that is the problem with foil tapes they fail..doesn't take long!
I use a mastic tape made by Hardcast gray masic backing with a foil facing available at hvac supply.
surfaces must be clean and dry for mastic tape to adhere.
cost is about $35 per roll.

also check where supply boxes penetrate the building envelope this is another leakag site. removing the suppy grill and sealing the box to the sheetrock with mastic tape will eliminate this leakage..

best of luck

So I need to seal the foil backed insulation seams with mastic tape, right? The ductwork itself doesnÂt have any sealing issues because I used mastic on almost every seam when I was installing it.

Correct me if IÂm wrong but I assume that since the main return trunk ductwork is insulated with JM Microlite® Formaldehyde-Free Fiber Glass Duct Wrap Insulation I do not need to replace it but should be able to let that dry out since it has fungus and bacteria resistance. I just noticed the working temperature limits for that insulation only goes down to 40 deg F and I live in MN. Is that a problem?

I used caulk to seal the ductwork against the sheetrock wherever an opening into the ceiling (envelope) was but what you are saying is that I need to use mastic tape instead to seal the openings to prevent condensation from forming inside the sleeve insulation? I am currently using Deflecto brand TFGX6 6" round plastic diffusers in the ceiling that you turn to open or close. I thought that should be sufficient to keep warm air from rising into the ductwork.

Will I still have to add another layer of insulation to seal up this ductwork and add insulation into the openings of the air ducts every fall to prevent the warm air from rising into the ductwork and creating condensation as well or will sealing with mastic tape fix this problem?

I am grateful for your advice because it made me think about my bathroom venting and how IÂve used the same insulation sleeving around those ducts and do not have any condensation issues there.

I'm a little puzzled--the source of the moisture is ordinary humidity in your house that is working up into the air return, condensing, and then leaking out of the duct? Are you sure that's the only possible source of moisture?

If so, what about running the fan (not the AC) for a little bit each day when it gets really cold? That will move the air through and presumably dissipate any condensation that's collecting.

I posted this in another thread but it might apply here. Do you heat the house with the same system that's in the attic? If not, condensation will form in the ducts when the heat hits the cold duct. Also whenever you cook and especially when you take a shower, this will dump additional moisture into the attic ducts. All you need is a sag in the line or a level area, and water will accumulate. Here's what I did (posted from another thread.)

Cover both the supply & returns in the winter months. Here's my story.
New house. Northeast US. Baseboard heat, A/C in attic. First winter condensation formed in the attic vents. So much that it ruined my ceilings. Installer came back & replaced entire system after 2nd year. Installed all insulated ducts and installed supply registers that close and have a rubber gasket that seals them shut. For the returns, I made up some covers out of White board that i put a self stick foam gasket and then glued some quarter size magnets to. I then put these on the returns. The magnets (4 corners) hold with no problem.

This fixed the condensation problem 100% and also keeps the heat from escaping into the a/c system.