked questions
What is interstitial condensation?
Condensation that forms inside the layers of a wall, roof or floor rather than on its surface. Warm indoor air carries water vapour outwards through the construction; where the temperature inside the build-up falls below the dew point of that vapour, moisture condenses within the structure — wetting insulation, corroding fixings and rotting timber, often invisibly.
How does the condensation risk calculator work?
It runs the monthly assessment method of BS EN ISO 13788 (the Glaser method) on the exact build-up you enter: each layer’s thermal resistance and vapour resistance sets the temperature and vapour-pressure profile through the construction for every month of the year, using BS 5250 vapour resistivities (cross-checked against ISO 10456) and monthly UK climate data. Any interface where predicted vapour pressure meets saturation is flagged, month by month, with the annual balance of condensation and evaporation.
Is a low U-value enough to avoid condensation risk?
No — they are different questions. A build-up can hit its Part L U-value target and still put a condensation plane in a dangerous place: internally insulated solid walls and flat roofs are the classic cases, because adding insulation moves parts of the construction colder. That is why U-Monkey runs the condensation screening alongside every U-value rather than treating it as an optional extra.
What do the verdicts mean?
Pass: no condensation in any month, holding with each assumed vapour value tested across its plausible range and each vapour control layer at a third of its rating, one at a time and then together. Pass (seasonal): condensation forms in winter and dries out in summer, which BS EN ISO 13788 does not count as accumulation. Conditional pass: it passes only while a named value holds, so enter the manufacturer’s declared value. Not a clear pass: it dries out, but lands on timber or on a non-absorbent material in a worrying quantity. Not a pass: BS 5250:2021 says a calculation isn’t the right test — cold pitched and cold flat roofs, or a warm pitched roof without an air and vapour control layer. Screening only: internal insulation on a solid wall, where a BS EN 15026 assessment by a specialist is the next step, or a humidity class 5 building such as a pool, which needs its own design conditions. Fail: condensate builds up year on year, or the inside surface is cold enough for mould. Every verdict is for the build-up exactly as entered: it is not a design review.
What if my build-up fails?
The check tells you how to fix it. It tries the usual remedies on a copy of your build-up — a vapour control layer on the warm side (added, upgraded or moved), a more vapour-open outer layer, a ventilated void under a roof underlay, turning a cold flat roof into a warm deck, extra insulation for a mould fail — and lists only the ones that pass the whole check, with the value needed, such as a VCL rated at least 120 m sd. Make the change and the verdict updates. They are calculated options, not a design: check them against the manufacturers’ instructions and the rest of the design before you specify them.
Is the check really free?
Yes — the on-screen screening runs automatically on every build-up, at no cost and with no sign-up. You only pay if you want the full written report: £25 +VAT (£30 inc VAT) for the layer-by-layer ISO 13788 working, the monthly interface analysis and a clear conclusion, to submit alongside your free U-value PDF.
When is a Glaser-method screening not enough?
The Glaser method is a steady-state monthly assessment — the standard screening tool, and honestly labelled as one. It does not model rain absorption, thermal mass or moisture storage, so for historic solid walls, unusual constructions or high-risk designs a full hygrothermal simulation (BS EN 15026) and specialist advice are the right next step.
Will Building Control accept the report?
That is building control’s decision, not ours: what they ask for depends on the project and the construction. The report is written so they can check it — it shows the full ISO 13788 method, every layer, every month, every interface, the climate station and humidity class used, and a printed report ID verifiable at energycount.co.uk/verify. It is a calculation of the build-up you entered, not a review of your design, and where the construction needs more than a calculation (a cold roof, internal insulation on a solid wall) the report says so rather than calling it a pass. If a question comes up, we’re a real consultancy on 01202 623236.
Start from the main U-value calculator, browse the common build-ups, or read what U-values actually measure.