It 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 U. S. Department of Energy's has more information about vapor barriers and vapor retarders .
