Abstract: The device holds the ends of insulating mats against the inside walls of air ducts at the ends of the ducts where frame pieces form external glands on the ducts. The frame piece has a stay which rests on the outside of the air duct wall and a stay which rests on the inside of the air duct wall. The inner stay is bent back on itself and with multiple bends forms a longitudinally opening pocket which has a transverse-running catch strip with a v-shaped groove form at its free end, pointing towards the inner stay. A cross-connecting spot weld connects the outside stay, the duct wall, the inside stay and the bent back portion of the inner stay.