What is a perm rating?

Home Energy May 21, 2014|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 Class I impermeable vapor retarder (which is also considered a vapor barrier); perm rating between 0.1 and 1 is considered a Class II semi-permeable vapor retarder; a perm rating between 1 and 10 is a Class III permeable vapor retarder; and a perm rating greater than 10 is highly permeable and not considered to be a vapor retarder.. The U. S. Department of Energy's has more information about vapor barriers and vapor retarders .

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