Been thinking about different finishes for the vaulted ceiling in our new extension, and I've come up with the idea of timber cladding with gaps in between (a bit like what people use on the outside of buildings), think it will look nice in there, and a little different from the usual plaster.   Obviously the roof trusses are timber, and the insulation is PIR with foil face.   Question is, do I need to have any sort of fire retardant barrier behind the timber cladding on a ce