Flex Duct Sealing

by Paul LaGrange

Seal Ducts for Enhanced Energy Efficiency

Depending upon the size of the leak, tremendous amounts of air can be lost from the duct work.  Repairing those leaks and making sure that they don’t return is relatively easy.

Sealing the connections between flexible ducts, plenums, junction boxes and boots/registers keeps the HVAC system running clean and efficiently.   Keeping the system as tight as possible also gives the HVAC equipment a longer life, saves energy and lowers utility bills.

Seal in this order:
• Disconnected components
• Connections between air handling unit and the plenums
• Return takeoffs, boots, and other connections, especially site-built items
• Joints between sections of branch ductwork

Materials Needed

Sealing ductwork is a simple process that homeowners can accomplish themselves. Start with a visual inspection of the system. Check connections at both ends of the duct when possible.  Also check the entire length of duct to make sure the vapor barrier is in good condition and that there are no holes in the duct itself.

Duct tapes do not provide a permanent seal. Their adhesive dries out. Experts recommend duct sealing using mastic. Mastic is a thick paste which can be used on all duct materials and provides a permanent seal. Mastic comes in tubs and tubes and costs around $10 per gallon. Any ducts that appear to be “sealed” with tape should still  have mastic applied.

Duct showing air leakage

This is an unsealed connection at a junction box.  Note the mold and rust that have formed around the perimeter due to condensation from cool air leakage.

Losing air means losing money!

Duct leakage.  Has NO tape or mastic which would seal the duct.

Duct leakage. Has NO tape or mastic which would seal the duct.

The flexible duct material and collar should also be joined together with mastic for an airtight seal.

Both the interior duct collars and the connections to the junction boxes should be sealed with mastic and fiberglass membrane to prevent leakage.  The instructions included below illustrate the correct method of sealing duct connections and how a finished, properly sealed connection should look.  Materials needed for sealing ductwork are readily available at local home improvement stores and AC supply houses.

Details of proper duct sealing

Details of proper duct sealing

Material List

•    Mastic (UL Rated 181B)
•    Inexpensive paint brush
•    Foil tape (UL Rated 181B)
•    Tie wraps or clamps

Cross-section of flexible ducting.  The inner core is usually made of plastic

or vinyl  and should be sealed to the collar with mastic.

Cross section of duct

Cross section of duct


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