Installation of insulation for the top floor ceiling (storage room)

I do not personally have experience with the finished boards. However, the manufacturer declares them as either walkable or not. You can rely on that.

And no, I didn’t mean 12mm but 12cm. You want to insulate the upper floor ceiling. Partially walkable. The non-walkable part is straightforward. For the walkable part, you have to come up with something.

You can take the finished boards, lay them on your concrete ceiling, and everything is great. - seriously. It just costs.

But then there is also the DIY option. The problem is that you have to make the approximately 24cm insulation load-bearing. Wool alone is, of course, not enough. So you could:

Get 12cm thick beams. Stand them upright on the concrete floor. Put the wool in rolls between them. Then cross another 12cm thick beams over that and again wool between.

Finally, as a walkable surface, put down tongue and groove boards.

I honestly cannot say anything about the final costs of this solution. You have to calculate that yourself.

I deliberately did not design my attic as a storage area, so it is completely not walkable.

Now I understand you. Thank you.
That means I could generally also use the cheap glass wool felt for the areas that are not walkable, right?
And, if I build with beams and boards myself, then also, right? Because the stone wool is considerably more expensive than the glass wool...

thank you

Yes, you can. I myself went with stone wool, but I'm also picky. But really calculate it beforehand. If you are busy with it for two weeks, saving €200 probably isn't worth it.