ᐅ Insulation of the Upper Floor Ceiling – Vapor Barrier or Vapor Retarder?

Hello forum,

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

Construction from warm to cold:

12.5mm (0.5 inch) plasterboard
30x50mm (1.2x2 inch) battens to hold the plasterboard
40x60mm (1.6x2.4 inch) furring strips on the rafters to create a larger air space for recessed lights
Vapor retarder or vapor barrier
220mm (8.7 inch) rafters spaced about 70cm (28 inch) apart with 220mm (8.7 inch) 035 mineral wool insulation from Isover
Tongue-and-groove boards to walk on in the attic
The roof itself will not be insulated.

Now to my question:
What kind of foil should/must I use?
The building materials supplier says his standard vapor retarder with a 100m (330 ft) roll is sufficient for new builds.
Roofer A says I should use a vapor barrier foil.
And my roofer says that usually Isover or Siga climate membranes are used.

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

I hope someone can help me.

Regards
Amin

We are doing exactly the same (with 200mm (8 inches) compressed mineral wool) and have been recommended the standard climate membrane from Isover (not the duplex).

Hello,
the roofer working on my building project always uses a vapor control membrane but says that a standard vapor barrier is completely sufficient.
Two other roofers, a drywall installer, and an expert have now told me that a vapor barrier with an SD value greater than 100 is the only correct choice and also significantly more affordable.
The vapor control membrane would be more suitable for roof insulation but not for insulating a ceiling between floors. In addition, the layers should always go from airtight on the inside to less airtight on the outside.

Regards