Transmembrane evaporation (often called membrane distillation) carried out in a countercurrent flow module, in which incoming cold seawater is heated by the condensing product water flow, is a promising technology for low-cost seawater desalination. This paper presents a model for preliminary design calculations for such a module. The model calculates temperature profiles alongside and across the module, heat and vapour fluxes through the membrane and the total product flow. Mass transfer of the water vapour through the membrane and the air gap is described with molecular diffusion through stagnant air. Estimated properties and constants have been used to calculate the energy transport. Calculations with variable temperatures, membrane fibre diameter and diffusion distance have been performed. From the calculation results can be concluded that the model is able to describe the process of interest in a qualitative way. The calculations indicate that the highest productivity will be obtained with high temperatures, small membrane fibres and a small air gap.