We have a 2-floor house built in 1978 in the DC area, which is hot and very humid in the summer. We've recently had a slight moldy smell, and water on the 1st-floor ceiling. One of the upstairs closets has a hatch in the floor that lets us look in between the lower ceiling and the upper floor. It turns out that the air-conditioning ducts in that space are covered in condensation which is dripping down onto the drywall and wood below.

I have measured humidity in this inter-floor space to be 80+% and the temperature is in the high 70's which is hotter than the air inside the house. This leads me to believe that hot, humid outside air is getting into this space somehow, and this is causing the condensation. There are a couple issues that an HVAC guy and a contractor noticed that may be causing outside air to get into this space:

-the 2nd floor overhangs the 1st floor a little in the back and there are a couple vents under the overhang. They appear to vent into the inter-floor space. An HVAC guy said he thought that was unusual, that attics normally have vents but he doesn't think there should be vents between the floors. He covered these vents with duct tape as experiment to see if that stopped it, but it hasn't stopped the condensation problem after a day and a half.

-there is an air return duct that runs down from the 2nd floor though the inter-floor space, right next to the hatch that lets us look down in there. A general contractor noticed that there is an opening in this return, covered by a grating. He thought this was unusual. This vent is sucking air from between the floors, into the return duct and the AC system. If it's sucking air out of this space, air must be coming into this space somewhere else. So, it seems likely that the negative pressure from this return vent is sucking air from the outside into the space between floors. The contractor suggesting having this vent covered up.

For a little history:
Last year we had big mold problems on the ceiling of the first floor. We had mold remediators and other come to the house and they believed it was caused by a leak in the roof around a bathroom exhaust vent. They re-did some bad flashing around that vent and fixed other roof issues that they thought were causing the mold. The first floor is slightly bigger than the 2nd, so it has a roof area above part of it and that's where they thought water was leaking in. It appears that they were wrong about the source of the moisture, or that there may have been a roof leak but it wasn't the only source of moisture. We spent a bunch of money on mold remediation.

We need to definitively identify and fix the source of this moisture so don't end up repeatedly spending thousands of dollars on repairs every summer. Nobody seems to be an expert at finding the causes of moisture that lead to mold. The mold remediator does repairs but doesn't seem to have much knowledge of how to determine the cause of the moisture other than obvious liquid water leaks. He told as to consult an HVAC company. The HVAC guy and a general contractor were basically scratching their heads and making guesses.

I've read some stuff about insulating AC ducts, but to do that would require ripping apart much of the house. This condensation is happening in several places over a large area, and it may be happening in other areas that we can't see. It seems like a better approach would be to fix whatever is causing the humid air in this space.

Does anyone have any ideas? Should we close up that return vent between the floors, like the contractor suggested, or do something else?