One of the DOE building experts I worked with back in the early 1990s stated that a major energy problem facing the nation was leaking ducts (although it sure sounded like "ducks"). He was referring, of course, to the ductwork  that most of our homes have to distribute the hot air in winter and air conditioned air in summer throughout the house. All but a miniscule percentage of homes built with ductwork have the ducts running outside of the building envelope , either into the unheated attic or into the unheated basement or crawlspace.
I never forgot what this DOE expert told me, although I never fully understood the reason behind his statement. It never affected me directly because the first house I bought nearly 20 years ago, and where I still live, never had ductwork. I would read ads in magazines and newspapers aplenty about companies warning of the health hazards of dirty ducts, offering services to clean them. I would chuckle to myself every time I saw those ads, because I don't have ducts. My house at first was heated by baseboard electric heaters, and air conditioned by window air conditioners. Then, a half dozen or so years ago, I had the first of now four ductless heat pumps  installed. I love them. They are ultra-efficient and ultra-quiet. The new ones I had installed downstairs, one on each end of the house, have a Heating Seasonal Performance Factor (HSPF)  of 10 and a Seasonal Energy Efficiency Ratio (SEER)  of more than 20. Now that's energy-efficient!
I am now taking a series of courses online to receive a green builder certification. The latest topic the course covered was on leaky ducts. Leaky supply ducts, the part of the ductwork that runs from the furnace, air conditioner, or heat pump, to the rooms to supply the warm and chilled air, can leak the conditioned air into the attic or crawlspace. You'll have to make up for that lost heated or air conditioned air, costing you money.
However, there are other, potentially more serious, consequences.
Leaky supply ducts create a negative pressure within the house, causing outside air to force its way into the house. If you have a fire burning in the fireplace, this could cause backdrafting, drawing dangerous carbon monoxide down the chimney, instead of letting it escape up the chimney. If this happens enough, it could make the homeowners sick and perhaps even lead to death. Backdrafting can also occur in homes that have oil- or natural gas-fueled appliances, such as stoves, furnaces, boilers, and water heaters. Not a pretty sight!
This pressure difference pulls outside air into the house. In the summer, when the air conditioner has cooled the inside walls, warm moist outdoor air is sucked into the house through cracks in walls and around windows and doors. When this moist air hits inside chilled walls, it can condense into water and cause mold and mildew.
The opposite occurs with leaky return ducts, the section of the ductwork drawing inside house air back to the furnace or air conditioner. Leaky return ducts cause positive pressure within the house. This pushes air out of the house via the paths of least resistance—doors, windows, and pathways around electric and plumbing penetrations and gaps in insulation. In winter, when we humidify the inside air, this humid air is forced outside through cracks and openings. When this warm moist air hits portions of the cold outside wall, it can condense and cause mold and mildew.
At a green building conference I attended years ago, I saw a presentation of a "house from hell" —a newly constructed house in Arkansas. During the first winter, it literally rained inside the house. This baffled the builder. A number of experts were called in to help solve the problem. It baffled them. One building science expert finally correctly diagnosed the problem. The builder had installed dozens of downlights in the ceiling of the house, models that were not air-tight. Leaky return ducts in the heat pump system created positive pressure within the house. This forced moist indoor air from cooking and showers up through the downlights into the attic, where it condensed and formed frost on the underside of the metal roof. Every morning when the sun rose and heated the roof, the frost melted and fell to the ceiling below, producing "rain" from around all of the downlights. When the expert sealed the ducts, the interior "rain" immediately stopped. Who would have thought? Only a building science expert!
So, in a nutshell, leaky ducts raise our heating and cooling bills, quite a bit in some homes. They may cause mold and mildew problems, causing in turn decay and adverse health effects. Leaky ducts may cause potentially life-threatening backdrafting problems. So, if you're up in the attic or down in the crawlspace where your ducts run, you may want to consider sealing  those ducts. Now, when it's not too cold, and before it gets too hot, may be a good time to do it. Use mastic, a special type of adhesive designed for ducts, rather than duct tape. Numerous tests have shown that most duct tape is ineffective over time at sealing ducts. There are a million and one good uses for duct tape, but sealing ducts is not one of them.
John Lippert is an employee of Energy Enterprise Solutions, a contractor for EERE. He assists with technical reviews of content on the Energy Savers Web site.