A double-walled vessel (1) is disclosed, which has a pressure equalization opening (24). Said opening is closed by a stopper (4), which has at least one gas passage channel (41). Said channel is configured such that it allows air to pass, while it prevents the penetration of water into the intermediate space. The stopper is preferably made from a silicone-based plastic and fastened to the vessel by means of an adhesive based on acetoxy silicone. For fastening purposes, it may comprise a disk-like fastening flange (43).