Silicon heterojunction (SHJ)-solar modules-when encapsulated with ethylene vinyl acetate (EVA)-are known to be extremely sensitive to water ingress. The reason for this is, however, not clear. Here, we explain the root causes of this degradation mechanism specific to SHJ, proposing a detailed microscopic model. The role of EVA is instrumental in facilitating a faster water uptake in the module. However, additional observations led us to consider the role of glass in the degradation process. The moisture at the glass/encapsulant interface promotes a glass corrosion process, releasing sodium (Na) ions that, in combination with water, forms molecular Na hydroxide. This can percolate through the EVA, eventually reaching the solar cell. Na ions may act as recombination centers in the passivating layers or at the a-Si/c-Si interface, reducing the cell's passivation properties. Finally, we propose strategies to reinforce the water resistance and overall reliability of SHJ solar modules.