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The underside of a cold steel roof at first light: beads of condensation along the sheet, a drip forming at the purlin, and a rooflight glowing beyond.

Systems and specification

Condensation risk analysis: the roof is proved to dry before it is specified.

Insulating a roof changes where it gets cold, and so where moisture can form. Before HOLVEM specifies a build-up, it checks whether that roof will trap moisture: first with the simple method, and where that is inconclusive, with a dynamic simulation of ten years of real weather. The result is part of the specification.

BS EN ISO 13788, then BS EN 15026 where it is neededA real HOLVEM roof, worked through below

01The question

Will this roof trap moisture?

Warm indoor air carries water vapour, and vapour moves outward through a roof all winter. Wherever it meets a surface colder than its dew point it condenses. In an insulated roof that surface is usually inside the build-up, on the cold side of the insulation, where nobody can see it. If more water forms each winter than dries out each summer, it accumulates year on year: wet insulation, a decaying deck, staining that appears long after the work.

Check 1 · The inside face

Surface condensation and mould

Is any part of the inside surface cold enough, often enough, to grow mould? Judged by the temperature factor fRsi against a critical value.

Check 2 · Inside the build-up

Interstitial condensation

Does vapour condense on a layer inside the roof, and where? For a warm deck the plane to watch is the cold side of the deck, under the sheet.

Check 3 · The year

Does it dry out?

Winter condensation is not the failure; failing to dry by the end of summer is. A roof that ends the year drier than it started is a roof that lasts.

02The simple method

First, the Glaser method: month by month, on averages.

BS EN ISO 13788 takes each month's average outdoor and indoor conditions, works out where the vapour pressure meets the saturation point inside the build-up, and adds up how much water condenses there, month by month. It is quick, standard, and deliberately cautious. The roof below is a real HOLVEM roof: a metal standing-seam flat roof in South East England, with 50 mm of closed-cell spray foam under the deck and 250 mm of mineral wool between the joists.

Condensate at the vapour control layer under the deck, accumulated, g/m²Rises from October, peaks in May, does not clear by September

Surface · fRsi
0.970above the critical value

No risk of mould growth on the inside face.

Interstitial · peak in May
103g/m²

Still 79 g/m² in September. On this method the roof does not fully dry within the year, so it is flagged.

03Why a flag is not a verdict

The simple method is where the standard says to start, not where it says to stop.

A flag on the simple method means one of two things: the roof really will accumulate moisture, or the method's simplifications are hiding a roof that dries. BS EN ISO 13788 says itself that it simplifies dynamic processes. BS 5250:2021, the UK code of practice for moisture in buildings, points to dynamic hygrothermal simulation where the simple method is inconclusive. What matters is what happens next.

Response 1

Ignore the flag.

Build it anyway because it has always been fine. The deck sits under the sheet where nobody looks, and a roof that does accumulate moisture fails slowly and expensively.

Response 2

Over-specify.

Add a vapour barrier, a ventilated void, more depth or a different system to make the simple method pass. More cost, sometimes a worse roof, and a design driven by the limits of a method rather than by the building.

HOLVEM's response

Prove the design.

Run the dynamic simulation the standard points to, with real weather, real materials and the membrane behaving as it does. If the roof dries, the specification stands on evidence. If it does not, the build-up changes before anything is sprayed.

BS EN ISO 13788The simple method. Monthly averages, vapour diffusion only, no moisture storage. Its own text notes that it simplifies dynamic processes.

BS 5250:2021The UK code of practice for the management of moisture in buildings. Where the simple method is inconclusive, it points to dynamic hygrothermal simulation to BS EN 15026.

04The dynamic method

Then ten years of real weather, hour by hour.

WUFI Pro simulates heat and moisture moving through the build-up together, hour by hour, to BS EN 15026. Materials absorb water and release it. Rain, sun, wind and night-sky cooling act on the outside; the room's moisture load acts on the inside. The same roof was run for ten years, set up for the worst case it will face.

  1. 01Standing-seam steel sheet, 2 mm
  2. 02Ventilated air layer, 10 mm
  3. 03Air and vapour control layer
  4. 04OSB deck, 18 mm
  5. 05Closed-cell spray foam between timbers, 50 mm
  6. 06Mineral wool between joists, 250 mm
  7. 07Humidity-variable vapour control membrane
  8. 08Plasterboard 12.5 mm and skim

Section, not to scaleThe dashed line is the plane the simple method flagged: the vapour control layer on the OSB deck.

Total water content in the build-up, kg/m²Dips each summer, never above the start, lower every year

Total water content · ten years
2.19 → 1.34kg/m²

The maximum is the starting value. Nothing accumulates; the assembly dries.

The OSB deck · the flagged layer
83 → 56kg/m³

Peaks at 92 kg/m³ each season and trends down year on year.

05What you receive

The check, and the proof, as part of the specification.

Neither the flag nor the fix is taken on trust. The analysis decides the build-up, and the build-up you receive is the one that was analysed.

Every specification

A condensation risk analysis

For the build-up as specified, on the simple method to BS EN ISO 13788: the surface check, the interstitial check and the monthly result, with the climate and the indoor conditions it assumed.

  • Surface condensation and mould: the temperature factor against its critical value
  • Interstitial condensation: where, how much, and whether it clears within the year
  • The U-value calculation it belongs with, to BS EN ISO 6946
Where the simple method is inconclusive

A WUFI report

Dynamic hygrothermal simulation to BS EN 15026, set up for the roof's worst case: the climate, the orientation, the moisture load, the materials and the membrane, and the result over ten years.

  • Total water content of the build-up, year on year
  • The water content of the layer that was flagged
  • The set-up, so the result can be checked and reproduced

How the U-value itself is calculated, with the same roof worked through: U-values and Approved Document L.

Your next step

Your roof.
Your brief.
The analysis.

Tell us about the building and the roof. The moisture check is part of how HOLVEM specifies, not an extra you have to ask for.

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