Water vapor permeability and water vapor transmission rate

Water Vapor Permeance :- The time rate of water vapor
transmission through unit area of flat material or construction
induced by unit vapor pressure difference between two specific
surfaces, under specified temperature and humidity conditions.

Water Vapor Transmission rate :- The steady water vapor
flow in unit time through unit area of a body, normal to specific
parallel surfaces, under specific conditions of temperature and
humidity at each surface. 

Water vapor permeability test :- The purpose of these tests is to obtain, by means of
simple apparatus, reliable values of water vapor transfer
through permeable and semipermeable materials, expressed in
suitable units.