The duct-work in the basement for my forced air heating system has been modified at various points over its long history and as a result had quite a few leaks when I moved in recently. I used foil tape to fix most of them, but I'm having trouble with the biggest leak:

enter image description here

  • The red arrow is the main branch from the furnace, showing the direction of airflow.
  • Blue shows the branch leading to the register. This register is the second one on this branch, and the most "direct" (the other leads through a section of smaller duct-work). The register is located directly above the floor joists shown.
  • Green shows where the leak is -- the duct (blue) butts against this joist, and where it meets the main branch (red) there is a gap about 1/4" (0.5 cm) wide, for the width of the duct. Between the joist (green) and the main branch (red) there is just enough space to get my fingers. This is the space that needs to be filled or sealed to stop the leak.

Since I have failed to get tape in to seal this joint, how else can I seal it? I'm thinking of getting a piece of foam insulation or perhaps tube wrap to shove in the gap, but would this be wise?

Some additional details:

  • Moving the main branch to get better access is probably more trouble than it's worth.
  • Location is an unfinished basement.
  • The space is already warm enough given the small size and the fact that the furnace and lots of duct work is radiating enough heat to keep it warm.
  • 1
    Can you try mastic? Get a long paint brush, and shove/gob it in there.
    – Greg
    Commented Dec 30, 2018 at 23:18

3 Answers 3


A 1/4 " gap is way to wide to fix with even metal tape. I would cut a hole large enough in the side of the trunk line so I could add a piece of sheet metal on the inside held in with some Silicone and a couple of screws. Then patch the access hole. I have had to do similar where folks have shut to many registers off and the main trunk split due to the large area and increased pressure.


I've used duct sealer in the past to seal up my attic air handler, it applies like a paste and dries hard. You can try that, but you options of globbing it on may be limited.

Is this duct in a basement or crawlspace?

The handler and ducts in my unfinished basement leak a bit, but I don't bother sealing them up as it helps keep the temp in the basement that much more liveable.

  • The system is poorly designed -- there's only one poorly placed cold air return on the whole second floor. I'm trying to get the pressure up to the second floor boosted, then I'll use vents off of other branches to supply the basement with heat. I'll give the duct sealer a shot.
    – LShaver
    Commented Dec 31, 2018 at 5:10

A good duct-sealer with fibers can be applied thickly with an old paintbrush. It also strengthens tape that covers gaps. If needed, a metal patch can be screwed on from the outside, then duct-sealed. If you want you can add duct-seal to the back of the patch, but the thick product forms a membrane on top of almost anything. Give it 6 hours to dry before pressurizing the ductwork.

  • I wasn't familiar with duct sealer -- I'll give that a shot.
    – LShaver
    Commented Dec 31, 2018 at 5:11

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