Units of permeability.
Class 1 vapor barrier - 0.1 perms or less
Class 2 - 1 perm
Class 3 - 1-10 perms
Built-in barrier in underlayment - can be used instead of dedicated vapor barrier - [1]
Perm rating - vapor barrier vs vapor retarder. A vapor barrier is a Class 1 vapor retarder. [2]
