I'm about to crack on with the garage ceiling that is below a new bedroom. The plans state 150mm celotex between joists and 50mm underdrawn. The plans don't detail the plasterboard. Would normal 12.5mm board suffice, or does it need to be fireline? And is 1 or 2 layers going to be required. Take some pretty long screws to get through 25mm plasterboard and 50mm pir.