© 2020 Elsevier B.V. Due to their outstanding properties such as thermal insulation, optical transparency or adhesion for instance, thermosetting coatings based on epoxy prepolymers offer many attractive advantages. In the framework of investigating their ageing under wet conditions, this study exposes an original approach taking into account the water penetration depth. This allows the derivation of local permittivity using mixing rule, ultimately yielding a physically-based explanation of the globally measured capacitance of immersed polymer coupons placed between circular electrodes and its link to gravimetric results. Theoretical predictions show good agreement with experimental measurements on epoxy-amine coupons both in terms of mass uptake and dielectric constants (including frequency dependence), bringing new insights in uptake mechanisms and associated measurement techniques.