Polyethylene sheeting has had its ups and downs as a preferred building material over the last 20 years.
At one time, it was routinely used in wall assemblies as a vapor barrier. As building scientists learned more about air and moisture movement through walls and ceilings, however, they began to advise builders that an interior vapor retarder is better than an interior vapor barrier, and the perceived usefulness of poly plummeted.
In most climates, air movement, not vapor diffusion, came to be recognized as a bigger threat to buildings. Air barriers, which can be vapor-permeable, became a more important priority. Builders also realized that because of its very low permeability, polyethylene had the potential of trapping moisture inside walls.
Even though poly has gradually lost its luster, a GBA reader nicknamed Minneapolis Disaster wonders whether it could have a place in an outbuilding he’s putting up. In a a Q&A post at GreenBuildingAdvisor, Minneapolis lays out plans for incorporating a 6-mil poly air barrier in both the walls and ceiling of his planned building. In the exterior walls, the poly will be sandwiched between the exterior plywood sheathing and 5 1/2 in. of layered and taped polyisocyanurate foam insulation. In the ceiling, Minneapolis plans on adding poly between two thick layers of polyiso.
He’s encouraged by the practices of Alaska builder Thorsten Chlupp, who has successfully used poly in cold-climate buildings. In addition, the poly is available in 12-ft. wide rolls, which will make it easier to lap and seal than 10-ft. wide housewrap. “I’ve come to the conclusion that for the small building that’s underway in my yard, I’ll see good air-sealing results from 6-mil poly, adhered at edges and openings, around the walls and another layer lapped over, across the ceiling,” Minneapolis writes.