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

Hello everyone,

I have a technical question for you, with the request and hope that I will receive some answers. I have a new building that was completed in 2020.

Topic:
It concerns the upper floor (bathroom).
At that time, I had planned to install an exhaust fan in the bathroom to possibly remove the moisture from the bathroom. I had also laid the piping then, and a ventilation (roof vent) was installed on the roof by the roofer for this purpose.
The ceiling in the bathroom was closed with drywall except for the small cutout where the fan was supposed to be installed.
Later, I decided against installing the fan because I have two windows for ventilation in the bathroom and I didn’t find it very nice to have the fan hanging in the ceiling.
I then subsequently closed the small cutout on the exhaust pipe, which was still open, with drywall.

In the winter of 2023/2024, I noticed that the ceiling in that small area had a different color tone and became slightly damp. At first, I thought this was caused by the small drywall piece that I installed later and hadn’t properly finished.

However, it has gotten worse over the past few months. A stain formed, and later it even started dripping from that spot on the ceiling.

My assumption:
1. Moisture is entering through the exhaust pipe.
2. Moisture from the bathroom is penetrating the small closed drywall piece and accumulating there.

Since the moisture got so bad that it dripped down from the ceiling, this mostly happens when taking hot baths or when the girls take hot showers, and predominantly in winter.

I opened the area last week and found out that unfortunately the moisture is coming from the ventilation pipe and not from improper sealing of the small drywall piece.
Now I suspect it is sweat water or condensation when hot showers are taken. But this is only a suspicion.
I would like to simply fill the pipe from the inside since I don’t use it, either with foam insulation or mineral wool, or both, and seal it at the bottom with Styrodur. Afterwards, close the open ceiling with drywall.

Question for you: Can this be done this way and the problem (moisture) would then most likely be resolved?
Or does the roof in that area even have to be opened and sealed from above?

Thank you very much in advance.

Attached are two photos of the opening in the ceiling with the ventilation pipe.

Does the pipe run through the house up to the roof vent? If yes, it is clear that warm air rises along the pipe, eventually condensing at a cooler point and causing droplets to run down. Hot showers regularly accelerate this process. Seal the pipe with several appropriately cut, round pieces of EPS insulation and observe whether condensation continues to form behind the insulation. If yes, then the roof vent should also ideally be sealed near the outlet at the roof.

The pipe runs through the house up to the roof ventilation? If so, it is clear that warm air rises along the pipe, eventually condenses at a colder point, and the droplets run down. Hot showers regularly accelerate this process. Seal the pipe with several appropriately cut, round pieces of EPS insulation and first observe whether condensation continues to form behind the insulation. If so, then the roof ventilation should ideally also be sealed near the outlet on the roof.

Hi Allthewayup,
thank you very much for your answer.

The ventilation pipe only runs a short distance and then it is already up at the roof at the roof ventilation. The roof ventilation was installed specifically for the pipe by the roofer. When I reach into the pipe with my hand, I touch two flaps with my fingertips, which I can slightly open.

Regarding the insulation, EPS insulation. Would it be better insulated with stone or mineral wool + then sealed with foam at the bottom and then drywall board attached and plastered over?

I mostly had the problem in winter when the temperatures are low in the basement. So I will no longer be able to observe it now.

Best regards

EPS because it does not absorb moisture like mineral wool. Actually, it doesn't matter what you insulate it with, but I wouldn't use foam sealant. You first have to check whether it still condenses inside the pipe. Only when you can say with certainty that condensation has been prevented can you permanently seal it and close the ceiling again.

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. However, it is not airtight where the pipe penetrates it. Actually, it would have to be completely sealed, preferably with a sleeve (although I don’t believe there is anything officially approved for OSB boards). Can you access it from above? So, can you look at the pipe (and the OSB board) from the top of the attic?