A method in the manufacturing of an insulated electric high voltage DC termination or joint includes the steps of providing an insulated electric high voltage DC cable including an inner conductor; a polymer based insulation system, the polymer based insulation system comprising an insulation layer and a semiconducting layer; and an outer grounding layer; removing the grounding layer and the semiconducting layer in at least one end portion of the high voltage DC cable, thereby exposing the insulation layer in the at least one end portion of the high voltage DC cable; covering the insulation layer of the polymer based insulation system in the at least one end portion of the high voltage DC cable by a cover impermeable to at least one substance present in the insulation layer of the polymer based insulation system in the at least one end portion of the high voltage DC cable in a non-homogenous distribution; subjecting the insulation layer of the polymer based insulation system in the at least one end portion of the high voltage DC cable for a heat treatment procedure, while being covered by the cover, thereby equalizing the concentration of the at least one substance in the insulation layer of the polymer based insulation system in the at least one end portion of the high voltage DC cable; and removing the cover. Instead of using a temporary cover, which is later removed, as the impermeable barrier, a field grading adapter or joint body mounted at the end of the DC cable during the manufacturing of the high voltage DC termination or joint may be used.