You may have already caulked and weatherstripped the envelope of your house to the point that it's airtight – and you may still be wasting energy. That's why it's important to examine your home's ductwork when you plan for energy efficiency.

Think of your ductwork as huge hoses, delivering hot air instead of water into your house. Mostly out of sight, ducts can leak for years without you knowing it. They can become torn or crushed and flattened.

Old duct tape – the worse thing to use to seal ductwork, by the way – will dry up and fall off over time, allowing junctions and splices to open, spilling heated air into your attic or under the house. It's wasteful.

According to field research performed by the California Energy Commission, you can save roughly 10 percent of your heating bill by preventing leaky ducts.

Leaky ducts waste heated and cooled air even before it arrives inside your carefully weatherstripped building envelope. There are plenty of duct horror stories.

One of the bathrooms in California Energy Commission staffer Claudia Chandler's Northern California home, for example, had no heating vent. "The room was always uncomfortable – too hot in the summer, too cold in the winter," she complained. So she hired an air conditioning company to install a heater outlet in the bathroom.

When the technician crawled under the house, he discovered ductwork was already in place leading to the bathroom. It snaked across the ground, open and unconnected at the far end.

"The original contractor who installed the central air conditioning system apparently forgot to cut the hole and install the bathroom vent as he planned," said Chandler. "As a result, conditioned air was pumping out of the end of an open duct. It turned out that for more than a year we'd been wasting money by heating – or air conditioning – our crawl space and the great outdoors!"

When the open duct was attached to a properly working vent, Chandler's bathroom was finally comfortable. Better yet, her summer electricity bill dropped by $15 a month.

Another Energy Commisson staffer had a central furnace and air conditioner replaced. The ducts under the house were checked, and duct tape supposedly sealing and holding two duct sections together had dried up and failed. The two sections had fallen apart, and that hole was also heating and cooling the crawl space under that person's house. It was fixed properly with sheet metal screws holding the sections together and mastic covering the entire seal.

Here are some common problems to look out for:

• Leaky duct connection
• Return leaks
• Furniture blocking register
• Leaks at furnace and filter slot and duct tape failure
• Fallen duct insulation
• Supply leaks
• Kinks in ductwork restricting airflow

Check Your System Yearly

Each autumn before cold weather begins it's always a good idea to get a routine maintenance and inspection of your heating system to make sure it is in good working order. At the same time, have the air conditioning contractor make at least a visual inspection of your duct system. If you've noticed any problems – rooms that don't get warm, even though the rest of the house is fine, for example – make sure the inspector knows about it, so he can look for a problem.

Depending on how accessible your attic or crawl space is, you may be able to inspect your ductwork yourself. Check for loose connections and joints that have come apart. Look for rips or tears, or for crushed ducts.

It's also a good idea to clean and inspect your vents where they enter the room. Sacramento resident and novelist Patti Berg did so after she and her husband moved into a new apartment. Very little heated air came through the register in their bedroom, leaving the room too cold in winter. When she removed the grill, she discovered that a previous renter had blocked off most of the duct with cardboard. "I guess it was the other tenant's way of trying to adjust a duct system that wasn't very well designed, but it made for an unpleasant house," said Berg.

Closing off a vent in that fashion could be harming your heating and air conditioning system.

Simply put, the point is this: today's homes are better insulated, with improved weather stripping to keep heat inside during the winter. Windows have become increasingly more sophisticated, with high-tech coatings and gases between multiple panes of glass that improve their insulating value. As a result, energy savings can be startling – providing your duct system isn't squandering the benefits.