A capacitor assembly that contains a solid electrolytic capacitor element positioned within a multi-layered casing is provided. The casing contains an encapsulant layer that overlies the capacitor element and a moisture barrier layer that overlies the encapsulant layer. Through careful control of the materials employed in the casing, the present inventor has discovered that the resulting capacitor assembly can be mechanically stable while also exhibiting electrical properties in the presence of high humidity levels (e.g., relative humidity of 85%). For example, the encapsulant layer may be formed from a thermoset resin (e.g., epoxy) that is capable of providing the capacitor element with mechanical stability. The moisture barrier layer may likewise be formed from a hydrophobic material.