Most homeowners don't realize that leaky ductwork can cause increased energy bills. It can also cause your furnace to generate more dust. Ducts must be sufficiently airtight to ensure economical and quiet performance of the system. All transverse joints, longitudinal seams, and duct wall penetrations, should be properly sealed. This can be done with a duct mastic, aluminum foil tape, or a chemical sealant forced through the inside of your ductwork.