The active components of the read-out planes in the CMS Preshower work at a temperature of about -10oC. An assembled Preshower detector appears as a closed vessel. Electrical services have to cross this container in a gas-tight manner, and they have to be disconnectable from both Preshower outside and inside. To this end, special gas-tight feedthroughs have been designed. As they are located on the cold/warm barrier, their heat inleak can be a potential source of problems. Condensation on the outside surface shall be avoided. This paper presents the behaviour of two different configurations is presented and resulting temperature values at the external surface are compared to the guaranteed maximum dew point temperature
