A vapor barrier is a material, often polyethylene sheeting or a coating with a low permeability rating, installed in a wall or foundation assembly to slow the movement of water vapor through the masonry rather than blocking liquid water directly.
Vapor moves through masonry differently than liquid water does, it diffuses through the material itself over time, following temperature and humidity differences on either side of the wall. A vapor barrier is placed to control which direction and how much of that diffusion happens, which matters most in below-grade spaces like crawlspaces and basements where ground moisture constantly pushes vapor toward the drier interior air. Getting the placement wrong, on the wrong side of an insulated wall assembly, can trap moisture inside the wall instead of keeping it out.
This is a detail worth getting right rather than guessing at, because a vapor barrier installed on the wrong side of a cold-climate wall assembly can cause condensation to form inside the wall cavity every winter, which does slow, hidden damage that's harder to diagnose than a straightforward leak. On any project where we're closing up a foundation or crawlspace wall, we account for which way that vapor is actually trying to move before deciding where the barrier goes.
More masonry vocabulary from the glossary.
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