Single-family house without a basement - generate parking space elsewhere

After airing with such outside temperatures, I have 20 degrees and 30% humidity. The dew point is then 1.9 degrees. So water condenses when I open the pull-down ladder under these conditions. If the hatch remains open, the humidity in the attic rises to 99%....

For a cold floor, the normal indoor humidity in winter can already become a problem. Your rooms retain more moisture at 20 degrees and 40% humidity than the attic at 1 degree and 99% humidity.

That means if the attic is left open for too long in winter and the room air has warmed the attic, the air has a high absolute humidity, which is no problem at 20 degrees at first. Now the hatch is closed and the temperature drops... with decreasing temperature, the maximum absolute humidity that can be absorbed decreases. When 100% is reached, water condenses... and we are happy about mold in the insulation.

Here is a nice calculator for this
rechneronline.de/barometer/Luftfeuchtigkeit

So opening the roof hatch is rather counterproductive...? Otherwise, we wouldn't be able to ventilate at all.

But good to know that if you have to get something, you should keep it as "short" as possible so that the hatch is not open for too long.

In the next life

In summary, it can be said:
- warm attic (ceiling and between-rafter insulation) must be connected to [Kontrollierte-Wohnraumlüftung] and requires heating?
- cold attic (only ceiling insulation) does not require either of these?

We need a warm attic because the [Kontrollierte-Wohnraumlüftung] and heating will be located there. We have a concrete ceiling. Recommendation from the energy consultant: 18cm insulation Styrofoam (flat insulation board EPS 035) + OSB 315mm. Between rafters with compressed fleece, vapor retarder, gypsum fiberboard (alternative drywall board). He even denied that we need a vapor barrier. I would take one as a precaution anyway.
Now to the question: Can we use OSB without concern? Can it warp due to moisture? I would rather use tongue and groove boards, but according to the energy consultant, this requires a complicated substructure. Or is it easy to create? And with this insulation variant, do we still need heating and connection to [Kontrollierte-Wohnraumlüftung]?
The second variant from a dealer would be wood fiber insulation boards (100mm Pavatex Isolair WLF 0.043 + 80mm Pavatex Isolair WLF 0.046) + OSB 22mm. Between rafters identical to the first variant. According to the dealer, the OSB should no longer warp then. Is that true? Is heating and [Kontrollierte-Wohnraumlüftung] still necessary with the second variant?

The practice after a winter. Cold attic, access by stairs, separation from the apartment by an insulated hallway upstairs plus terrace door. Terrace door, because it copes with the enormous temperature difference between floor and apartment without warping and because it is well insulated. Attic ventilated through a Velux. Stored there are cushions, boat and garden, blankets, suitcases, tools, odds and ends, chairs and much more. Everything intact, zero mold, everything dry. No insulation needed. Really not. Karsten