CSLB General Building (B)Finishes and Building EnvelopeMedium
A contractor is selecting a vapor retarder membrane for a wall assembly and reviews a manufacturer's data sheet showing a permeance rating of 0.5 perms. According to standard vapor retarder classifications used in building codes, how is this material classified?
- AVapor permeable membrane (not a retarder)
- BClass II vapor retarder
- CClass I vapor retarder
- DClass III vapor retarder
Show answer & explanationAnswer & explanation
Correct answer: B. Class II vapor retarder
Vapor retarders are classified by permeance: Class I is 0.1 perms or less (e.g., foil, polyethylene sheet), Class II is greater than 0.1 up to 1.0 perms (e.g., kraft-faced batts, some vapor-retarder paints), and Class III is greater than 1.0 up to 10 perms (e.g., latex paint). A rating of 0.5 perms falls within the Class II range.
Why the other options are wrong
- A. A material this low in permeance still functions as a retarder, not a permeable membrane.
- C. Class I requires 0.1 perms or less, well below 0.5.
- D. Class III covers 1.0–10 perms, above this material's rating.
Vapor Retarder Classes
Vapor retarders are grouped into three classes based on water vapor permeance, which determines how much moisture diffusion they allow through a wall assembly.
- Class I: ≤0.1 perms (foil, polyethylene)
- Class II: >0.1 to 1.0 perms (kraft-faced batts)
- Class III: >1.0 to 10 perms (latex paint, some primers)
Memory trick: 'I is impermeable, III lets it through' — lower Roman numeral, lower perm rating.