Styrodur - the ideal insulation material for everything?

The OP is probably just confusing EPS (Expanded Polystyrene (Styrofoam)) and XPS (Extruded Polystyrene (Styrodur)).

Personally, I see no problem insulating the roof with XPS; it just doesn't make much sense because you pay for properties that are not needed there, while lacking those that would be desired. Both insulating materials offer less sound insulation and heat protection than, for example, the often used mineral wool or the "Mercedes" wood fiber and are also significantly more difficult to work with.

The insensitivity to moisture, on the other hand, is useless because you have to work with a vapor barrier anyway. By the way, it is by no means true that moisture would not condense on EPS/XPS; it simply does not harm the insulation material (but very much harms the surrounding substance).

Hello and thank you very much for your answers!

MOLD is for me the big problem or one that I absolutely want to avoid! Therefore, I consider hydrophobic insulations to be a good choice.

The vapor barrier is placed OVER (towards the roof tiles) and UNDER (towards the room) the insulation. This vapor barrier is supposed to prevent the insulating material (e.g. glass wool) from getting damp. This would happen if warm room air can penetrate into the insulation (glass wool) and meet the dew point there. Have I understood this correctly?

The vapor barrier is, in turn, a film that is vapor-permeable but does not allow direct water through. Is this to prevent condensation forming directly on this film, or why is it like this?

Would you also need this vapor barrier with hydrophobic insulating materials?

Back to the first post. Are you doing interior insulation on the wall?

No, the vapor barrier (actually vapor retarder) belongs under the insulation and it is called vapor retarder because it retards vapor and not water, which usually runs from top to bottom and not from bottom to top. Whether an insulating material is moisture-resistant has nothing to do with that.

@hanse : the inner shell is of course insulated from the outside. With Styrodur. I think putting a hydrophilic insulation material in the cavity is quite dangerous! What would you install there?