Recent studies showed that silicon heterojunction (SHJ) solar cells can be prone to potential induced degradation (PID) when encapsulated with ethylene vinyl acetate (EVA). Here, to gain understanding in the role of EVA, we perform PID test in humid conditions (85 degrees C/85% RH) under a negative bias (-1000V). We study the effect of moisture ingress and cover materials by using different module structures. We focus on studying both sides of the cell for modules packaged in a glass/glass scheme after 500 hours of test (corresponding to similar to 5 times the duration foreseen by the corresponding IEC standard). The front-side degradation is dominated by a reduction in short-circuit current (J(SC)), whereas the rear-side degradation is driven by a loss in fill factor (FF). EQE measurements show that increased front-surface recombination is largely responsible for the observed degradation of the front-side. From TEM and EDX measurements, it seems that the degradation at the cell level is predominantly caused by diffusion of sodium into the cell, which is triggered by humidity and low encapsulant resistivity. Assuming that the Na is also accumulated at the rear-side of the cell, this would create defects at the p-n junction, leading to the loss of FF observed.