Vapor Barrier? Retarder? What’s the difference?

The effectiveness of a material to control diffusion is measured by its permeability or perms.  A perm is defined as the ability to pass one grain of water vapor per hour through one square foot of flat material at one inch of mercury (gr/h*ft²*in.Hg).  One grain of water is 1/7000 of a pound or 0.0022 ounces of water.  Many building materials are tested to measure permeability, the result of this test is perm rating.

A perm rating is a standard measure of the water vapor permeability of a material. The higher the number, the more readily water vapor (in the gaseous state) can diffuse through the material. A perm rating of less than 0.1 is considered a vapor barrier; perm between 0.1 and 1 is considered a vapor retarder; a perm between 1 and 10 is semi-permeable; and a perm rating greater than 10 is considered permeable.  The four general classes of materials based on permeance are as follows:

Air/vapor barrier material manufacturers all report the permeability of their products. Design professionals use that information to specify materials.  And now you know.