Test procedure for determining vapor permeance class is ASTM E-96 Test Method A (the desiccant or dry cup method).
▪ Class I: Materials that have a permeance of 0.1 perm or less (Note: This is 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 of greater than 10 perms

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