According to a preferred development of the present invention the electrical insulation layer is glued into the heating rib frame. It is true that the insulation layer is exposed in the window. Nevertheless, an overlapping portion between the electrical insulation layer and the surfaces of the heating rib frame accommodates a sealant, normally a sealing adhesive through which the electrical insulation layer is connected to the heating rib frame so that the recess surrounded by the frame is sealed towards the environment. Alternatively or in addition, the surrounding front face surfaces of the electrical insulation layer are received in an electrically insulating mass. The electrical insulation layer is thereby also sealed in the heating rib frame. The mass is preferably a mass which ensures a tight connection between the heating element and the heating rib frame. This mass may particularly be silicone, specifically an addition-curing 2-component silicone which cures at room temperature and cures in a forced way under heat. In the cross linked state the component of the mass that forms the liquid phase should have a Shore A hardness of about 10-40 and/or a dielectric strength CTI>600.