The equation used in calculating water
vapour diffusion through materials is given as:

(10)

where m = mass diffusion rate of water
vapour or vapour transmission

A = cross-sectional area for water vapour
diffusion

m = **vapour permeability**
defined as the property of material that determines the rate at which vapour passes
through it under the influence of unit pressure gradient across the material.

pw = vapour pressure

x = distance along the flow
path

Therefore, m/A is mass of
vapour diffusing through unit area in unit time and dpw/dx is vapour pressure
gradient.

If permeability is taken as
the average permeability coefficient applicable to the varying conditions along the flow
path of length L, and its value is assumed to be independent of vapour pressure and
temperature, Eqn. (10) becomes:

(11)

where M = total mass transmission rate of
vapour, ng/s

A = Area of cross section
of flow path, m2

D Pw = difference of vapour
pressure between ends of flow path, Pa

L = length of flow path or
thickness of wall, m

m = average permeability, ng/(s m Pa)