Few topics in building science seem to have caused as much confusion as the use of a polyethylene vapor barrier in exterior walls.
Once routinely used by builders to prevent the migration of interior moisture into wall cavities, polyethylene is no longer recommended for houses unless they’re built in extremely cold climates.
But related questions keep coming, including this query from Ed in Chicago, which was posted in the Q&A forum at GreenBuildingAdvisor: “It’s well known that improper use of vapor retarders can prevent walls from drying out properly and leading to moisture related issues such as mold and rot,” Ed writes. “Is it uncommon to have moisture issues in walls due to air leaks or vapor diffusion if the vapor retarder is left out? Would there be a concern with moisture issues for a standard construction wall (siding, plywood sheathing, 2×4 or 2×6 framing, fiberglass or cellulose insulation, drywall without vapor retarding paint) if the vapor barrier is not used in cold climates?”
The house under construction (not Ed’s, but the house of someone he knows) won’t include a poly vapor barrier — but the builder doesn’t plan any special effort to make it airtight, either.
“It seems like every story I hear about wall moisture problems is because of improper use of vapor barriers, not because it was left out,” Ed wrote. “Is there concern that leaving out the vapor barrier without making the house reasonably airtight lead to a moisture problem?”
That’s the subject of this month’s Q&A Spotlight.
Don’t forget, vapor barriers had a purpose
Malcolm Taylor reminds Ed that problems associated with the movement of interior moisture is what prompted builders to begin using vapor barriers some 40 years ago, and that walls built in the…
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