In this paper, we develop a discrete approach to describe the transport of condensible vapors through a microporous substance. We consider only isothermal water migration under uniform atmospheric air pressure, at temperature lower than 100C with negligible gravity. The pore-structure which is supposed to be representative of the material is built on a 2D random network of tubes. The basic phenomena (adsorption/desorption, diffusion, condensation) that occur during the water vapor transport in a single cylindrical pore at the steady state are taken into account. We underline that capillary condensation can be expected to cause vapor flow amplification of some few orders of magnitude. This phenomenon provides us with a possible explanation of some experimental results.