Abstract: A damper device insertable into a slotted duct or flue without cutting completely through the duct, including a sleeve conforming to the duct cross-section and adapted for insertion through the slot to nest within the un-cut duct portion. A damper plate with stiffener ribs is rotatably mounted within the sleeve. An apertured cover plate slightly larger than the duct slot size is located to overlap the slot for mounting the damper device to the duct, and a spacer plate may be placed intermediate and rigidly attached to the cover plate and the sleeve for reinforcing the support at one end of a damper shaft. The damper shaft may be rotated manually or by means of a motor mounted by a bracket to the cover plate.