Attic expansion visible beams / proper insulation installation

Hello everyone,

all statements and questions refer to the attached and hopefully sufficient drawing.

I have the following questions for the experts among you:

[*]Is the setup I have indicated basically correct?
[*]Can a roof with the vapor barrier/retarder be made airtight when the beams are left visible on the inside, or is it better to completely close the roof from the inside?
[*]The insulation lies on the bottom/inside on the vapor barrier and is limited on top/outside by the sheathing and the underlay membrane. Does space need to be left here for air circulation or is the setup fine as it is?
[*]Is the insulation with 155 mm sufficient? It is a bio-insulation (I cannot specify the exact name or manufacturer due to lack of knowledge) that has been stored uninstalled in the attic for 15 years.
[*]Would you install one or two drywall boards (12.5 mm) stacked between the rafters?
[*]If I decide against visible beams, I would have a total height of 205 mm that I could fill with insulation. Would I need to fill this height completely or could I leave air space at the bottom or top?

I look forward to helpful answers or questions.

Regards and thanks from Blautopf,
Galsan

In short: if the rafters are to remain visible, one installs the vapor barrier and insulation from the outside, i.e., in front of the roofing, everything else is nonsense and doesn’t work. When doubling up, you do exactly what the name says, apply the same thing again and not some plank stuff. 15 cm is far too little. "Biostuff from 15 years ago" surely has a poor insulation value, so you might need about 30 cm to barely meet the energy saving regulation.