How to perform insulation on an old saddle roof

Hello

we have an old gable roof without insulation, up there there should only be parking spaces for the tenants. The top floor ceiling, thus the floor of the attic, will of course be insulated as required. My question now is should I just leave it open like that; should I install a board sheathing between the rafters. Or maybe just staple a roofing underlay.

I would prefer not to have it so drafty up there.

Does anyone have helpful tips

As a landlord, you should have more interest than just "underlay, so that there is no draft," right? Most of the energy is lost through the roof. Therefore, do it completely and properly. Even if the upper floor ceiling is already insulated. I don't consider anything else future-proof, also regarding consumption requirements. At the moment, the attic has outside temperature = inside temperature. Especially in (old) buildings, I would exhaust all possibilities to keep as much heat as possible INSIDE the building.

The question is what the tenants want to store. Due to the large temperature differences, it can only be robust items. If then a wood fiberboard is used, I prefer that over foils.