Vapor diffusion and vapor permeability

Vapor diffusion is a physical property in the field of hygrothermics that describes the movement of the water vapors from the areas with bigger pressure towards the areas with lower pressure. The Vapor Permeability Factor, μd, represents the number of times one certain material is less permeable than another. The Vapor Permeability Coefficent, μ and the Restistance to Vapor Permeability, Rv is the capacity of the construction element to oppose to the humidity transfer. Rv=d(μd)M [m/s], d being the width of the element and M being a constant value.