Vapor Permeance: Classification

Class I: Materials that have a permeance of 0.1 perm or less.,

Note: this is also the definition of a "vapor barrier".

Class II: Materials that have a permeance of 1.0 perm or less and greater than 0.1 perm. 

Note: This is the definition of a "vapor retarder".

Class III: Materials that have a permeance of 10 perms or less and greater than 1.0 perm.

Class IV: Materials that have a permeance greater than 10 perms. 

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