Ive used a foil type on the inside of all exterior walls, its battened on, to which the slabs will be attatched. We followed the manufacturers advice and no mention of gap on wall side of foil. The foil is directly on the wall. We also have an insulation in the cavity. We are awaiting the skim finish to be applied. Its a dormer so we used fibre glass in the rafters and then placed the foil accross the rafters, battened up the rafters and slabbed them.