Vapor retarders are meant to protect our building assemblies from getting wet, but they can also slow drying. When humidity levels differ drastically between indoors and outdoors,
vapor diffusion can result, sending water vapor into wall cavities and other building assemblies. A dedicated vapor retarder manages this “vapor drive.”
But moisture seldom enters a wall or roof cavity just because of vapor diffusion—leaky roofs and poorly detailed walls that leak vapor-laden air are much more common culprits—and once things get wet, vapor retarders can trap moisture, contributing to mold and rot.
This is particularly problematic in mixed and cold climates; a vapor retarder should be on the “warm” side of the wall assembly, but in these climates, the warm side alternates—a problem that is imperfectly solved by putting the vapor retarder on the interior, warm-in-winter side of the cavity insulation.
With vapor permeance that varies based on humidity level, a “smart” vapor retarder helps resolve the dilemma: it retards vapor diffusion in less-humid conditions while allowing drying when conditions are more humid. These vapor retarders are
hygroscopic, which means they absorb water when exposed to ambient humidity. Water molecules lodge between the molecules of the material and become like open valves, allowing water vapor to travel freely through the membrane.
Although many building materials have the property of variable permeance, three membrane products are designed specifically as vapor retarders.
Nylon has highly variable vapor permeance; MemBrain is the best-known nylon brand.
polyethylene copolymer (not to be confused with impermeable polyethylene-only membranes) is a hygroscopic blend of polyethylene with one or more other plastics. Intello is the best-known brand.
Paper-based membranes can also be used as smart vapor retarders. DB+, reinforced with fiberglass, is the best-known brand.
Any type of smart vapor retarder must be properly placed. If drying to the inside of the building is intended, for example, then all interior finish materials, including paints and wall coverings, must be vapor-permeable.
September 1, 2012
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