I am not sure were this question belongs, that why I post it here in the hope that somebody has an answer for me.
I am reading/attending the Attic, Insulation, Ventilation and Interior course. I am currently reading the section “Vapor Diffusion vs Vapor Barrier” the confusing part is:
quote:“A vapor barrier, as defined by the International Building Code (IBC), is classified as 1 perm or less …
Four General Categories of Perm Ratings
Class I vapor diffusion retarder
≤ 0.1 perm
Class II vapor diffusion retarder
> 0.1 perm and ≤ 1 perm
Class III vapor diffusion retarder
> 1 perm and ≤ 10 perms
Class IV vapor diffusion retarder
> 10 perms
Class I is often referred to as a vapor barrier and is considered impermeable. An example would be a sheet of polyethylene or non-perforated aluminum foil.
Class II vapor retarder is considered semi-impermeable. And example would be Kraft facing on fiberglass batt insulation or extruded polystyrene greater than 1 inch thick." quote end
and here the big question:
Class I and class II are both < perm1 and should be by definition both vapour barriers, yet class II is only a vapor retarder.
Any ideas why that is?