Master Flow's Rectangular Stack Duct is used for HVAC system applications between wall studs as supply and return ducts, and to vent range hoods, kitchen and bath fans between walls as well . Installing rectangular duct between wall studs saves space, and is an easy way to route air up or down a wall and to other stories of a home. It also works in overhead spaces between floor joists or in attics. Sold in "L" sections. Two pieces must be ordered to produce a full section of duct.