Glossary
Vapor
Any gas at a combination of temperature and pressure placing it very close to its dew point (saturation).
Vapor Barrier
A material that has a permeance of 0.1 perm or less. A vapor barrier is a material that is essentially vapor impermeable. 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.
Vapor Permeance
layer property that describes the ease with which vapor molecules diffuse through it. It is defined as the quantity of vapor flow across a unit area that will flow through a unit thickness under a unit vapor pressure difference. It is to vapor diffusion what conductance is to heat transfer. The unit of measurement is typically the "perm" (gr/h•ft2•in. Hg); in metric/SI units, it is stated in terms of ng/(s•m2•Pa).
Vapor Permeance Classes
Test procedure for determining vapor permeance class is ASTM E-96 Test Method A (the desiccant or dry cup method). ▪ Class I: Materials that have a permeance of 0.1 perm or less (Note: This is the definition of a "vapor barrier.") ▪ Class II: Materials that have a permeance of 1.0 perm or less and greater than 0.1 perm (Note: This is the definition of a "vapor retarder.") ▪ Class III: Materials that have a permeance of 10 perms or less and greater than 1.0 perm ▪ Class IV: Materials that have a permeance of greater than 10 perms
Ventilation
The intentional flow of air into occupied spaces or behind cladding/roofing to move heat and moisture in a desirable manner.
VOC
abbr. Volatile Organic Compound. Often considered a pollutant detrimental to indoor air quality.