Thickness of installation plates for attic?

IMHO: Hardly any moisture will penetrate from the house into the insulation (the house is sealed), but the insulation, despite OSB boards above it, is exposed to the atmosphere, and eventually somewhere the dew point will be undershot (at night, weather change), and once you have mold inside ...

Why are the boards exposed to the atmosphere? We also have 22mm thick OSB. On top of that, about 300kg of tile leftovers are placed on an area of approx. 1m^2. According to the construction company, 18 or 19mm would also suffice, it also depends on the distance between the battens/beams. We have also left about 5cm of air circulation all around, so far everything is fine...

Why are the plates exposed to the atmosphere?

Because the roof is not "sealed".

Because the roof is not "sealed".

That is not generally the case; a warm roof, which I would always prefer, should be sealed. Even if you only want to store wooden decorations, I would be reluctant to expose them to the temperature fluctuations of an uninsulated roof...

I assumed the structure vapor barrier - insulation - attic. If the attic is within the airtight, insulated building envelope, the problem does not arise (but then I would be consistent and properly convert the attic).

That is not generally the case, a warm roof, which I would always prefer, should be airtight. Even if you only want to store wooden decorations, I would be reluctant to expose them to the temperature fluctuations of an uninsulated roof...

However, the OP clearly did not speak of a warm roof, nor do they have such:

"we are getting a prefab house at the end of the year. The floor of the unfinished attic will still be open. That means there is the"

In prefab houses, it is almost standard to have a cold attic.