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Vapor barrier I have read through the full thread and Im still confused! Ok I have a 35 yo ranch style home that has steel siding then 1/2 black (Buffalo?) board. Looks like a thicker type of tar paper board. The inner wall board is OSB. Walls are framed with 2X6 studs then standard pink insulation with plastic vapor barrier and drywall.
The headline of your question is "Insulating an old house with no vapor barrier." Perhaps the first thing you need to know is that the presence or absence of an interior vapor barrier is irrelevant. (If you want a vapor retarder on the interior side of your walls, you can always apply vapor-retarder paint. But that's not usually necessary.)
The original justification for a warm-side vapor barrier was to solve the wall moisture problem that arose with the advent of filling the walls with insulation, which at that time was fiberglass. The insulation lowered the temperature of the inside surface of the sheathing below the dew point of the outward vapor flow.
Rigid foam board, especially foil-faced polyisocyanurate, creates a vapor-impermeable barrier, so the wall would have limited drying potential. Even more permeable types of insulation, such as expanded polystyrene, are vapor barriers when the installation is thick enough.
My basement has a 3.5-foot high concrete block wall, with a standard framed wall above. The concrete block is currently finished with 2x4 framing, fiberglass insulation and poly vapor barrier covered with gyproc, which I know is not what is recommended but that is how we bought the house.
You don’t need a vapor barrier when insulating a ceiling between conditioned space and a vented attic. You sometimes need a vapor retarder (you usually don’t want a vapor barrier) in some types of walls. I think what you have is something like the “Devil’s Triangle” area of story and a half type homes.
Often, the air barrier goes somewhere in the middle of such a wall. That’s a really good place to put the air barrier because it’s protected from damage from both sides. Where’s the insulation? The second perspective of air barrier location has to do with its relation to the thermal control layer (i.e., insulation).
The wall you describe -- one with 4 to 6 inches of rigid foam on the exterior side of the peel-and-stick -- is a type of PERSIST wall. This type of wall works for the reason given by Kevin Dickson -- the vapor barrier is on the warm side of the rigid foam insulation, not on the cold side as it is at the Posluszny house.
The 2 inches of spray foam will help maintain a higher temperature in the wall assembly, keeping surfaces above the dew point during the winter. However, some overzealous builders install a polyethylene (plastic) vapor barrier on the interior. If the wall has a vapor retardant layer on both sides, it has limited drying potential.
Regarding the combination of a vapor barrier and rigid foam boards on the walls of a crawlspace, I agree that the combination of the rigid foam and vapor barrier on the walls is redundant if diffusion is the mechanism for moisture movement into the crawl space, but if there is any risk of bulk water intrusion, a vapor barrier on the wall (with ...