Few topics in building science have been as thoroughly haggled over as vapor retarders, the materials designed to keep water vapor from migrating into wall and roof assemblies. In a paper published nearly 20 years ago, Joseph Lstiburek, a principal at Building Science Corp., summed up the problem this way: Building professionals may have come to understand the challenges of water in liquid and frozen form, but water vapor is “in a class of craziness all by itself.”
The problem is that water vapor doesn’t always stay in vapor form. When it comes into contact with a surface that is below the dew point, vapor becomes a liquid. This can cause a host of problems inside wall and roof cavities. Vapor retarders are intended to stop water vapor from moving into a wall or roof assembly in the first place so this can’t happen.
“The fundamental principle of control of water in the vapor form is to keep it out and to let it out if it gets in,” Lstiburek wrote. “Simple, right? No chance. It gets complicated because sometimes the best strategies to keep water vapor out also trap water vapor in.”
That was the problem with polyethylene sheeting, once a widely used vapor retarder in cold climates. In technical terms, poly is a Class I vapor retarder with a perm rating of no more than 0.1, on a par with sheet metal and glass (a “perm” is a measure of how much water vapor can pass through a material). Poly was typically applied on the warm side of a framed wall. In a heating climate, and when detailed carefully, the poly prevented warm, moist air from traveling into wall cavities in the winter where it could condense on the back side of cold sheathing. But if water…
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