Bathroom ceiling moisture + inadequate insulation

I don't believe in protocols with our developer... unfortunately, it doesn't leave such a professional impression in hindsight when it comes to such topics

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Steam diffusion-tight foil (windproof bonded) and
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I find "tight" here unfortunate!

No, being airtight there is not unfavorable. There is rather no error visible in the pictures. Whether the drywall or ceiling spotlight is airtight or not is irrelevant, as the drywall does not represent a sealing layer. The most common error (but there can be 100 other reasons) is the user. Having no controlled residential ventilation is already very bad – if the bathroom is not consistently ventilated by hand thoroughly after every shower, morning, evening, and midday, that can already be the cause.

First of all, thank you for your answers

I have now also read a bit more about it. "Dicht" does not seem to be wrong here. I did not know that the drywall is so permeable until now. So I will let the drywall installer dry everything, insulate, and redo the ceiling today and hope that the problem does not occur again.

Our builder unfortunately did not offer controlled residential ventilation. But I had not really dealt with it beforehand either. However, it really seems to have many advantages. Maybe I will have it retrofitted in a few years.

Hi, if no controlled residential ventilation is installed at the moment (it can also be decentralized in individual rooms ;-) only a tiny core drilling + power required) then at least forced ventilation in the bathroom would be appropriate. Just like back in my student flat with the light switch and overrun timer coupled... Then the humid air is vigorously pushed outside for 10-15 minutes.

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only a tiny core drilling + electricity required)
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Ideally a 30 cm core drilling. The fan can then be inserted there. The advantage of spending a bit more money here lies in heat recovery through a heat exchanger. The "better" models can also transport air out and in.