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.