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
Many thanks