Degradation of PV modules reduces their operational lifetime, resulting in an increased levelised cost of electricity (LCOE) and shortening the useful lifetime of valuable materials. One of the leading cause of degradation is moisture. Understanding how this moisture diffuses through PV materials and in different conditions can help prolong modules’ lifetime. This thesis investigates moisture ingress in PV modules, specifically looking at non-Fickian diffusion and material degradation as alternative, and potentially more accurate, ways of modelling it. To simulate non-Fickian diffusion, a dual-transport method is used; the study finds the approach to deliver more accurate results for EVA, but not for PET. To simulate material degradation, an adapted version of the diffusion coefficient equation is proposed, incorporating a degradation constant based on the materials’ properties. The findings are then used to analyse the behaviour of other PV materials and behaviour in different climates. The simulations find very slow moisture ingress for ionomer under non-Fickian diffusion and a strong deviation from Fickian diffusion. In EVA/PET simulations, non-Fickian behaviour is found to deviate more from Fickian behaviour in warmer climates. Degradation constants are found for the other PV materials. The approach shows promising results for the TPO/PET and ionomer/PET simulations, showing degradation in proportion with their material properties. However, simulations that include EVA appear to strongly limit moisture diffusion, indicating a revision of the EVA degradation constant should be made. TPO/PET degradation simulations in show minimal degradation over 20 years in different climates, but more material degradation in colder climates is found.