Bathroom ceiling moisture + inadequate insulation

Does anyone have any idea how the moisture could have gotten between the drywall and the foil? Could it actually come from the ceiling spotlights? Or through the joints between the wall and ceiling (sealed with acrylic, but cracked in a few places due to the movement of the house)?

Maybe the cracks are only in the paint? Because almost every house has that (well, my experience is limited to 3-4 houses in my circle of acquaintances with this type of construction ;)). Do you know what sd value your vapor barrier has? Maybe it’s actually a vapor retarder? Because vapor retarders are supposed to be able to transport a certain amount of moisture to the outside. There are also so-called humidity-variable retarders. You should read up on that.

In my opinion, it depends on what kind of membrane it is. Is it a vapor retarder or a vapor barrier? A vapor retarder should be able to control water vapor, similar to a Gore-Tex membrane. With a vapor barrier – that is, a water vapor impermeable membrane – condensation might occur on the membrane, as is currently the case. However, I am not an expert in this matter.

We installed an Isover KM Duplex UV on the upper floor. This membrane can regulate moisture to a certain extent by opening or closing its pores. I was unsure whether this was necessary, but ultimately I decided in favor of it.

So far, fortunately, we have had no visible moisture problems in the bathrooms, and we only have an uncontrolled residential ventilation system (UKWL), meaning: windows and the conscience to open them appropriately.

: same thought :D

Thank you for the answers. I will try to find out and inform myself further on this. The more I read about it, the more worried I become that the roof and ceiling construction were really done badly and that it will turn into an even bigger issue.

Don't such works also have to be documented or at least stated in the execution description?

In the construction description it says:

for the roof: ... vapor-permeable underlay membrane

for the ceiling: ... mineral fiber insulation (200mm WLG035) vapor diffusion-tight foil (sealed windproof) and plasterboard...