Membrane-based enthalpy exchangers are an upcoming technology in energy efficient building ventilation systems. Evaluation of system performance and potential energy savings require appropriate theoretical models. This is challenging since the favored module geometry combines areas of cross- and counter-flow. With CFD simulation tools it is possible to accurately discretize such geometries. However, these models cannot be applied in process simulations. To overcome such limitations, we replace the complex module geometry by a combination of standard cross- and counter-flow units. Transition terms linking 1D- and 2D-discretization are introduced. In addition, governing equations of heat and mass transfer are presented. The final set of equations is solved using Aspen Custom Modeler (R), which is a commercial tool comprising an extensive fluid property data base. After the model is validated by means of experimental data, the final set of equations is used to investigate the impact of boundary layer resistance on water vapor transport.