Duct systems can be leaky in many areas and most are. Studies have shown that a typical duct system leaks from 20% to 40% of the heated air that should be getting to the various rooms of the house. This is bad for energy bills and worse for health and safety. A leaky duct system can create pressure differences in a home’s heating system which can draw in contaminants like dust from the attic, soil gasses, or flue gasses. A well-designed, well sealed duct system should be a fundamental part of an energy-efficient home. The duct system should be sealed at all joints and penetrations with brush-on, low/non toxic mastic, not duct tape.