Term: Vapor Barrier

Also known as vapor closed, a vapor barrier is 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.

Source: BSD-106: Understanding Vapor Barriers