water vapor transmission of a body between two specified parallel surfaces, induced by the vapor pressure difference between two surfaces. The test conditions must be stated. An accepted unit of water-vapor permeance is the perm. Note: Permeance is a performance evaluation and not a property of a material. See perm; rep.

1. the portion of atmospheric pressure exerted by entrained water vapor, at a specified temperature.

2. pressure due to water vapor.

steady vapor pressure difference that induces unit time rate of vapor flow through unit area of a flat material (or construction that acts like a homogeneous body) for specific conditions of temperature and relative humidity at each surface. Note: Vapor resistance is the reciprocal of vapor permeance. It is the arithmetic product of the resistivity and thickness.

steady vapor pressure difference that induces unit time rate of vapor flow through unit area and unit thickness of a flat material (or construction that acts like a homogeneous body), for specific conditions of temperature and relative humidity at each surface. Vapor resistivity is the reciprocal of vapor permeability 

steady-state vapor flow in unit time through unit area of a body, normal to specified parallel surfaces, under specific conditions of temperature and humidity at each surface.