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A vapor barrier is anything which completely stops the movement of water vapor. The question arises, do we need to completely stop the movement of water vapor in our homes? Is it ever like horseshoes, can close be good enough to win?
One simple answer is that sometimes we don’t want to stop the movement of water vapor at all. In spite of all our efforts, sometimes houses get wet. When they do, we want them to dry out. In a case like this, the last thing we want is a barrier to stop the movement of water vapor.
More to the point, when we do want to stop the movement of water vapor, close is often good enough. In many situations, rather than use a vapor barrier, we can get away with a vapor retarder.
The ability of a given building material to retard the diffusion of water through the material is called permeability. This ability is measured in the laboratory and materials are assigned permeability factors, or perm factors.
A material with a perm factor of zero will completely stop water vapor from diffusing through the material. This material is referred to as a vapor barrier.
Materials with perm factors between zero and .1 are called Class I vapor retarders.
Materials with perm factors between .1 and 1 are called Class II vapor retarders.
Class III vapor retarders are problematic.
Some materials that are water vapor permeable are unfinished drywall, cellulose and fiberglass insulation, and house wrap.
So there we have it. When selecting materials to create a vapor barrier, don’t hesitate to use vapor retarders with low perm factors. The results can be excellent.