Ducts are used to distribute conditioned air from your furnace and central air unit throughout your house, and when properly installed can make your home more comfortable and energy-efficient. However, in a typical house, about 20% of air traveling through ducts is lost due to leaks, holes, and poor connections. This can add hundreds of dollars a year to your energy bills and make rooms stuffy and difficult to heat and cool.Ducts are often concealed in walls, ceilings, attics, and basements which can make repairing them a challenge. It is a good idea to work with a professional when making duct system improvements. If you choose to take the project on yourself, start by sealing air leaks using mastic sealant, metal tape or other heat-approved tapes (not duct tape). Insulate all the ducts that you can access and make sure that the connections at vents and registers are well-sealed where they meet the floors, walls, and ceiling.