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My HVAC system has panned returns. In one location, there is a jumper duct. It is unfortunately necessary because the main supply trunk is between the return "boots" (e.g. square holes in the floor leading up to grates) and the main return trunk.

A picture to explain the setup: enter image description here

You can see where I've (temporarily) removed the header at the top of the picture - where the arrow for "Main return trunk" is. I have removed this header because I've observed a great deal of air infiltration (using a smoke stick). This is in the garage, which is unconditioned (and either way, I don't want garage air getting into the HVAC).

Here is what I can see (looking at the top of the "main return trunk"):

enter image description here

There are significantly large gaps (a) between the main return trunk and the panning connecting it to the jumper duct, and (b) on the sides the main return trunk. (For some reason they don't look too bad in this photo - they are significantly worse in real life.)

There was a a patchy, unholy combination of mastic and foil HVAC tape "sealing" these gaps on the sides, but nothing in the back at the panning (which means air was mostly being sucked in from the bottom between the main return trunk and main supply trunk). Some was still coming in from the sides, despite what seemed like a good few pounds of mastic.

Is there a better way to seal this connection?

I can definitely slather on a ton of mastic, making sure to get between the main return trunk and panning... but it seems less than ideal. I would sure love to get a piece of sheet metal over the existing squares in the trunks, install 6" (or bigger) takeoffs and directly connect the jumper duct to the main return trunk - but there's not quite enough room / solid duct remaining for me to do that, without first pulling the whole thing down.

I am almost tempted to fill the gaps with spray foam, or several tubes of caulk... but neither situation seems ideal.

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    Mastic and/or proper foil HVAC duct tape are the normal tools to seal duct joints with. Sounds like yours just was done poorly. Since you seem to have cleaned it up well, I'd suggest simply reapplying it and doing a better job of it.
    – FreeMan
    Jan 12 at 18:32

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Solid wood makes a fine duct material; it isn't any more leaky than sheet metal. Well-detailed joints make all the difference. Don't shrink this down to one or two 6-inch ducts; their cross section is much smaller. If you really want it metal ducted the whole way then use a rectangle.

Along the sides, where the joists cross the width of the return ducts, we can see that the sheet metal is not held tightly to the bottom of the joists. That's a good thing to fix; tape and mastic aren't great for sealing a joint that isn't mechanically secure. Screw the top wall of the duct to the bottom of the joist in all four locations.

Make access holes in the bottom of the return and jumper ducts. One could use a hole saw, say 1/2 to 1 inch diameter, and then a suitable extension and magnetic holder on the drill/driver to reach in from the bottom, cross the height of the duct, and drive a screw through the top of the duct. Or, keep reading - there might be good reason to cut a large rectangle hole rather than several small rounds.

The seal between the side walls of the return and supply, and between the jumper and supply, is another problem area you noted. Foam caulk backer rod, or weather strip, or even just tightly bunched fiberglass insulation, all could improve this area. Prepare a piece that is longer than the space between the floor joists and slide it up into the gap from below. Use something thin to slide/pack the foam up to the top edge of the ducts - a straight edge, a framing square, a sheet of thin plastic or wood all would work to push the material up. If it's within reach, you could reach in from the camera's viewpoint and pack the material down into the gap instead. (For that matter, your suggestion of spray foam here is not a bad idea.)

The leakage may be reduced enough just with that, but if not, you could clean the dust and debris and then apply caulk, mastic, or tape over the foam.

If better access is needed to accomplish the sealing cut a rectangle hole in the bottom of the duct. Make it plenty large, the full width of the joist bay or so. This allows you to reach up through the ducts. When the work above is finished patch the access holes. The best way to do this is to use a new piece of metal cut about 1-3/4" larger than the hole so that you can use S-cleats to join the patch to the original ductwork.

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