Insulation layer and stud wall

[Zebra579]

I am building a stud wall to cut my bathroom in half. The room has a flat roof. Between the roof joists I am fitting 100mm deep of rigid foam insulation; beneath this 25mm deep continuous layer of foil-backed rigid foam insulation, with the joints foil-taped to act as an airtightness layer and prevent moisture getting into the void above the insulation. The question is: if the wall plate for the stud wall is fixed directly to the roof joists, then there will be a gap in the insulation/airtightness layer, but if I put the wall plate under this layer, then it wouldn't be as securely fixed to the joists. What is the standard way round this?
Many thanks

 

[AronSearle]

Not sure about the standard way.

The wall plate fixed through the insulation, into the joists. Insulation is non compressible, so not sure why you consider this insecure?

I'm assuming you are putting the wall plate up, then attaching studs, rather than lifting it in place. so if it fights in between insulation, why not VCL to face of stud, as you would in a timber frame wall?

 

[noseall]

Are you venting the roof space?

The vcl membrane gets punctured by the plasterboard fixings. So by screwing your header plate through the wood, through the membrane and through the insulation into the joists, ain't gonna make much difference.

You will still need to ventilate because this is not a warm roof method of insulating.

 

[Zebra579]

Yes, I'm venting the roof space. In fact, I had a disaster already when I fitted all the ceiling insulation, taped and sealed the joins, but somehow some air got up there from the bathroom, and on looking above I found the entire roof deck had gone mouldy underneath, so I took it all down again, made more ventilation, and now want to be super-sure I'm getting the airtightness right.
So, under the insulation layer is OK then?
Thanks very much.