How do you seal leaky ductwork?


How do you seal leaky ductwork?

Asked by denise mack


David Edwards PhD's picture

That all depends on what type of ductwork you have. Sometimes what you have is not worth saving because the time and/or money required to fix it is more than it would cost to replace the whole system.

First thing you should know is that the job is, in almost all circumstances, a really lousy way to make a living, or to spend your free time if you do something else for a living.

  • The ducts are usually either in the attic or in the crawlspace, and if you live in a two-story home, they are almost always in the walls as well.
  • These places do not usually see human visitors, so they tend to be dirty, hot or cold, critter-infested and very cramped.

Replace old sheetmetal & duct tape ductwork

If your ducts are old sheetmetal, they were most likely put together with duct tape -- not the approved tape for ducts, but the silver fiber tape that everybody calls duct tape but that really isn't supposed to be used for ducts because it dries out, oxidizes, and falls apart, leaving whatever it was taping untaped.

  • If your ducts were taped with this tape, which is really just a strong packing tape, then your ducts are now probably covered in dried-out adhesive, falling apart at the seams and leaky as all get-out.
  • You can often tell where the leaks are on these old systems if they are covered in insulation by looking for the dirty insulation.
  • So much dirt has blown through the insulation since the time they were installed that the insulation is actually brown or black.

If you have this kind of ducts, I highly recommend pulling them out and replacing them with plastic-lined, wire-wound, pre-insulated flexible duct lines. They are easier to run and are airtight except at the seams, where it is your job to get them as airtight as possible.

The old guard will tell you that the insides of the old sheetmetal ducts are smoother and therefore allow better airflow, which is true, but they weigh more and tend to fall apart over time (unless someone has screwed the joints together like you are supposed to), they're a bear to seal, have many more joints that can leak and, because you have to wrap them with that loose blanket fiberglass insulation with no plastic wrapping, they are not going to retain their heat as well as the preinsulated ones will. Besides, they are much more work and more expensive to maintain.

Installing new flex ducts

As far as installing the new ones, you should really have someone do the Manual J, D and S calculations to determine the best duct sizing and layout in the house so you get good heat distribution.

Here is a list of what we do at every junction, termination or origination point with the flex duct:

  1. Pull the inner pipe around the sheetmetal fitting, pulling two wire winds onto the fitting flange.
  2. Place a minimum of 3 self-tapping sheetmetal screws behind the wire wrappings so that the head of the screw holds down the wires to the fitting, so there is no way that the flex duct can separate from the fitting.
  3. Tape the joint with 181B-approved foil duct wrapping tape.
  4. Place water-based mastic all over the tape so no air can leak out of the joint.
  5. Make sure all joints in sheetmetal fittings are sealed with mastic, including plenums, air return ducts and register boxes, etc.
  6. Make sure all duct runs are as straight as possible, are supported every 4 feet with plastic pipe wrap, with no big curves or turns, which will ruin airflow.
  7. Tape all register boxes to the surrounding wall or floor surface to keep air from leaking behind the register cover and into the crawlspace, wall cavity or ceiling space.

Tighter ductwork

If done correctly, you can expect to take your duct leakage from 35%, which is average, to 8-10% or better if you are good and seal up the furnace itself.

Best of luck.

For more information:

Read Daniel Glickman's Q&A "Should we replace our PVC-lined flexible heating ducts with another type of ductwork?"

Also, read Rick Goyette's Q&A "Should I replace metal duct in a 1950s home with the current flex duct? Or is cleaning better?"