Insulate shed roof + floor ceiling (year of construction 1967, unrenovated)

Hello everyone,
in another post, you already gave me very good advice regarding the generally expected investment costs for our house. Now I need your help again with planning specific measures.
The house is from 1967 and has an uninsulated shed roof. In the single-story building part, the ceiling also serves as the roof – so there is no attic in between. In the two-story part of the building, there is a low attic, but it is not used as such – so it does not have to be walkable.
According to the Building Energy Act, the ceiling must be insulated – and my energy consumption practically forces me to do so. Therefore, it is now about the execution. The roof was newly covered with bitumen just last year because it was leaking. A sarking insulation was not financially feasible at the time and is basically ruled out now because you would have to do everything again.
Therefore, I would like to insulate from the inside in the form of insulation between the rafters. In the pictures, you can see that in the single-story part there is currently simply glass wool between the rafters, without a vapor barrier, etc. Apparently, this was not done in 1967. How would you proceed? In principle, remove the ceiling covering, take out the boards, remove the glass wool, put in suitable insulation material (?), vapor barrier underneath, new covering on top?
The ceiling of the two-story part is similar. There, the glass wool also lies between the rafters, but would be accessible from above via the attic, so the ceiling would not have to be removed.

(The beam in the first picture has recovered well since the roof was sealed. I only added the picture so you get a better idea of the construction.)

Thank you in advance ;)

Difficult to see everything. For the single-story part, I agree with you. I would use stone wool and OSB boards there. That provides mass, offers better heat protection, and creates a good windproof layer. For the second part, you then have to remove the wool from above and insert the vapor barrier from above. Then put the wool back on, as much as possible in my opinion.

Therefore, I would like to insulate from the inside in the form of cavity insulation between the rafters. In the pictures, you can see that currently in the single-story part, there is simply glass wool between the rafters without a vapor barrier, etc. Apparently, this was not done in 1967. How would you proceed? In principle, remove the ceiling covering, take out the boards, remove the glass wool, put in suitable insulation material (?), vapor barrier underneath, new covering on top?

I agree with . But depending on space, I would suggest PUR as an alternative.

In the second part, you then have to remove the wool from above and install the vapor barrier from above. Then the wool on top, and in my opinion, as much as possible.

Yep. 2x12cm laid lengthwise/crosswise, if there is space for it.

Thank you for your feedback.
A question regarding the construction of the two-storey part with a "small" attic. I sometimes read articles stating that a vapor barrier is not necessarily needed there if I don’t lay a floor or similar over the insulation, so the insulation can breathe upwards.
I wouldn’t even know how to apply a vapor barrier from above. I could either only "lay it out" between the beams, but then it would be interrupted by every beam. If I lay it over the beams, they can absorb moisture from below but cannot release it upwards anymore.
And how does it actually work with the "roller shutter boxes"? If they remain uninsulated, the insulation is pointless. But I don’t read anything about that in the Building Energy Act. I will add pictures of the "attic" later.