Open Cell Foam Insulation: Can it ever function as a vapor barrier?
ADUinProgress
| Posted in General Discussion on
Maybe I’m misunderstanding or missing information but I was under the impression that only closed cell foam insulation forms a vapor barrier. I am being told by someone quoting on a job that 4″ of open cell will form a vapor barrier. What am I missing?
Thanks in advance.
Replies
Everything I’ve ever read says open cell foam allows moisture to pass through, no magic thickness number suddenly stopping it. Every site I just looked at says the same thing.
It sounds like the only thing you are missing is a reason to trust other things this particular person told you. (like timing, price and materials they would use on the job)
Before insulating my shed roof with cut/cobble polyiso (from above while redoing the roofing), I checked out prices for closed cell (wowsers!). Armed with every bit of GreenbuildingAdvisor's advice I could hold in my head, I contacted the recommended "best in the area." The one my contractor friends said was the best gave me the closed cell price, and then said, "well, of course you can do 8" of open cell." I asked "but I can't use open cell in an unvented assembly!" "Well, why not? We do it all the time."
https://www.energyvanguard.com/blog/will-open-cell-spray-foam-insulation-really-rot-your-roof/