Proper sealing can reduce duct leakage to near zero. Even though contractors have been sealing ductwork for years, leakage is still very common. The leading cause of ductwork leaks is duct tape. Contractors continue to use duct tape to put ductwork together and it does not work. Duct tape works for a lot of things, but it never worked on ducts and shouldn't be used for them despite its name.
Experts recommend mastic and fiberglass mesh tape to seal joints in ductwork.
Mastic is an adhesive that can be painted onto many surfaces, including metal and flexible ducts. The fiberglass mesh provides additional reinforcement to prevent cracking from vibration or expansion and contraction caused by changes in temperatures. When building a ductwork system, think of it as a garden hose. To keep water flowing at full pressure, you want to keep the path as straight as possible. The more bends you have in your ductwork, the harder it will be for air to pass through.