Some of the air duct from the furnace to the registers are several shorter
ducts joint together. A while ago I found some of those joints failed and
separated in the crawlspace. The ducts look to be 8 to 12 inches in
diameter, and they want to pull away from each other at the joints, perhaps
the ducts were taut when installed. I duct taped them back. Although they
hold, they seem to be leaking air.

What is the best way to seal the joints?