During the last years, the technology of HVDC became increasingly important. In comparison to AC applications, the electrical DC field is controlled by the partially temperature-dependent conductivities of the insulating materials. During operation, the current-carrying conductor causes an inhomogeneous temperature distribution both along the insulator and in the gas compartment. To understand the influence of the temperature on the electrical field, both phenomena have to be investigated separately. Thus, this paper describes a method to calculate the breakdown voltage of a gas insulated system with heated electrodes, exclusively with respect to the effects in the gas. Finally, a decrease in the breakdown voltage caused by a lower gas density can be stated. Besides, additional measurements on an experimental test set-up confirm the calculation results.