The long-term performance of glass fibre reinforced polymer (glass-FRP, GFRP) materials used in civil engineering applications is influenced by several factors, such as their constituent materials, their production processes and quality level, and environmental factors, such as temperature, moisture, or ultraviolet radiation, among others. In the design of GFRP civil engineering structures it is important to ensure that the design values of material properties adequately account for the potential property reductions that may stem from ageing and exposure to environmental factors during service life. The first part of this paper presents an assessment of existing design guidelines for FRP structures regarding the recommendations they provide about the reduction of FRP mechanical properties due to moisture. The second part of the paper presents a survey of test data available in the literature concerning the effects of exposure to moisture in the mechanical properties of FRP materials with E-glass fibres and either unsaturated polyester or vinylester resins. The review comprised accelerated ageing tests carried out in water or saline solution immersion. The mechanical properties assessed included strength and moduli in tension, compression, and in-plane shear, as well as interlaminar shear strength. The third part of the paper compares the gathered experimental data with Arrhenius-type prediction models found in the literature, to ultimately derive a conversion factor to account for the material degradation in environments with high- and continuous exposure to moisture for a reference service life of 50 years, typical of civil engineering applications.