For electronic devices that work at high humidity (such as implants), but for which only a moderate lifetime is required, it is possible to use polymer packages for protection. However, these materials are porous and allow moisture to diffuse through the package. Fick's laws applied to water diffusion have a solution which can either be solved numerically, or approximated accurately with the single exponential quasi-steady-state (QSS) model. This paper adapts the so-called QSS model to the case of moisture ingress into a package which has walls or elements of different thicknesses or properties. Using an electrical analogy, we propose a model which allows estimating the change in relative humidity with time inside the enclosure cavity, using a simple calculation method. © 2012 IEEE.