On a recent trip up into my attic, I found out that the air conditioner was blowing cold air into the attic. In several places at the AC unit, the foil tape had become unsealed. In the interest of not cooling another room, I plan on re-taping it, but I have the following concerns:
On the plus side, the people who did your ductwork made an attempt to seal it! On the minus side, they used tape instead of mastic. Tape can come off, as you've discovered. If you really want to do an A+ job, seal those joints with a generous application of mastic. It's basically a sort of putty-like substance intended for duct sealing.
I just had a new furnace installed. They sealed the ducts with 3" aluminum backed mastic tape. It's expensive stuff, this roll costs $40 on Amazon. It might be cheaper your local HVAC dealer, and I doubt you will find it at Home Depot or Lowes. Carlisle HVAC manufactures a few different mastic tapes, they have a tool to help find a local distributor.
Here is a good read about sealing ducts and with mastic vs tape. A particularly interesting paragraph:
One of mastic’s chief advantages is that, unlike some tapes, it performs well without clamping. Yet mastic will not prevent a joint from opening up. “Mastic is not a mechanical fastener — you still need sheet-metal screws, and scrap metal or fiberglass drywall mesh for big holes,” notes Davis. “You need to be sure that everything will hold together on its own merits. But, unlike with tapes, you don’t have to worry about whether the surfaces are clean.”
