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Making Floor Vents air tight

by Lorenz Prem
published on December 1 2011 8:16 pm

One of the most common areas of heat loss in residential heating systems are the duct joints. The building code calls for all joints to be either taped or covered with mastic. Older homes like mine often do not have this work done. The homeowner can greatly reduce his or her heating costs by sealing these joints.

The floor vents in my house were particularly bad. The installers did not take much care to create an airtight seal. A significant amount of air escaped into the subfloor and the crawlspace. It was time to fix the problem.


You'll need the following supplies.

Foil tape (UL 181 rated)NA
Duct masticNA

Both are available at a home center.

1. Fix the vent box

The repair begins by bending the metal vent box back into shape. The foil tape and duct mastic that we'll apply later cannot span large distances or withstand stress. The vent box must be installed securely.

Bend any misshaped metal pieces back into place. Secure them in place with nails driven into the subfloor or an adjacent framing member. The vent box should not move under stress.

2. Route a channel

If you are adding another layer of flooring, route a shallow channel around the vent opening. This channel allow the tape and mastic to be wrapped around the edge of the vent without creating a bump in the subfloor. The channel does not have to be particularly neat. It'll get covered by the finished floor.

If you are not altering your subfloor skip this step. Either apply the tape inside the vent over the gap between the vent box and subfloor, or accept the fact that the tape creates a ridge around the vent (which is not a problem in carpeted rooms).

3. Apply foil tape

Next apply foil tape around the opening. Use only tape with a UL 181 rating. Only this type of tape will create a joint that will last as long as the heating system will. UL 181 rated tapes are certified to withstand continuous cycles of heating and cooling. They won't catch fire either.

Duct tape is no suitable for this application. It will fall off soon after it is applied.

The subfloor and walls of the vent must be clean for the tape to adhere perfectly. Make sure the tape adheres to both the subfloor and the vent. Cover all visible gaps with tape. Pin holes can be filled with mastic alone. Overlap individual pieces.

At this point the vent is air tight. Applying a layer of mastic over the tape will ensure the repair will last.

4. Apply duct mastic

Duct mastic is a wonderful substance. It's basically paintable rubber. Combining it with the tape takes advantage of the strengths of both products. The tape cover large areas particularly well. The mastic flows into small crevices and shield the more fragile tape from mechanical impact.

Cover the foil tape with a generous amount of mastic. Let it dry over night. Once cured, the mastic and foil tape create an air tight and durable joint.


My new floor vents have already made a big difference in the comfort level in the bedrooms. They produce noticeably more airflow than they did before the repair. I feel good about not heating the crawlspace anymore. Repairing your vents is a fast and cheap way to make your home more livable.

About the Author
"Lorenz is the founder of Hingmy. When he is not reviewing power tools or improving the site, he is building things in his workshop or playing hockey."