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I found a large hole (~10cm x 10cm) cut into one of the HVAC ducts that supply the bedroom. It was cut between the duct and the joist and is not visible from the ground, I found it attempting to diagnose efficiency issues with the system.

The duct itself is not insulated and has close to square cross section.

How should I go about making a repair?

The results I've found online tend to be related to fixing small / minor cracks with mastic or tape.

My thought is to purchase a length of ducting, tape it in place then insulate it.

From below

From above

  • Edit 1: Add two pictures of the duct. From below and from above (between the duct and the joist. Add a description of the duct.
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  • 1
    Is it a round metal duct? Metal rectangular? Metal flexible? Non-metal? Insulated?
    – Matthew
    Aug 14 at 17:07
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    I'd add to both great answers below that a piece of hvac sheet ducting is perfectly fine, screws are necessary if it's not perfectly flat (and preferred otherwise), and sealing it with metallic duct tape will be fine. Mastic is usually used if you also need to seal around folds, cleats etc...., but it's fine for the patch too. For seams, tape is easier and less messy compared to mastic.
    – P2000
    Aug 14 at 17:24
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    Do you know why the hole was cut and not used? Could it have been a cleaning access, to extract a pet/animal, or part of a future extension that never happened?
    – Criggie
    Aug 15 at 1:55
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    @Criggie Unsure why it was cut. Was done by the previous owner, and due to the location not caught by either the inspection or us at any point. Aug 15 at 22:37
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The best is to cut a piece of metal larger than the hole and attach with sheet metal screws. You can cover this with duct tape but if the metal fits properly this should not be necessary. If you do not have the material, you can possibly salvage a large coffee can. You will need a tin snips to do this unless you can purchase a piece of metal of the proper size.

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  • ...and if you are buying metal cutting shears, alow me to suggest investing in "aircraft snips" rather than "tin snips" - they cost similarly and work a lot better, IMPE. Aircraft snips can be had optimised for straight, left curves, and right curves (yellow, red, green handles, respectively, for most brands) cutting. Olive oil cans are even better than coffee cans, if you happen to have an empty one.
    – Ecnerwal
    Aug 14 at 17:19
  • Additionally, the cut has exposed bare metal, so OP would benefit by protecting that cut edge with some zinc-based spraycan primer before going too far.
    – Criggie
    Aug 15 at 1:53
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    Worth being explicit that "duct tape" means aluminum foil duct sealing tape and not the general purpose gummy grey-fabric stuff.
    – J...
    Aug 15 at 9:45
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Commonly (for metal ducts) a sheet metal patch a bit larger than the hole, mastic/sealant around the edges of the hole, and short sheet metal screws.

Most tape does not hold up well in the long run.

Might depend on what access you have or the shape of the duct as to how practical that is for your specific repair job. i.e. if a round duct, your plan of a section of duct, rather than flat sheet metal, might be preferable. Or just a section of duct, and replace the whole section of duct that's been cut, rather than patching it.

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    I think it is worth emphasizing the importance of properly sealing the patch. I agree with Ecnerwal that mastic is more fool-proof (as long as it is coated thickly enough). However, I also think a high-quality, UL-listed tape (e.g., Nashua 324A) works well if applied correctly. Do not use the gray fabric "duct tape" (sometimes marketed as "duck tape") as it is not, in fact, approved for sealing ducts! If you apply a tape (instead of mastic), be sure to use a tool to apply pressure (an old credit card works well), otherwise the tape may not properly adhere.
    – stmp945
    Aug 15 at 1:01
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You can buy a piece of sheet metal at most home improvement stores or even local HVAC or sheet metal shops.

Cut out a sheet slightly larger than the existing hole and screw it on with sheet metal screws.

You can also tape up the edges, using proper HVAC foil tape, to prevent air leaks but I presume your ducts are not sealed anyways.

If you apply mastic then make sure to sandwich it between the metals. If you do just the perimeter then wait for the mastic to cure before using your heating/cooling.

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