Polyethylene as an air barrier in very cold wall assembly
My question pertains to the use of a polyethylene air barrier (6 mil) within a double wall assembly (and ceiling) in a very cold climate. My concerns when we came up with this wall were obviously wall performance, related to moisture buildup and drying, as well as keeping the build relatively inexpensive both in materials/labor while at the same time providing me with a system provides high R-value while decreasing air leakage.
My designer who I trust and who has many years of success with double wall assemblies in cold climates, has provided me with the following assembly from out to in: siding, vapor permeable housewrap with drainage plane behind siding, wall sheathing, load-bearing 2×6 outer wall with Roxul insulation, 3″ XPS foam board between walls, poly air barrier to be fastened to the inner side of the XPS, then a non load-bearing 2×4 inner wall with Roxul, and finished with painted drywall.
Based on my knowledge, the air barrier is placed deep enough into the wall to not be affected by dewpoint, thus not having a condensation issue. If rain would get into my assembly from the exterior, my understanding is that it can dry to the outside, and any moisture on the inside of the poly could dry to the interior (having to move through 3.5″ of Roxul and painted drywall). My question is with XPS, which is quick to erect, provides nice R-value, and allows for a firm surface to attach the poly air barrier layer. Could moisture easily form between the poly and foam (there is ~R-40 separating the exterior to this layer). If the poly could be easily attached to another surface such as Roxul to fill the void between walls, I would do it. Just not sure how.
Any thoughts? Thanks in advance.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part