This might be a fun problem for some of you to think about. It is way off your usual situations or maybe it isn't.

This is something that I recently uncovered at work. We have a cold room work space with a Munters desiccant wheel dehumidifier. The temperature is controlled by hot gas injection so the evaporator probably condenses nothing if the dehumidifier is working and it is humid in there if it is not. The system has been there for some time. I noticed that it was not working right and I got tired of waiting for the building guys to get around to it while my stuff got moldy inside. There has been some construction work done around it due to extensive renovations in the floor above. I found that the plumbers disrupted the ducting that returns air to the chamber from the dehumidifier so it is working OK now, but they have to reseal it.

While I was up there I noticed that the insulation around the cool duct leading to the dehumidifier is filled with water! If I poked it with a pencil, I am sure water would drip out. There is a vapor barrier on the outside of this duct. The humidity on the inside is set to 35%, outside is certainly higher most of the time, yup, condensing temperature. The question is, how to fix this? I would think, ideally, one would install a duct that is not a vapor barrier and could dry to the inside. I highly doubt that such a duct material exists off the shelf. Does it?

Is this fundamentally different from cold ducts in a humid attic or crawl space?