In this study, we analyze the effects of membrane properties, namely water permeability, solute permeability, and structural parameter, on the overall performance of an FO membrane module to extract water from simulated seawater (0.6 M NaCl). By considering the thermodynamic limit of operation, we demonstrate that the maximum achievable water recovery is practically independent of membrane properties, and higher maximum water recovery is achievable with counter-current compared to co-current mode. Analysis of the module-scale model indicates that reducing the support layer structural parameter offers substantial reductions in the membrane area required to achieve a specified water recovery. For example, a 25% reduction of the structural parameter of a state-of-the-art thin-film composite (TFC) membrane (from 400 to 300 mu M) yields a sizable 20% reduction in membrane area In contrast, quintupling the water permeability coefficient (from 2.0 to 10.0 L m(-2) h(-1) bar(-1)) of a modern TFC membrane generates only a modest 10% saving in membrane area In addition, because of the permeability-selectivity trade-off that governs current polymeric membranes, doubling the water permeability coefficient would cause crippling similar to 7-fold increases in forward and reverse solute permeation. This quantitative study models the potential performance of a module-scale FO desalination process and firmly highlights the need to prioritize the reduction of support layer mass transport resistances over water permeability increases in membrane development. (C) 2015 Elsevier B.V. All rights reserved.