A model is developed for the diffusion of moisture into a solar module package and the subsequent degradation of copper indium gallium di-selenide (CIGS) solar cells. It is based on time-dependent mass and energy balances governing module temperature and the diffusion of water through a module package subjected to weather conditions. The moisture ingress calculations are coupled to an experimentally determined degradation model for CIGS solar cells, where the degradation rate is proportional to the encapsulant degree of saturation. Extensive results are presented for module conditions of temperature and relative humidity for four benchmark climates. Detailed predictions are presented for moisture ingress, module degradation and life as functions of the climate, mounting, packaging materials, configuration, and the kinetics of cell degradation. The results clearly show how the moisture ingress and degradation scale with the climate and the characteristic package diffusion half-time. This time constant determines the rate at which the module approaches the environment's average humidity. In order to keep enough of the moisture out of the package to significantly slow the degradation for a target number of years, diffusion half-times approximately equal to the target years must be achieved, which typically translates into moisture barriers of 10−4 g/m2 day or better at 25°C—a formidable challenge. In the diffusion-controlled regime, the life depends on the square root of package permeability and cell degradation rate. Estimated acceleration factors for damp heat (85°C/85% RH) versus Miami are nonlinear and range from 10X to 700X, depending on the package and cell degradation kinetics. Copyright © 2011 John Wiley & Sons, Ltd.