According to the national Department of Energy, “Typical duct systems lose 25 to 40 percent 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 or dropped ceilings may lose somewhat less than this, while some other types of systems (such as ducts in attics or underneath raised homes in our hot, humid climates) often lose more.”