ᐅ Insulating a ceiling made of aerated concrete blocks / cellular concrete blocks

Hi house building forum community,

let me introduce myself first.
My name is Steffen, I am from southern Hesse, 35 years old, and I have a background in electrical work.

We bought a house from the 1980s and are currently removing all the wooden ceilings.
Below that, there are concrete steel beams installed. Between them are aerated concrete or cellular concrete blocks.
Above is the roof with a 3% slope, which is not walkable.
Now the question is how to insulate it effectively.
Removing the roof covering, insulating, and then re-roofing is not an option.
My idea would be to insulate between the first battens with glass wool, then add a vapor retarder, then counter battens, and finally drywall panels.
The walls are not plastered but covered with drywall panels.
I should actually place the vapor retarder behind the drywall, directly against the structural wall, right?

Regards
Steffen