A material that has a permeance of 0.1 US perm or less. A vapor barrier is a material that is essentially vapor impermeable (e.g., metal, glass, thick plastics, unperforated epoxy paint). A vapor barrier is a Class I vapor control layer. The test procedure for classifying vapor barriers is ASTM E-96 Test Method A—the desiccant or dry cup method.

Related terms: Permeance, Vapor Control Layer

Related resources: BSD-106: Understanding Vapor Barriers; RR-0004: Air Barriers vs. Vapor Barriers; CP-1101: The Influence of Low-Permeance Vapor Barriers on Roof and Wall Performance