Moist spot in the drywall ceiling in the bathroom, caused by pipe ventilation

Insulation alone will not be enough, you need a vapor barrier from the interior to the attic. That is probably the OSB board on the rest of the roof. But the penetration for the pipe is leaking. Actually, it should be completely sealed, preferably with a sleeve (although I don't think there is anything officially approved for OSB boards). Can you access it from above? That is, can you look from the top of the attic at the pipe (and the OSB board)?

Hi Tolentino,
in that area I specifically used a small piece of OSB board, which was actually meant to mount the exhaust fan. The rest of the ceiling consists of drywall panels, and I wouldn’t be able to mount the exhaust fan there.
I also cannot access it from above. The ventilation pipe runs straight up a short way to the roof, where there is a roof vent.

Regarding the vapor barrier in the interior:
- I could cut the pipe flush with my blue foil on the ceiling from the inside, since the pipe still protrudes a bit into the room.
- Then insulate the pipe from the inside (with the diameter of the pipe) with XPS boards (of which I still have some).
- Then I could seal the pipe opening with blue foil and green vapor barrier adhesive tape (Rissan).
- Additionally, I could roughly cut an OSB board and insert it into the open ceiling. Seal the open areas of the OSB piece with acrylic or silicone.
- Insert a drywall piece into the open ceiling and plaster it 4 times. I would apply joint tape at the transitions after the first plastering.
- Sand the area and then paint the entire ceiling white.

Question: Is an OSB board or an OSB ceiling also a vapor barrier, or do you only achieve a vapor barrier with the blue foil?
Many thanks again to you both.

OSB or the like is not vapor-tight. Only the foil achieves that.

Okay, would the individual steps I described in the last message be sufficient?