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 :)
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.