Moisture transmission can be a factor in the selection of materials for load-bearing structural composites. This article considers the methods currently available for measuring the permeability to moisture of membranes, and their applicability to thick laminates. It summarizes the experimental results available in the literature, and discusses the available means of reducing moisture permeability without resorting to metallic barrier films. New experimental results are presented, comparing the three main categories of thermosetting laminating resins (epoxy, polyester, and vinyl ester) and two moisture barrier coatings. The vinyl ester resin showed the lowest permeability. Both barrier coatings reduced the water transmission rates, although their robustness and long-term adhesion to the substrates have yet to be evaluated.