What is a perm rating?

Home Energy November 03, 2010|Print
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 .

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