ᐅ Vapour retarder or smart vapor barrier when the exterior cladding is tongue-and-groove boards or slate?

Hello dear forum members,

I am currently converting an attic into a living space after the roof was renewed and three dormers were added. Outside the dormers, in the regular roof area, a vapor barrier will be installed from the inside, fully bonded with OSB after the insulation between the rafters.

The dormers to be insulated have the following construction from the outside:

- Natural slate
- Breathable underlay membrane (Delta Maxi or similar)
- Boarded sheathing (rauspund)
- Timber studs (insulated with mineral wool in between)
- OSB as vapor barrier or variable-permeability vapor retarder???
- Plasterboard

My question is as follows:

I understand that for a roof equipped with an underlay membrane, a fully bonded OSB panel as a vapor barrier is acceptable. Moisture can dry outwards if necessary.

But what about the dormers, which have the boarded sheathing + underlay membrane + natural slate on the outside? Is the use of OSB also suitable here, or should a variable-permeability vapor retarder be preferred due to the reduced drying potential to the outside?

Of course, I would prefer OSB here as well to add some stability (e.g., to properly mount wall cabinets or similar items). For the ceiling area of the dormers, I plan to use a variable-permeability vapor retarder since there is a bitumen membrane roofing above.

I look forward to your expertise.

Thank you very much and best regards,
markus90

Does anyone have any ideas or suggestions?