Seal leaky heating and cooling ducts in the attic, crawlspace or garage
Added
If you have forced-air heat, check the ducts you can reach (attic, crawlspace, unfinished basement, garage) for kinks and disconnected joints. ENERGY STAR says to seal leaks with mastic sealant or metal tape, never duct tape, and insulate accessible ducts.
The result
Result: ENERGY STAR: in a typical house, about 20 to 30 percent of the air moving through the duct system is lost to leaks, holes and poorly connected ducts.
Source
ENERGY STAR: Duct Sealing (opens the original source in a new tab). Check it for the latest numbers before you act.