Insulation of the upper floor ceiling - vapor barrier / brake?

Hello Forum,

we have decided to insulate our top floor ceiling (directly under the roof) ourselves to save costs.

Regarding the construction (from warm to cold)

12.5mm gypsum plasterboards
30x50mm battens to hold the gypsum boards
40x60mm battens on the rafters to create a larger air space for the recessed spots
Vapor retarder or vapor barrier
220mm rafters at about 70 cm spacing with 220mm 035 insulation wool from Isover
Tongue and groove boards to walk on the attic floor.
The roof will not be insulated.

Now to the question:
Which foil should/must I use?
The building materials dealer says his normal vapor retarder with 100m is sufficient for new construction.
Roofer A says I should use a vapor barrier foil
and my roofer says that in most cases they use the Isover or Siga climate membrane.

Now I am standing here with my (half) knowledge and don’t know which foil is the right one.

I hope someone can help me?

Regards
Amin

We are doing exactly that right now (with 200 clamp felt) and have been recommended the regular climate membrane from Isover (not the Duplex).

Hello,
the roofer of my BT always uses a climate foil, but says that a normal vapor barrier is completely sufficient.
Two other roofers as well as a drywall installer and an expert have now told me that a vapor barrier with an SD value of >100 is the only right choice and also significantly cheaper.
The climate foil would probably be more suitable for roof insulation but not for insulating a floor ceiling. In addition, one always goes (from inside to outside) from tight to less tight.

Regards