Upper floor ceiling - insulate additionally? What makes sense?

Hello everyone,

I have bought a house built in 1991 and have been living in it for several months now.

The house has one and a half stories and the upper floor ceiling (about 40 sqm) to the attic is so far only partially (right half of the house) covered with chipboard (about 24mm thick). Between the rafters there is Isover Rollisol-SB (WL-B1/040) insulation with a thickness of 120mm. At the bottom of this insulation there is an aluminum vapor barrier (or retarder?). It is not properly stapled to the rafters everywhere and in those places there are black spots (mold?) on the chipboard. I noticed this because I wanted to lay insulation on the rest of the floor and removed a board for that.

The goal is actually to make the entire attic walkable to store "stuff" (Christmas decorations, children's clothes, suitcases, etc.).

I was now planning to remove the insulation, lay new foil (Isover Vario Duplex) from above over the rafters, put the insulation back in without the aluminum "foil" and lay OSB boards or chipboard on top again.

Now I am wondering whether I should double the rafters from above and add extra insulation (e.g., 120mm Isover Integra ZKF 1-035).

Does that make sense – meaning the additional insulation – and is it worthwhile?
Is it even necessary to install the new vapor retarder everywhere?
The costs for everything are manageable (several hundred euros). However, the work itself is daunting :-(

I am grateful for any advice, ideas, and recommendations,

Regards
Patrick

Patrick, do not line walk-in attics with OSB boards, but with formwork boards instead. The whole thing becomes noticeably airier, not so dense. And that prevents mold. Simply nail board to board. Slight gaps are intentional. Karsten

That may be true, but no moisture comes through the properly installed new vapor barrier - right?

Moisture gets through because of a brake, just more slowly.
And what if it doesn’t get through? Where should it go? Causing mold in the insulation?

Do not use a vapor retarder but a vapor barrier from below and above on the insulation a vapor-permeable foil which allows moisture to pass upwards through the OSB boards in case moisture does get into the insulation.

lets out upwards through the OSB boards

OSB boards are not diffusion-open, hence the objection from