Wooden floor and wooden ceiling, insulation

the room:
about 12sqm separated by lightweight walls in a storage floor.
The floor consists of rough wooden floorboards. in total about 4 – 5 cm height. Ceiling looks the same.
The lower floor is not heated and the upper floor neither. And there is no insulation either upstairs or downstairs. Only wood. Room air is always nicely dry. Temperature difference when heating could be 10 – 15°.
I plan to lay new solid wood floorboards and insulate the room floor and room ceiling.

Floor:
I was thinking of the following structure:
1st layer – wooden battens and wood fiber insulation in between
2nd layer – vapor barrier on the old floorboards
3rd layer – screw the floorboards onto the battens

My questions about this:
A. should I use a vapor retarder foil? or rather diffusion-tight?
B. is the structure correct?

Ceiling:
1. clamp mineral wool between the beams
2. create ceiling with vapor barrier foil (tack the foil to the beams)
3. screw wooden battens across the beams to
4. screw plasterboard panels onto the wooden battens.

My question about this:
C. should I use a vapor retarder foil? or rather diffusion-tight?
D. does the foil help if the walls have no vapor retarder?

Many thanks in advance