ᐅ Moisture on OSB Panels after Installing Air Conditioning Unit

Hello dear house building community,

I have a problem: after installing an air conditioning unit in our attic, I noticed moisture on the OSB boards.
We installed two ceiling cassette units because our attic is unfinished, so this seemed like the best solution for us. The attic is only insulated on the ceiling side.

The installation went smoothly. After summer, I sealed the cooling unit. Before that, it was only installed without the vapor barrier or anything else sealed.
My thought was that I need to close the vapor barrier again to prevent cold air from coming in during winter, but now I’m wondering if that was a mistake.

Here is what I did, some of which you can see in the photos:
I first used sealing tape as much as possible to stick the drywall to the air conditioning unit below. I thought this would be the first seal because the drywall had previously been flush. Unfortunately, this can no longer be seen in the pictures.
Then, I extended the vapor barrier with sealing tape so that I could connect it to the air conditioning unit on the other side. I also taped the vapor barrier to the unit with sealing tape. So I believed I had sealed the vapor barrier again, with the unit in between.

After that, I packed glass wool insulation all around the air conditioning unit to ensure continuous insulation.
After some time, I went back to the attic and noticed that moisture was forming under the OSB boards—not enough to drip yet, but some areas were wetter than others. The first layer is already somewhat soft.

I then checked the vapor barrier right below it; it seems dry. Also, the space between the vapor barrier and drywall appears dry. There is definitely no visible water. It’s harder to feel because everything is sealed and I cannot reach between layers.

Next, I removed the cover of the air conditioning unit from the inside to check for moisture there. So far, I do not see any moisture. The drywall does not feel damp either.

Do you have any idea what I might have done wrong or what I should do differently?
I’m concerned this will eventually lead to mold growth and the boards becoming saturated, or even moisture affecting the joists.
There could also be water forming between the vapor barrier and drywall.

Best regards,
Sebastian

Could there be a thermal bridge built in? The device is warm inside the room, but its top side is in the cold attic. With the day-night temperature changes, the cold attic experiences varying temperatures (e.g., 15°C and 0°C (59°F and 32°F) in autumn), which leads to condensation forming on the top side.

It has to be done somehow. I might be mistaken, but even if the vapor barrier is not completely airtight and allows warm air to pass through, there should still be enough space above for it to escape. So why is it better to place it just to the left instead?

I also felt the glass wool insulation on both sides, and yes, it’s warmer there, but it’s not yet damp. Then I opened the vapor barrier in two places and checked with my hand. Between the vapor barrier and the drywall, I couldn’t feel any moisture, but of course, I can’t reach further inside.

The question is whether it would have been better to apply the sealing tape on the drywall on the room side and then stick the vapor barrier on top, rather than taping from the outside on the drywall and then applying the vapor barrier. From the inside, I can better see if there are any gaps. If done correctly, no moisture should normally pass through there, as I understand it.

No moisture penetrates through the ceiling.
A cold roof is ventilated from the outside; moisture condenses from the air when the temperature drops at night.

However, the moisture issue is only in the area around the air conditioning unit; everywhere else, the OSB boards are dry. Therefore, warm, humid air is probably rising upwards.

The area around the air conditioner itself is not warm. And if heat passes through the air conditioning unit, shouldn’t it be able to escape easily above the unit? As you can see, the top is open.