Abstract: A collar for connecting a duct to a wall having a hole therein bounded by an edge having a predetermined thickness. The collar is constructed from a sheet of material having top, bottom, left and right edges, the sheet forming a tubular structure in which the left and right edges overlap. The sheet includes a groove for engaging the edge of the hole. The sheet also includes first and second detents spaced apart thereon. The overlap of the left and right edges at the top edge changes when the spacing between the first and second detents is changed. The collar also includes a first fastener for fixing the overlap of the left and right edges at the top edge when the edge of the hole is engaged in the groove and a second fastener for fixing the overlap of the left and right edges at the bottom edge. In the preferred embodiment of the present invention, the groove includes a layer of a sealing material for forming a seal between the sheet of material and the edge of the hole.