ᐅ Converting an attic: cavity insulation

I am finishing the garage loft. The space will not be used frequently, so the project should be cost-effective.

Since the roof is very flat, I plan to install walls made of OSB panels to separate the sloped area from the rest of the room.

Currently, there is only a membrane on the rafters, followed by the roof tiles.

The usable area will be properly insulated and completely sealed with a vapor barrier.

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

Is this possible, or could mold form in that cavity? The cavity has effectively been exposed to outside conditions for 20 years, as air can pass through the roof tiles and membrane into the space.

Dear OP, this sounds very complicated to me. If you don’t get a satisfactory answer right away, photos or sketches and similar materials would be helpful.

A precise sketch of the roof construction 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.

The use of a vapor barrier or vapor retarder depends on whether the underlay membrane is also vapor-permeable and ventilated from behind.

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 builds, but it is less susceptible to mold if the construction is not carried out correctly.