3 Steps to Saving Duct Energy

How to Seal Air Ducts

by Mike McClintock

High-efficiency furnaces and AC systems can milk almost every penny from a dollar’s worth of fuel. But to get where you need it, conditioned air has to flow through the duct system, and that’s where losses add up. The Dept. of Energy says typical installations lose from 20 to 40 percent of their energy. The two culprits: direct air leaks, and radiation through duct walls.

Leaks are likely wherever duct connections are not fully seated, and not sealed with tape. Worse yet, joints without screws can separate and dump the entire flow inside wall cavities.

In unconditioned spaces, loss through duct walls can be just as bad. In winter, hot air in ducts without insulation will lose most of their heat traveling through a 30-degree crawl space. In summer, even greater losses occur where cool air is ducted through attics reaching 140 degrees under blistering sun. Another trouble spot: where part of a second-story room overhangs the first floor. If ducts run in the overhang there isn’t enough room for adequate insulation- for the ducts or the floor above.

Uninsulated ducts can also trigger condensation. Expose the cold metal surface of an AC duct in a vented attic full of steamy summer air, and it can sweat enough to soak surrounding surfaces and foster mold growth.

An HVAC contractor can check for those problems by measuring temperature loss and pressure drop in the airflow. But DIYers can solve them with these three improvements.

First, make sure that duct connections are properly aligned, fully seated, and joined with screws. If screws are loose, tighten them. If there are none, add three self-tapping sheet metal screws. (Ducts over 12-inch diameters get five.)

Second, seal the connections. Contractors often brush on a gooey mastic sealer. DIYers usually get better results with pressure sensitive, foil-backed duct tape. No, not all-purpose duct tape. It doesn’t hold up as well, particularly to heat, and where strict codes control duct sealers it’s not approved.

Third, insulate ducts in unconditioned spaces. Specialized wraps come in a variety of thicknesses, typically in four-foot-wide rolls applied in sections with taped joints. But you could use standard wall batts in a spiral wrap. Foil facing and tape provide a vapor barrier, though contractors may use long plastic sleeves slipped over insulated duct runs on new installs.

The National Assoc. of Homebuilders Research Center says these three improvements can save 20 to 30 percent on heating and cooling costs.

Saving Duct Energy
Ducts on a split-loop heat pump mounted in an uninsulated attic have bulky R-8 insulating sleeves covered with plastic vapor barriers.
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