The Excellent Adventures of Vapor Barriers, Vapor Retarders, and Perm Factors

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.

Understanding Perm Factors

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. 

• Examples are glass, rubber and sheet metal.   These Class I vapor retarders have perm factors that are so low, for all practical purposes they are vapor barriers.  The amount of water vapor that diffuses through these materials is negligible.  Using these should give excellent results with no adventure.

Materials with perm factors between .1 and 1 are called Class II vapor retarders. 

• Class II vapor retarders are often good enough to accomplish the job of keeping water from diffusing from one side of the material to the other.  Examples are:  vinyl and oil-based paints.  Again, the results can be excellent.

Class III vapor retarders are problematic. 

• These materials have perm factors between 1 and 10 and, while they may do a serviceable job of stopping the movement of water vapor, materials with lower perm factors can usually be used.  Examples are plywood, kraft paper, and latex-based paints.

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.