This research focuses studies of water permeation potential through a polymer membrane with the help of a standard field and laboratory emission cell. Special efforts are devoted to finding a correlation governing the relations between the number of transfer units (NTU) and the moisture exchange effectiveness. As a first step, moisture diffusivity in the hydrophilic polymer membrane is experimentally measured. In combination with mathematical modeling, the moisture concentration distributions in the cell, the water uptake gradients in the membrane, as well as the local vapor emission rate on membrane surface, are investigated. The results are that the emission rates show a non-uniform character and a polynomial equation governing the moisture exchange effectiveness and the dimensionless number of transfer units, could be used to inversely estimate the diffusivity of water vapor in hydrophilic membranes. The form and the value of constants in the equation are obtained. (c) 2005 Elsevier Ltd. All rights reserved.