Moisture ingress into photovoltaic (PV) module results to degrade the power. To predict the lifetime of the PV module under actual temperature and humidity (T&H) conditions, the solubility of the cured ethylene vinyl acetate (EVA) and the maximum water vapour transmittance rates (WVTR<SUB>max</SUB>) was evaluated. Using the solubility, WVTR<SUB>max</SUB> values, and PV module-temperature data (Miami, Arizona, Seoul), the relative humidity history of the interface between the cell and EVA sheet (RH<SUB>cell</SUB>) was calculated for 1 year. The effects of the solubility on the RH<SUB>cell</SUB> were also determined. We are experimenting the outdoor PV test depending on environmental conditions in Seosan (International place) in Korea at the moment.