How to avoid sealing duct joints inside walls?

Question

I am trying to replace a very short amount of duct work for my range hood and am having a bit of an issue figuring out how to seal/tape the joint between the 6" elbow duct and the wall cap.

Currently, for the 6" elbow duct to be centered correctly onto the range hood the elbow duct and the wall cap ducting must meet inside the wall. Which also means the joint where the elbow duct and the wall cap meet must be taped inside the wall.

What I'm trying to avoid:

If the taping happens inside the wall, then any time anyone needs to work with the ducting they will need to tear out a large section of dry-wall so that they can tape the joints inside the wall.

Options I've researched so far:

Mastic

Seemed promising as you could coat the two ends of the joint and then push them together. However, everything I read about mastic indicates you need to also screw or tape the joint.

What if I could remove the wall cap?

If I could remove the wall cap I could tape 6" elbow duct to the straight ducting, then push the straight ducting through the wall from the inside.

Reattaching the wall cap seems daunting in this situation. How difficult would it be to get things flush? If I get the system all setup before taping so that it is flush, then reassemble it I think the problem is solved.

Main problem with this one is I don't think there's anyway to remove the wall cap! Maybe there's a better product I don't know of?

Answer 1

First, I straightened out the entire elbow so it was easy to reach the most interior joint. It's really important that before taping each joint you can get it into the correct final position.

I had to undo my tape work after I realized I needed to rotate the most interior joint 180 degrees so that the final product would seat correctly. Do a dry run without tape to ensure all the pieces are rotated correctly. Don't be afraid to check after taping each joint! It will save time.

My first attempt I used really bad tape an HVAC team left at my house last time. Fortunately, I had bought some really nice Nashua tape for another job and it seamlessly joined to the duct work.

The tape works pretty well, but there are definitely still leaks. I plan to pick up some mastic tomorrow and just coat everything as well as I can.

Currently finished product looks like:

Thank you so much for your help Kris!