perm inch

perm inch
   a traditional unit of water vapor permeance. The perm value (see above) does not depend on the thickness of the material used as a water barrier.
   The permeance is the product of the perm value and the thickness, measured in inches. One perm inch equals about 1.453 x 10-12 kg/(m·Pa·s) at 0 °C or about 1.459 x 10-12 kg/(m·Pa·s) at room temperature. The SI unit kg/(m·Pa·s) actually simplifies to seconds (s).

Dictionary of units of measurement. 2015.

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