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 pitched roof | Under-ventilated loft meets warm moist air | Saturation risk at the underlay in the winter months |
| 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 |
Two of these have ready-made worked examples: solid brick wall with internal insulation and the warm flat roof — both open in the calculator pre-filled.
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. It is a screening to the standard method, honestly labelled as one.
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.
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.
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, ready 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?
The report shows the full ISO 13788 method — every layer, every month, every interface — so a building control body can check the working line by line, and it carries a printed report ID verifiable in seconds at energycount.co.uk/verify. If anything raises a question, 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.