The main aging mechanism of electrical cables with polymeric electrical insulation is the growth of water trees. Water trees are initiated if the relative humidity (RH) in the electrical insulation is above a critical level. Delaying the water ingress into the electrical insulation system delays the water tree initiation and reduces water tree growth, thus extending the service life of the cable. For a cable without any metallic water barrier, the water ingress can be significantly delayed by the use of an outer sheath material with low water permeability. An even greater delay in the water ingress into the electrical insulation can be achieved using a layered sheath system. To explore the possibilities of a layered sheath system, calculations of water ingress into a typical cable cross section has been performed using a finite element method. The water diffusion and sorption data used in the calculation has been measured for typical cable materials. Calculations have been performed for uniform temperature conditions and for a temperature gradient due to resistive current heating. The time to reach critical humidity levels and stationary humidity levels in the insulation system has been determined for several different arrangements of the sheath system. A sheath system with an outer layer of a material with low water permeability and an inner layer of a material with a high water absorption capacity is shown to give a significant delay of the water ingress into the electrical insulation. For the sheath materials used in this study, there is an optimum distribution of thickness of each layer. The calculations also show that a temperature gradient across the insulation system of a cable in operation gives an advantageous RH profile. With a temperature gradient the equilibrium RH level in parts of the electrical insulation can be lower than the critical value for water tree initiation. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 2127-2133, 2011