When properly sealing ducts, it’s critical to look beyond long seams and major joints. Pay attention to these important, often overlooked, duct sealing locations

Register boots
Seal the seams in the boot itself, and attach and seal the boot to the floor or ceiling. Sealing these connections can improve air distribution and pressure balancing. Proper register boot sealing also helps improve Blower Door test results by sealing openings to the crawlspace and/or attic.

Plenum take-offs
Plenums experience high air pressure and movement at joints, so sealing these joints properly can drastically reduce duct leakage. Sealing plenums during assembly is ideal, when awkward 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 (note this is the only location where mastic tape is allowed). For additional holding power, apply mastic to the tape from the inside of the cabinet as well.

In all cases, remember that mastic is meant to keep air in the ducts—not to hold ducting together. Always secure ducts and components with mechanical fasteners such as screws, rivets or nylon draw bands. When ducting is secure, wipe the area clean and apply mastic—it needs to be about as thick as a nickel to be effective. Mastic can be applied using caulking guns, brushes, trowels or by hand with proper gloves.

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