Nate Dorr

The days when every builder stapled 6-mil poly to the inside of every exterior wall are long gone. Experience in the 1980s and 1990s made it clear that in many situations, impermeable plastic on the inside face of a wall caused trouble rather than preventing trouble.

So in this century, codes have evolved. And in today’s world, recommendations for what building scientists are now calling the “vapor control layer” have become considerably more refined. Depending on the climate and the other materials involved in the wall or roof assembly, a designer could choose to specify either an impermeable material, a...

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