This Ductwork isn't airtight. There are gaps all along the duct lines such as at collars, joints, and even where the line meets the floor. These gaps can be big enough to fit a finger in between and allow for dust and allergens to get into the system, and for air you're paying to heat and cool to escape. Leaky ducts can be indicated by high energy bills, an excess of dust in the home, or dirt around the registers.