Home appliances generally comprise a mechanical envelope, or enclosure, of electrically conductive material (steel, aluminum, etc.) that houses electrical and electronic circuits. That envelope typically includes all kind of apertures, openings, holes, slots, windows, etc., to connect—physically and electromagnetically—the external space with its internal space. The performance of that envelope is a key element to comply with surface current EMI/EMC (Electro-Magnetic Interference/Electro-Magnetic Compatibility) regulations for electromagnetic interference, both emissions and immunity. Here, we present a novel theoretical proposal that consists of establishing the mathematical relationship of the coupling between the resonant modes of an internal cavity (RMs) contained in a conductive body and the characteristic modes (CMs) on the external surface of its envelope through the openings that connect those external and internal spaces, and doing so by means of equivalent virtual surface currents located in those openings. The comparative results of simulations and actual measurements of immunity in an anechoic chamber (measurements originally oriented to other purposes) are presented as above-mentioned evidence of resonating modes coupling on the frequency range 40–1000 MHz. However, this theoretical proposal is novel and remains to be developed in greater depth and detail in future works.