Hi all I have finished building a 5x3m workshop which is made of 2x4 Timbers, wrapped in breathable membrane then batterns on top of that and finally cladding. I now want to start on the inside. This workshop will having some basic heating and will be used all year round. I have already brought celotex to put between the rafters in the floor and ceiling. so I have a few questions : Once I have put celotex in the floor can I just put the osb straight down or should I cover over with polythene sheets first? Same question for the ceiling but I will be plasterboarding this. And for the walls, which again will be plasterboard, I'm planning on putting 100mm polystyrene sheets between the timber (cheaper than celotex) will this be OK? and again should I just plaster board over this or put polythene sheets over first? Many thanks.