Problem with condensation on HVAC ducts inside house
I'm having a pretty serious problem. There is an old sheet metal HVAC duct in my home that is getting condensation that is dripping onto the ceiling of a pantry. This duct is completely internal to the house, there is no exterior wall. The home was built in the mid 80's, modern style construction with lots of little half floors, so there is quite a bit of open space. There is a picture attached to this message.
I cut a hole into the ceiling and got rid of the damp drywall so I could access the duct. My hope was to wrap duct insulation around it, but it butts directly against an interal wall and header, making a wrap around nearly impossible.
Any suggestions on how I can get rid of this issue without breaking the bank or gutting the inside of the house?
In the image, the duct in the foreground runs horizontally, the other part is actually vertical up to the air handler, which is in a closet.