Leaky ducts are a serious problem in homes that use forced-air heating and air conditioning. Distribution losses amount to 5 percent to 30 percent of the fuel consumed.
Duct sealing yields the biggest savings when the ducts are located in an unconditioned area that is well connected to the outdoors. The outdoor air enters the return duct leaks, and heated or cooled air exits the supply ducts. This leakage wastes energy. It also pressurizes and depressurizes areas of the home, providing a driving force for air leakage throughout the building shell. When ducts are located in conditioned areas, duct leakage leads to some inefficiency and local temperature differences, but isn't a major energy problem. Return leaks are the trickiest to find and the most important ones to seal, especially return leaks near the furnace. It is very important for the safety of the residents to seal ducts thoroughly and to relieve pressure problems near combustion furnaces. A large return-air leak near the furnace can draw flue gases down the chimney into the living space.
Duct sealing is an extremely important and often neglected energy management measure. The force-air supply and return ducts should be an airtight, closed system joining the furnace to the building. Duct joints should be sealed with duct mastic between the furnace and ducts, between registers and floor, wall, or ceiling, and between duct sections.
Fiberglass is the most common insulation for ducts. Seams should be tight between pieces of insulation.
It is important to avoid insulation gaps and voids. The insulation should wrap all the way around the duct. Seal the seams with a high-quality tape. Metal fasteners hold insulation in place better than tape. Fiberglass duct board and insulated flexduct are duct materials with built-in insulation. They are not as durable as metal ducts but are easier to build and install. Duct board and flexduct ducts must have a larger cross-sectional area compared to metal ducts, because they are rougher inside and therefore create more air resistance. Flexducts should not be used for long runs.
Hot water or steam pipes should also be insulated.