How “Smart” Vapor Retarders Work

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.

A 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

DISCUSSIONS

Reader-contributed comments related to How “Smart” Vapor Retarders Work - EBN: 21:9. Comments are listed with newest at the top.

correction/clarification

Posted by Greg La Vardera on Jan 5, 2013, 03:49 PM
You wrote: "MemBrain is the best-known nylon brand"

"MemBrain" not really a "brand" but is the trade name used by Certainteed for their nylon based membrane. This is a rebranding of the Isover product sold in the EU, where it does not have nearly so silly a name. Certainteed gained access to this product after it was purchased by Saint Gobain, the mother company for both the Certainteed and Isover brands.

Furthermore you wrote: "Intello is the best-known brand" and "DB+, reinforced with fiberglass, is the best-known brand"

Both Intello and DB+ are made by ProClima, a German manufacturer of building products. So both of these are from the ProClima brand. Your description made it sound like there were two different manufacturers here, but these are both the same brand.

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