Retrofitting insulation on top of a hipped roof bungalow

Hello,

my wife and I bought a bungalow from 1977 in 2015. In 1999, due to leaks, a hip roof was placed on top of the old damaged flat roof. The old chipboards and the old roof covering were not removed beforehand.
When we bought the house, we opened the ceiling and insulated it with 20 cm of glass wool plus a vapor barrier.
Now I wondered whether I can/should add insulation on top of the old flat roof.
At present, there is a focus on saving as much energy as possible.
The facade is very, very difficult to insulate properly; the windows have all already been replaced, as well as the front door.
My idea was to either glue styrofoam (10-12 cm) on top of the old flat roof or to install battens and then insulate with 10-12 cm of glass wool, followed by 18-22 mm OSB on top.
Would that be sensible, or should I rather forget the whole plan?

You have already insulated the ceiling/the old flat roof from below with 20cm of glass wool and want to insulate again from above, even though the roof screed is unoccupied? If so, an insulation between the rafters might be sensible, depending on the facade connection. How is your access to the roof, maybe a roof ladder, can something be optimized there? Do you have someone with a thermal imaging camera or can you rent one? I would now, with outside temperatures close to zero, inspect and specifically look for thermal bridges.

exactly, The roof was insulated from below. The attic is used solely as a storage room and is intended to remain uninsulated. Because the roof surfaces would be significantly more than just the floor. The hip roof was simply screwed onto the old flat roof. There is no special "connection" to the building. Insulating the intermediate ceiling from above was just an idea to increase the dim light. If it doesn't make sense, I can also leave it as it is. Upwards, we have a pull-out ladder. This is also insulated. With a thermal imaging camera, we could not detect any significant thermal bridges in the ceiling area.