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?