An air duct assembly includes upper and lower pliable sheets that can be assembled in a sequence that makes the assembly easier to install and later easier to remove for periodic cleaning. To install the assembly, the upper sheet can be installed first by inserting the edges of the upper sheet within a pair of tracks and then fastening the upper sheet to a vertical supply air duct. An optional airflow modifier (e.g., filter, airflow turning vane, or noise attenuator) can be install where the supply air duct connects to the upper sheet. Once the upper sheet is suspended from the tracks and fastened to the main supply air duct, the lower sheet can be installed separately. Pliable end panels and pliable fasteners (e.g., zipper or VELCRO fastener) can make most of the air duct assembly machine washable. The upper and lower sheets can have different degrees of air permeability.