Process for controlling the capacitance value (C1) of a tubular sheath (1) formed by extrusion of an insulation compound (2) on an electrical cable (3) in an extrusion head (4), a foaming agent (FA) being introduced in the insulation compound (2) in such a way as to improve the capacitance value (C1) of the tubular insulation sheath (1), this process being characterised in that: a predetermined amount of foaming agent (200) is used in such a way as to obtain a predetermined capacitance value (C1) for the tubular insulation sheath (1), and in order to control precisely the capacitance value (C1) of the tubular insulation sheath (1), a gas pressure (110) is applied on at least a portion of a face (100, 101) of the insulation compound (2) extruded by the extrusion head is (4), the value of the gas pressure (110) is changed in such a way as to control the capacitance value (C1) of the tubular insulation sheath (1).