Need help with recycled polyiso insulation in attic
I found a great deal on recycled polyiso boards that were once a roof on a commercial building with EPDM rubber over it. Each board is 4 foot by 4 foot and 4 inches thick.
I am going to use them in a 1830 post-and-beam house. The boards are going to be going in the attic. There is currently no insulation or vapor barrier in there. The house is in upstate NY and sees some seriously cold winters (at least in the past).
I got the foam and my plan was to cut each board to fit between the ceiling joists and great stuff around the edges. I have enough foam to do 8 inches which will be two layers and I will overlap the seams. This should create a good tight envelope for the ceiling. The ceiling joists are 4x6s, 24 in. on center.
My question that I need help with is with the vapor barrier/retarder. The foam I have is foil-faced but the foil is deteriorated and not easily repaired. Does it make sense to cut strips of 10-mil poly 24 in. wide and lay them in the joists bays against the sheet rock ceiling and staple them to the side of the joists then lay the foam in and seal it in place? Should I not do a vapor barrier at all and just rely on the deteriorated and perforated foil facing?
My latest idea was to lay in Reflectix then the foam for added thermal barrier which would also act as a vapor barrier.
The space above the foam will be a good sized open unfinished attic ventilated at the peaks.
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