In the 1970s and 1980s, the practice of installing polyethylene on the interior side of wood-framed walls became common in much of the U.S. These days, however, installing interior poly is nowhere near as common as it used to be; the practice has all but disappeared in the U.S. outside of Minnesota and Alaska.
In Canada, however, the use of interior polyethylene is alive and well.
Why did American builders abandon interior polyethylene? There were two main reasons:
The roller-coaster history of interior polyethylene leaves many builders, especially Canadian builders, with lingering questions. Here at GBA, we occasionally get questions from Canadians who ask, “Should I still be using interior polyethylene? Everyone up here uses it, and building inspectors want to see it.”
These are good questions. It’s time for a fresh look at the issue of interior polyethylene.
Why did cold-climate builders begin installing interior polyethylene in the 1970s? Back then, the standard answer was, “Polyethylene is a vapor barrier. Walls need interior vapor barriers to prevent the outward flow of moisture during the winter.”
In fact, carefully installed interior polyethylene often improved the performance of cold-climate walls. However, the main reason that interior polyethylene helped wall performance was because the polyethylene was installed as an air barrier. Even back in the 1970s and 1980s, conscientious Canadian builders were installing polyethylene with taped seams or seams sealed with Tremco acoustical sealant. Whether the builders knew what they were doing or not, they were effectively reducing air movement through their walls. This air-leakage benefit was far more useful than any changes in the rate of outward vapor diffusion. (For more information on vapor diffusion, see “All About Vapor Diffusion.”)
Although interior polyethylene was sold as a way to limit vapor diffusion, it was also acting as an…