Water permeability is an important property of films used in, for instance, protective clothing or construction applications. There are a number of standardised test methods available for evaluating the permeability of films. In one class of tests, there is a direct contact between the polymer film and water; in another class of tests there is an air layer between water and the film. This paper deals with the modelling of water permeability of two commercial types of segmented block co-poly(ether-ester). Water transport in the films is modelled assuming a simple Fickian diffusion behaviour, sorption isotherms are described in terms of either a Zimm-Lundberg based cluster model or Henry's law. Mass transport in the air layer is described in analogy with heat transport for the classical Benard problem. Using this analysis it is shown that water permeability for both films and for both types of tests can be predicted as a function of the film thickness. There is a good agreement between the model predictions and the experiment.