For a flat roof construction – be it a cold roof, a warm roof, or a warm inverted roof – the calculation method for year-round condensation risk set out in BS EN ISO 13788, known as the Glaser method, is almost always sufficient. As long as the roof accords with good practice then a more complex calculation method should not be necessary.
Good or best practice in terms of condensation risk means following the guidance of BS 5250 ‘Control of condensation in buildings’. Ensuring the roof includes a suitable vapour control layer on the warm side of the insulation layer is an appropriate starting point.
What does BS 6229 say about condensation risk in flat roofs?
For a flat roof design to be considered good practice, it should also follow the recommendations of BS 6229 ‘Flat roofs with continuously supported flexible waterproof coverings – code of practice’. We have written about the revised version of BS 6229, published in 2018, in this post.
Section 4.7 of BS 6229 offers the standard’s own summary of how to deal with condensation risk, and says the risk of surface and interstitial condensation should be analysed based on BS 5250 and BS EN ISO 13788.
The key part of this section, and the reason for this blog post, is where it says the risk should be assessed “using an external temperature of -5 deg.C for 60 days during the heating season, to allow for the cooling effect of clear sky radiation.”
In simple terms, and as the name suggests, ‘clear sky radiation’ refers to the radiative cooling that takes place when there is no cloud cover to restrict the emission of longwave radiation from the Earth’s surface.
Are winter parameters for flat roof condensation risk analysis new?
The recommendation to use an external temperature of -5 deg.C for 60 days during the heating season (i.e. winter) is not new. It has featured in versions of BS 6229 since the 1980s, but has only come to prominence with the substantial revision of the standard that took place in 2018.
Calculation software, which normally produces a Glaser method condensation risk analysis alongside a U-value calculation, does not automatically apply the lower temperature. The climate data used by software is location-specific, not application-specific. Since the recommendation to use a lower winter external temperature is specific to flat roofs, it is incumbent upon clients to ask for the data to be amended, or for software users to amend it if they want to follow the recommendation.
In truth, for cold, warm or inverted warm flat roof designs that follow best practice guidance, using a winter temperature of -5 will not make a difference to the outcome of a condensation risk analysis. It will change the output of the analysis slightly, in terms of the temperature and dewpoint charts, but it will not change the outcome of no predicted condensation risk.
Where a difference might be seen is in non-standard roofs that do not follow best practice, such as hybrid (or ‘over and under’) flat roof constructions. These types of build-up are not recommended as they heighten the risk of condensation anyway, so adherence to the winter external temperature recommendation will only serve to reinforce the unsuitability of these roofs.
What is Polyfoam XPS’s approach to winter condensation risk in flat roofs?
Since the revision of BS 6229 brought to prominence this recommendation for a lower winter external temperature, Polyfoam XPS have adopted the measure as standard. We always aim to accord with best practice and are happy to apply this measure to U-value calculations and condensation risk analyses for both conventional warm flat roofs and inverted warm flat roofs.
The standard does not specify which 60 days of the heating season the measure should be applied to, so we elect to amend the external temperature for January and February. Customers are also free to specify other parameters if their project requires. For example, we recently dealt with a technical enquiry where the external temperature was adjusted for three months of winter – but that is a rare occurrence.
For more information about our U-value calculation service, or to discuss your project, contact us.
Published May 2019.