PassivHaus Flat Warm Roof

[Architype]

Hi,

I'm currently looking at detailing a flat roof for a PassivHaus project in Wolverhampton. Usually we use a breathing roof (OSB/400 I beams & Warmcel/breathable board/vent/ply deck etc.)  on previous flat roofs with parapets we always end up with large vented ridges/huge firrings etc etc. But on this project we have decided upon a warm flat roof in order to avoid this situation (excessive onsite carpentry/tricky membrane installation etc).

At the moment we are looking at: Timber structure/OSB/Intello/EPS or XPS/EPDM - looking through the detaile flat roofing book it seems that there is a risk of condensation build up on the underside of the membrane (why we are thinking of using the Intello to let the building 'breath' back into the building instead of out)

I was just wondering if anybody here had any experience of warm PH flat roofs or this roof construction in general - primarily looking for tips/guidance/WUFI calcs?

Thanks in advance.

Tom Mason

[Mark Siddall]

Hi Tom,
I've no experience of this but I would tend to agree with the Detail book. The EPDM will seal in the moisture that diffuses through the construction. Using Intello will mean that it is not "breathable", in a the classical sense, but should help to reduce risk (it is simply more diffusion open that would otherwise be the case with other vapour retarders). Moisture migration will tend to occur in the winter and reverse diffusion, back through the Intello, during the summer. In my view inter-seasonal storage of moisture really calls for a WUFI calc (does your Warmcell have sufficient capacity to absorb all the moisture?). Ultimately you'd need fairly robust weather data (and this may be harder to get in the the urban heat island of Wolverhampton).

HTH
Mark