Properly sealing ducts is an important step to increase the energy efficiency of a home. It’s especially critical to look beyond long seams and major joints. Duct sealing locations that are often overlooked include:

• Register boots: Seal the seams of the register boot and attach and seal the boot to the floor or ceiling to improve air distribution and pressure balancing. Also seal openings to the crawlspace and attic to improve Blower Door test results.
• Plenum take-offs: Seal plenum joints, which experience high air pressure and movement, to drastically reduce duct leakage. Sealing plenums during assembly is ideal, when the joints are more easily accessible. Use mesh to cover any large gaps and follow with a coat of mastic.
• Holes and seams in the air handler cabinets: Many air handlers are shipped from the distributor with holes and unsealed seams. Because air handlers create a lot of pressure, pay special attention to these leakage points. Large holes and gaps can be sealed with mastic tape―the ONLY location where mastic tape is allowed. For additional holding power, apply mastic to the tape from the inside of the cabinet, too.

Mastic will keep air in ducts, but is not meant to hold ducting together. Secure ducts and components with mechanical fasteners such as screws, rivets or nylon draw bands, wipe the area clean and apply the mastic. Mastic can be applied using caulking guns, brushes, trowels or by hand and should be about the thickness of a nickel to be effective.

For more information about duct sealing and Energy Trust requirements, download the EPS Field Guide or watch the Duct Sealing and Testing module on our Online Trainings page.