Commercial Building Inspector B2Building EnclosureHard
An inspector is performing a rough-in inspection for a new commercial kitchen. The plans specify a wall assembly requiring a Class I vapor retarder. Which of the following materials would satisfy the requirement for a Class I vapor retarder?
- AAsphalt-impregnated building paper (Grade D)
- BPolyethylene film (6-mil)
- CKraft-faced fiberglass batt insulation
- DFiber cement board
Show answer & explanationAnswer & explanation
Correct answer: B. Polyethylene film (6-mil)
A Class I vapor retarder has a permeance of 0.1 perm or less. A 6-mil polyethylene film typically has a permeance well below 0.1 perm, making it a suitable Class I vapor retarder. Kraft-faced insulation (Class II or III), building paper (Class II or III), and fiber cement board (vapor permeable) do not meet this requirement.
Why the other options are wrong
- A. Asphalt-impregnated building paper is typically a Class II or Class III vapor retarder.
- C. Kraft-faced insulation is typically a Class II or Class III vapor retarder (0.1 < perm <= 1.0 or 1.0 < perm <= 10, respectively).
- D. Fiber cement board is a vapor-permeable material and does not function as a vapor retarder.
Class I Vapor Retarder
A material with a very low vapor permeance (0.1 perm or less) used to significantly restrict moisture vapor flow.
- Permeance of 0.1 perm or less.
- Often used in cold climates or high-humidity environments.
- Examples include polyethylene film, foil-faced insulation.
Memory trick: Class I is tight, no vapor takes flight.