Make cold attic accessible?

My understanding was that rough-sawn timber is vapor-permeable compared to OSB boards and thus moisture could pass through the wood.

I would be interested to know: What moisture do you mean? From where to where should it move?

Our new building also has an uninsulated roof with an insulated ceiling of the upper floor. Air passes through some cracks up here, so no moisture accumulates. Since it is a concrete filigree ceiling, I hope that nothing from the floor below penetrates upwards here. Best regards, Steffi.

To my knowledge, this is often standard, especially with all the city villas. The cold roof is sufficiently ventilated through a few cracks. It makes sense to work with tongue and groove boards on the top floor ceiling. Any moisture from the living floor passes through the insulation and releases it through the joints of the tongue and groove boards into the air in the cold roof. This will escape through cracks.

Any moisture from the living floor passes through the insulation

Normally, there is a vapor barrier there, so that the heated living space is sealed airtight and vapor-tight.

I understand it as follows: Above the insulation in the cold roof there is initially ambient air, warm and humid in summer. In case of a weather change or also during the day-night cycle, this air cools down. If the air layer above the insulation is practically sealed in and poorly ventilated, excess moisture condenses and wets the insulation and other building components*. Therefore, it makes sense to ventilate the roof space well. So either the insulation is left open or, if you want to make it walkable, you design the structure to be as "airy" as possible. This then somewhat excludes building materials that are very dense. (Whereas IMHO sufficient distance to the insulation and a sufficiently large free edge strip are more important than the question of tongue-and-groove or OSB.)

* For the same reason, desiccants are included in various packages for the expert or overseas shipping. Everyone has surely seen these small packets before.

Whereas IMHO sufficient distance to the insulation and a sufficiently large free edge strip are more important than the question of tongue and groove boards or OSB.

I absolutely have to disagree. The "ventilation" is ultimately ineffective because it has no "driving force." OSB boards act like a vapor barrier. In the garage, we have a corner where the flat roof (OSB board) is not well ventilated from below. There, the moisture from plaster and screed stayed so long until it started to mold. Our house roof has a full boarding made of tongue and groove boards, which is always dry. At Christmas, some water once settled there because the floor hatch was open – but the water dried quickly.

Our house roof has a full sheathing made of tongue-and-groove boards, which is always dry.

How do you know that? Can you look behind it?

I mean the visible side. For that, I made the comparison to the visible side of the OSB board. OSB is as dense as a foil, nothing diffuses through it, only ensuring air movement helps, and you can't do that under the attic. Building damage is then inevitable, IMHO.