Expanding the roof, cavity insulation

I am converting the garage attic. The room is not used often, so the project should be inexpensive.

Since the roof is very flat, I am installing walls made of OSB panels to separate the slant of the room.

At the moment, there is only a foil on the rafters and then the roof tiles.

The used space will be properly insulated and completely sealed with a vapor barrier.

However, I need advice here: My plan is not to insulate the unused part (the roof slope behind the OSB walls).

Is this possible, or can mold form in the cavity? The cavity has effectively been exposed to outdoor climate for 20 years, as air can enter the room through the roof tiles and foil.

Dear TE, to me it reads very exhausting. If you do not get the fulfilling answer right away, photos/sketches etc. would be helpful.

An exact sketch of the roof structure would be helpful. With OSB, it should be noted that unlike tongue-and-groove boards, it is not vapor-permeable and there is a risk of mold if condensation occurs.

A vapor barrier or vapor retarder would depend on whether the underlay membrane is also vapor-permeable or ventilated.

A simple solution would be a cold roof structure, where air can still circulate above the insulation wool. This is no longer common in new buildings, but it is less prone to mold if the construction is not executed properly.