Asked for a condensation risk analysis? Here is what it has to show

Updated 27 September 2026 — reviewed against the regs as they stand.

What building control is actually asking for

Requirement C2 of the Building Regulations in England and Wales deals with resistance to moisture. It says the walls, floors and roof of a building “shall adequately protect the building and people who use the building from harmful effects caused by” four things, and one of them is interstitial and surface condensation.

Interstitial condensation is the kind that forms inside a construction, between its layers, where warm indoor air working its way out meets a layer colder than its dew point — wetting insulation, corroding fixings and rotting timber where nobody can see it. Surface condensation is the kind you can see, and the mould that follows it.

Approved Document C, the government guidance on meeting C2, points to two standards for condensation: BS 5250 and BS EN ISO 13788. It still cites their 2002 editions, because the Approved Document itself was last fully revised in 2004; BS 5250 has since moved on to the 2021 edition, now called Management of moisture in buildings. A condensation risk analysis is the calculation that shows a build-up meets C2 on condensation — in practice, BS EN ISO 13788 run on your build-up and read against BS 5250.

In Scotland the equivalent is standard 3.15 of the building standards technical handbooks: no threat to the building, or to the health of its occupants, from moisture caused by surface or interstitial condensation.

When they ask for one

Not for every build-up. BS 5250 sets out arrangements that experience shows are safe — a warm flat roof with its vapour control layer under the insulation, a cold loft with a sealed ceiling and ventilated eaves — and the 2021 edition suggests that for the simplest, lowest-risk constructions a condensation risk analysis may not be necessary.

The request comes when a design steps outside those arrangements. For example:

• A flat roof with a twist — a cold deck, or a hybrid with insulation both above and below the deck. Split the insulation of a flat roof and leave the vapour control layer on the deck, and U-Monkey’s check puts condensation under the deck every winter; the flat roof insulation thickness page runs exactly that case.
• Insulation on the inside of a solid wall, which turns the old masonry cold.
• Insulation along the rafters, where the underlay and the vapour control layer decide whether moisture can get out.
• An unfamiliar layer order, or a membrane whose vapour resistance nobody on the job actually knows.

Warranty providers ask for them too, often on the same build-ups.

What the report has to show

No form is prescribed, but whoever checks it will be looking for the same things:

• The build-up, outside to inside: every layer with its thickness, its thermal conductivity and its vapour resistance (a μ-value, or an s_d in metres), and where each vapour figure came from. Published vapour data varies widely for membranes, foil facings and OSB, so a declared value from the product’s own datasheet carries more weight than a generic table.
• The conditions: the external climate used — monthly data for a named weather station near the site, not a default — the internal temperature and humidity class for the building’s use, and any allowance for a colder-than-average year.
• The results, month by month: which interfaces condense, how much accumulates, and when it evaporates again.
• A conclusion in the standard’s terms: no condensation; condensation that dries out completely over the summer; or condensation that accumulates year on year.
• The surface check: the temperature factor f_Rsi, which for a dwelling should be at least 0.75 if surface condensation and mould are to be avoided.
• The element and project it applies to, the date, and who produced it.

That is the whole document, and it covers one build-up: a wall and a roof are two analyses.

Reading the result

A pass with no condensation is the clean answer. Seasonal condensation — some forms in winter but all of it evaporates by the end of summer — is normally acceptable, provided the amount is too small to harm what it forms on. Timber is the thing to watch there, because decay becomes a risk as its moisture content approaches about 20%. Condensation that accumulates year on year is a fail, however small the first winter’s amount looks.

Two things are worth reading beyond the verdict. The first is the margin: a pass at 99% of saturation is a different animal from one at 80%, because a small change to a layer or a product can tip it over. The second is the vapour data: if the result hinges on an assumed figure for a membrane or a board, get the declared value and run it again.

What a calculation can’t prove

BS EN ISO 13788 is a screening method: monthly, steady-state, one-dimensional, and diffusion only. It does not model air leaking through gaps, rain, solar gain, moisture built into new construction, or moisture stored in and released by materials. For most build-ups it is still the standard way to show that C2 is met, but it runs out in two places.

Cold roofs. BS 5250:2021 says that the calculation methods — BS EN ISO 13788 and the more advanced BS EN 15026 alike — do not account for moisture carried on moving air, so they are not suitable for cold flat roofs or cold pitched roofs, and it relies on prescriptive rules instead: a sealed air and vapour control layer on the warm side, a ventilated gap and cross-ventilation. For a cold flat roof that means at least 50mm of air above the insulation and ventilation equivalent to a continuous 25mm opening on each side. BS 6229:2018, the flat roofing code of practice, notes that cold flat roofs are not now recommended, and BS 5250:2021 says not to use one where the void spans more than 5m, can’t be ventilated at both ends, or can’t have a sealed air and vapour control layer on the warm side. The cold flat roof page shows what that construction does to the U-value, and what the check can and can’t tell you about it.

Solid walls and heritage buildings. Insulating a solid wall on the inside changes how the masonry wets and dries — rain, stored moisture and all — which a diffusion-only calculation cannot see. For those constructions, and other high-risk ones, a full hygrothermal simulation to BS EN 15026 is the right next step.

When it is due

No regulation sets a date. The analysis is evidence that the design meets C2, so it belongs with the plans, or at the latest before the element it covers is built.

The sensible time to run it is earlier still — before the insulation, membranes and boards are ordered — because the fix is almost always a change of product or layer order, which is cheap on paper and expensive on a finished roof.

If the check fails

The fix is rarely more insulation. It is usually the order of the layers:

1. Put a sealed air and vapour control layer on the warm side of the insulation, continuous at laps, joints and penetrations.
2. Keep the outer layers more open to vapour than the inner ones, so moisture that gets in can get out.
3. In a flat roof, keep all the insulation above the deck — a warm deck — rather than splitting it around the deck. In an inverted roof the waterproofing itself sits on the warm side and does that job.
4. In a cold roof, fix the ventilation first, because no calculation will prove it for you.
5. Replace assumed vapour figures with declared ones, and run it again.

What it costs

• Manufacturers’ technical services will often run U-value and condensation calculations free for build-ups that use their products. They model their own products, so check the rest of your build-up is in there too.
• Independent consultants usually price per element. One specialist advertises £250–£350 per element for a Glaser assessment, typically in one to three working days, and from about £500 per element for a WUFI (BS EN 15026) simulation.
• U-Monkey runs the BS EN ISO 13788 check free on every build-up. The written report is £25 +VAT (£30 inc VAT) per build-up, or included on Monkey Pro Unlimited.

Whichever route you take, check what one purchase covers. A condensation risk analysis covers one build-up, so a job with a wall, a roof and a floor to evidence is three.

Do it yourself, free

Build the element in U-Monkey’s free condensation risk calculator and the check runs as you go, alongside the U-value: the verdict, the worst interface, the month-by-month balance and the surface mould check, free and without signing up. Pick the weather station nearest the site and set the building’s use before you trust the answer.

What you’ll need to hand: the build-up from your drawings, and for each product its declared thermal conductivity and vapour resistance — both are on the datasheet, the declaration of performance or the product’s certificate.

If building control needs it in the file, the written report is £25 +VAT: every layer, every month, every interface and a clear conclusion, with a report ID they can verify online. See an example report (PDF) first.

Or start from a worked example, each opening pre-filled: a warm flat roof, a cold flat roof, an inverted flat roof, insulation between the rafters, loft insulation, a timber frame wall or a solid wall insulated on the inside.