A perm is a measured amount of water vapor transmission through a medium - defined as grains of vapor transmitted per hour per square foot per inch of mercury pressure differential across the medium. Impermeable materials have a perm rating of less than 1. Semi permeable materials have a rating between 1 and 10. Materials with a rating of more than 10 perms are permeable to water vapor.
See the following for additional perm information:
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