Typical duct systems lose 25-40% of the heating or cooling energy put out by the central furnace, heat pump, or air conditioner. Homes with ducts in a protected area such as a basement may lose somewhat less than this, while some other types of systems (such as attic ducts in hot, humid climates) often lose more.
Duct repairs could be the most important energy improvement measure you can do if your ducts are in the attic. If only one half the typical loss of uninsulated and unsealed ducts that are in attics or crawl spaces were saved, it would amount to $160 off the total heating and cooling bill in a typical home. This savings is based on the national average use of natural gas and electricity for central heating and cooling at national average energy cost of 70-cents per therm, and 8-cents per kilowatt-hour.