Even if you have the most efficient furnace and air conditioner known to man, if the ducts that funnel that precious hot air or cold air around your house are in bad shape, you'll still lose way too much energy.
"Most air ducts are riddled with leaks," says physicist Max Sherman, head of the Energy Performance of Buildings Group at the Lawrence Berkley National Laboratory. Sherman says he's seen ducts that let a third of the air passing through the attic or basement escape.
Annual savings: About 10% to 20% on your heating and cooling bills.
Up-front cost: About $50 if you do it yourself; $500 to $3,000 otherwise.
Quick Fix: A programmable thermostat (costing about $100) could save you about $100 a year. Models like the Honeywell Seven Day Universal Thermostat take the chill out of lowering the temp: Your house will automatically be cool when you're out or asleep, warm when you're around and conscious (and vice versa in the summer). You can easily install this little device without calling a pro, and it'll pay for itself in a year.