The Difference Between Permeance and Permeability

The moisture transmission rate of a material is referred to as its permeability, stated in perm inches. Permeance is the number that should be used to compare various products in regard to moisture transmission resistance. 

Even though this number is not dependent on the materials' thickness, its' permeance, is dependent on thickness much like the R-value in heat transmission. Dividing the permeability of a material by its' thickness gives the materials' permeance, stated in perms. 

Water vapor permeance is also a measurement of vapor transmission. However it is a performance evaluation, and not a property of a material. 

U.S. units are typically reported as perm-inch (1 perm-inch = the passage of 1 grain of vapor through 1ft2 of 1 inch thick material in 1 hour under a pressure difference of 1 inch of mercury). 

Water vapor permeability is commonly used to describe the water vapor passage through bulk materials such as thermal insulation. U.S. units are typically reported as perms (1 perm = the passage of 1 grain of vapor through 1ft2 of material in 1 hour under a vapor pressure of 1 inch of mercury). 

Water vapor permeance is commonly used for reporting the water vapor that passes through thin materials such as insulation coatings. (This link was checked on Mar-01-2004Online source) [Note: 1 ng/(s.m^2.Pa) = 1.459 perm/inch]

Permeability is the rate that water or vapor will pass through a membrane. The test used is Moisture Vapor Transmission ASTM-E 96 or Moisture Vapor Permeability D-1653. The results are measured in “perms”. The lower the number the less moisture vapor will come through the membrane. This is extremely important in immersion conditions. Lower permeability is better.

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