I'm at the tail end of a new construction of a log home. The ceiling above the great room (24'W x36'L) has a 15/12 pitch, walls are 9' high and ceiling peaks at 21'. Presently, the roof consists of shingles, plywood deck, engineered I-beam joints (24" apart) filled with R-40 batts and finally 6 mil plastic vapour barrier. Now I'm wondering if I could add 2" R-10 rigid foam sheets on the furrings prior to adding my 5/8" pine interior wood siding, without removing the 6 mil plastic. Could a condensation problem develop with the above arrangement - the tongue and grove sheets will be taped, i.e., forming a second vapour barrier. The roof has a roof ridge ventilation design.
Answer:
That would certainly be a second vapour barrier and that is about the last thing you want in a cathedral ceiling. I wouldn't dare. R-40 in concert with a large diameter ceiling fan should do you quite well.
