The heat and mass transfer characteristics of a water permeable membrane were studied to determine appropriate selection criteria for such a membrane in a heat recovery ventilator (HRV). A general physical model was developed to analyze the performance of various types of membrane based HRVs. The theoretical results obtained using the model were validated experimentally. The advantages of such a system are: simultaneous recovery of the sensible and latent heat, high heat and moisture exchange effectiveness, no mechanical components and year-round energy savings in air-conditioning systems. The model and results can be used to develop membrane based heat recovery ventilators. (C) 2000 Elsevier Science Ltd. All rights reserved.