Water permeability is an important item for many applications of block co-poly(ether-ester) films. For segments, such as protective clothing or construction applications. water permeability is not only governed by the polymer film, but also by stagnant air layers surrounding the film. We interpreted this barrier effect by a series of permeability experiments in a standardised ASTM E96B test, varying systematically the air layer thickness adjacent to the block co-poly(ether-ester) film. Furthermore, we developed a mass transport model describing the water transport in the polymer film and air layer. Experimentally, we observe two regimes. For a relatively small air layer thickness, the water permeability varies strongly with varying air layer thickness, for a relatively large thickness there is hardly any dependence. With the model at hand, we rationalise these effects. For the first regime, the water transport in the air layer is diffusion driven, in the second regime it is of a convective nature. The strong water permeability dependency is interpreted in terms of the water sorption isotherm of the polymer material. The predictive description of the model is of a quantitative nature.