Insulating the concrete ceiling of the upper floor / What to watch out for?

Hello dear all,

the shell construction is finished - windows are still missing but have a delivery time.
In the meantime, I would like to insulate the upper floor ceiling (concrete) and cover it with OSB boards.

Facts:
- 2 stories
- upper floor ceiling made of concrete - 20 cm
- according to the Energy Saving Ordinance proof for KFW 55, the ceiling must be insulated with 18 cm of polystyrene
- attic will not be walkable, only storage space
- the purlins lie directly on the concrete ceiling
- the roof will not be insulated, the floor will be instead.

Questions:

1) What preliminary work do I still need to do on the concrete before I start the work? Clean thoroughly? Any ideas?
2) Do I also have to lay a vapor barrier foil on the ceiling in the upper floor?
3) I would lay the XPS boards with 9 cm each (2 layers) - crisscross so that the joints are closed
4) then the OSB boards on top (thickness - no idea - do you have recommendations)?
5) Do I have to lay the appropriate electric cables to the attic before all these actions (for light, power and possibly access point)?
6) Which boards would you recommend? Has anyone already had experience with them?
7) Any special features I haven’t considered yet?

Looking forward to your opinions/ideas/tips and tricks

Piotr

and one more thing: What exactly are these polystyrene boards? or is there also something cheap as an alternative?
What is the difference between EPS and XPS?

I would insulate the roof if I were you... You can't create more storage space! And the work is (almost) the same, I would say... Costs maybe a fraction more, but not in proportion to the generated storage space.

The boards would not come into the roof for me. If there's a fire, you don't need the fire brigade. Suggestion: Put 8cm of mineral wool on the concrete ceiling and the OSB boards. Additionally, an intermediate rafter insulation.

For this purpose, there is something from the company Bauder, called PIR DHW or similar, insulation material with an OSB board already attached.

I would insulate the roof if I were you... You can't create more storage space!

But you can store things in the attic even if the roof is not insulated. We've been doing it that way for years and are happy.

But you can also store things in the attic if the roof is not insulated. We have been doing that for years and are happy.

not if it is a KFW55 house. The Energy Saving Ordinance requires that.