Despite the extensive attention that water transport in Nafion membranes has been subject to, several phenomenal still remain unexplained. It is for instance not clear why desorption rate is much faster than absorption. The purpose of this paper is to suggest an answer to that problem. A simple but thermodynamically consistent model is formulated within the framework of Classical Irreversible Thermodynamics including swelling and interaction of the membrane with the surrounding water vapor. Numerical solutions to the model are in a good match with experimental data of sorption dynamics. By simplifying the model (excluding swelling and assuming instantaneous diffusion within the membrane), we are able to identify the source of the asymmetry between the absorption and desorption rates - the shape of the uptake curve. This natural explanation of the asymmetry is then discussed quantitatively (showing an agreement between both the full and the simplified model but also with the experimental data) and also in the context of active carbon sorption.