Okay, builders around here (Toronto) are starting to use 4×8′ sheets of either XPS or PolyIso (foil faced) insulation on outside of 2×4″ stud walls to increase insulation and decrease air penetration.  Great…except…PolyIso and XPS are vapor retarders. Perm rating < 1.  Interior already has poly sheeting vapor retarder.

Double vapor retarders.  This has always been a definite no-no to my understanding.

I asked one company (after seeing the logo in my father’s subdivision). Here was the response:

“The existence of the 2 vapor retarders is not a problem when the exterior sheathing and the interior vapor retarder are installed per their respective code requirements and code designated exterior flashing procedures are followed.

By properly excluding the intrusion of moisture from the outside of the structure, the moisture commonly present in building materials does not promote the growth of organisms or promote wood deterioration. Properly installed interior vapor retarders will prevent additional moisture from entering the wall system from the interior.

The intent is to prevent moisture entry from sources outside the wall structure – from either side. The equilibrium of moisture content in the new materials will be reached soon after the building is finished and will be at levels which will not be harmful to the wall system components. “

Any thoughts? 

Regards,

Tim Ruttan