If your home has a forced-air heating and cooling system, it probably has leaky ducts—a fact has been confirmed by countless researchers over the years. Fortunately, many U.S. jurisdictions have adopted more stringent building codes in recent years, and these code changes have nudged HVAC contractors (at least in some communities) to pay more attention to duct sealing and duct leakage testing. Because of these code changes, anecdotal evidence suggests that the ducts installed in new American homes aren’t as leaky as they used to be. Most existing homes, however, still have leaky ducts.

If you are an energy-conscious builder, you probably already know your duct basics:

• Duct leaks are very common; in many homes, duct leaks are responsible for significant energy losses.
• For ducts located in an unconditioned space—for example, an unconditioned attic or crawlspace—leaks in the supply system will depressurize a house, while leaks in the return system will pressurize a house. Either condition can cause problems.
• Duct leaks outside of a home’s thermal envelope waste more energy than duct leaks inside a home’s thermal envelope.
• Even if ducts are located inside of a home’s thermal envelope, duct leaks can still connect to the outdoors. For example, supply system leaks in a ceiling between the first and second floors of a two-story home can pressurize the joist bay, forcing conditioned air outdoors through cracks in the rim joist area.
• It’s much easier to seal duct seams during new construction than in an existing house.

Characteristics of a good duct system

A good duct system: