For homes with central furnaces, air conditioners, or heat pumps, the heated or cooled
air is carried to the different rooms in your home through a duct system.
To create the duct system, ducts are connected to other sections of duct and then
to the registers for the rooms. Ducts are often are hidden in the attic, crawlspace and
walls where they may or may not be insulated properly.
Thus, in addition to 20% of heating and cooling costs typically being lost through
leaky ducts (where they connect to each other, where they connect to the room registers,
and damage caused by age or during home repairs), another 10-30% of heating and
cooling costs are typically lost through uninsulated ducts, especially when they
pass through unconditioned spaces such as your attic or crawlspace. (Imagine, for
example, a duct carrying 55 degree cooled air through a 130-degree attic in the
summer. In that scenario the system must work very hard to keep the rooms cool!)