Abstract: A rotary brush is especially useful in vacuum cleaning of duct interiors. The brush has a hub with a center hole for receiving a drive shaft of a power tool, an annular collar positioned over the hub, and a low density strand set of polymeric flexible strands extending from the annular collar. The collar has a set of substantially uniformly spaced radial holes to receive the strands. The strand set of polymeric flexible strands is capable of being bent axially of the annular collar so as to access the duct's interior through a small opening. The strand set is also capable of bending radially during a cleaning process. The low density of strands ensures that efficient vacuuming of debris loosened by the rotary brush is accomplished.