An interdigital capacitive sensor has been developed to characterize the permittivity of wire insulation. A theoretical model has been developed to calculate the capacitance of such a sensor that is in intimate contact with the surface of a double-layer cylindrical dielectric surrounding a conductive core. The cylindrical form of the electrostatic Green’s function due to an exterior point source is utilized, with the final capacitance value being calculated using the Method of Moments. Example calculations are performed and a field optimization method is developed to improve the sensing efficiency of the electrodes.