I have the internal walls on the first floor going up and I initially thought I would build them on the joists but I have been told that I could just put the whole floor down, 22mm Kaber protect+, and build them on that - thus making things a bit simpler, somewhat safer when building and more fireproof when finished does that sound about right? (There are a couple of places where structural beams go up through and these will not sit on the floor panels as they may crush!)