Can a SIPs flat roof ever be called a warm roof???

[GrumpyBear]

Hi all,

 

We're currently building a 50m2 rear and side extension using SIPs.  The SIPs structure has been completed, and includes a flat roof structure also made with SIPs.  To make a long story short, I've been told by various sources that converting the flat roof from a vented cold roof design to a hybrid roof with tapered insulation on top is a feasible, though not officially approved solution.  I have had WUFI and DELPHI hygrothermal analyses of the proposed build-up sent to me by smart membrane manufacturers which show that this will work all the way down to 60mm of tapered PIR insulation.  The tapered insulation manufacturer has just come back, however, stating that the only AVCL they will permit is above the deck (above the SIPs), that they won't approve the use of their products if another Smart Membrane is used below the panels, and that the minimum thickness should be 120mm of PIR (most of which will end up being between 150mm and 255mm) The problem with this is that the structural panels will be exposed to humidity from below without protection from an AVCL, and will quite likely soak up moisture over time.  They are referring to our build-up as warm roof, despite the fact that the panels are insulated and sit between joists.  In addition, I'm pretty sure they're using the simpler glaser method dew-point analysis as opposed to the more sophisticated and accurate WUFI analysis.  I'm thinking of two options here:  

 

1.  Agree to get the tapered insulation with the impermeable membrane above the deck and below the tapered insulation. Seal all panel joins and joists with tape and/or sealant without using a smart membrane over the underside of the panels.

 

2.  As above, but use the smart membrane anyway, as the WUFI (from Partel) and DELPHI (from SIGA) analysis says that it will work.

 

Current build up as proposed is the following:

 

OUTSIDE

1.2mm EPDM

tissue backed, PIR tapered insulation (120mm minimum)

bituminous vapour check membrane

11mm OSB3

172 grey EPS

11mm OSB3

(optional smart membrane like SIGA Majrex, Intello Plus or Partel Vara Plus)

50mm service void

plasterboard

INSIDE

 

Happy to attach the analyses if helpful...

 

Many thanks in advance for the input and help.

 

GB