I'm seeing a crack in the corner of the ducts. Can I just tape it shut with boxing tape or is there a better way to do it?
Also, one register in an unfinished area is turned off but I can feel it leaking air. I looked inside and there's clearly a gap letting air past too. Can I also just cover the register in reflectix and tape that shut too or is there a better option? Thank you :)
I'll echo what others said about the aluminum tape. Make sure it is a good one, not a cheap one. You might want to take a damp rag to the area first. Any dust will stop it from sticking. Make sure to let it dry too.
Mastic could work as well. Get a tub of it and a chip brush or two... and don't forget gloves. It sticks to everything and is rubberized, so it holds up very well. The only downside is that it makes a mess very easily.
Yep foil tape works but id say mastic is the right way. A tub will be more than you need, they also sell it in caulk tubes which should be cheaper. Just use any extra on your other junctions too. Hit the ones nearest to your unit which is where your static pressure will be freatest.