Polymer based encapsulant materials such as Ethylene Vinyl Acetate (EVA) are commonly used in silicon-based PV modules and play a central role in the structural integrity and hence long term performance of the device. In spite of many advantages, however, water can di!use through them, leading to swelling and chemical degradation that can make the device susceptible to accelerated corrosion and failure. This thesis reports on e!orts to provide novel methodologies to investigate the opto-mechanical response of EVA due to water di!usion and on the quantification of water concentration in deployed PV modules. A novel non-contact time-resolved measurement technique based on Wavelength Scanning Interferometry (WSI) is proposed to determine thickness, refractive index, thickness change and refractive index change in EVA during water uptake. This is preceded by numerical simulations to inform the design of the experimental methodology and setup, such as thickness of EVA layer, maximum surface velocity during measurements, type of data filters to use for data analysis and also for validating the proposed approach. The measured time-resolved thickness profiles provide a unique opportunity to study the swelling behaviour of EVA. Using a novel inversion technique, the hygroscopic expansion coe"cient (HEC) as a function of water concentration is established for the first time. A hygro-mechanical Finite Element model is also developed to predict time-resolved hygroscopic expansion of EVA and compare it with experimental measurements. The hygro-mechanical stress is further studied through a set of deflection tests on EVA-glass bi-material strip specimens immersed in water and intermittently measured with an optical profilometer.