Passive House Modular Suspended Ground Floor Construction

[Diane Aspinall]

Hi all

We are involved with a Passive House modular building project with a suspended ground floor that is the standard construction of a well known modular building supplier.  Unfortunately, the Building Control Officer who will not accept this construction stating that it does not comply with AD C of the Building Regulations (resistance to moisture).

The suspended ground floor construction is described below.  The Building Control Officer is concerned that the radon barrier on the underside of the floor will cause interstitial condensation within the floor.  The modular building supplier argues that the vapour open EPS bead insulation will allow any water vapour that gets into the floor through the internal AVCL to evaporate through the open structure to the external walls and roof and then outwards through the external breather membranes than line these, but they are unable to provide any evidence to support this.

Does anyone have any ideas of how we could test or justify the modular building supplier’s argument in order to convince the Building Control Officer?  Would a WUFI calculation help, or is the moisture movement between the floor and external walls too complex?

At the moment our lovely 80 year-old client has a house that she cannot insure due to the lack of a Building Control Completion Certificate, so any help would be greatly appreciated.

The floor comprises:

Vinyl floor covering, 22mm T&G chipboard flooring, 25mm EPS insulation, 1200 gauge polythene, 18mm OSB 3 deck, 304mm Easi-joists with pumped Eco-bead EPS insulation (continuous with wall and roof insulation) , Monarflex Reflex Super Radon Barrier DPM, 15mm Versapanel cement bonded particle board over a ventilated sub-floor void over more than 150mm.

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