Is mold formation possible?

Leave it just as it is now. You don’t need an extra valve (heating circuit), better have an additional ventilation valve installed there if there is already a pipe, your GÜ and his heating technician are thinking ahead, that’s actually what you want... without heating and ventilation the room is a potential time bomb when it comes to mold.

We have a storage room, about 3m² in size, adjacent to the kitchen. This room is located at a corner of the house, so it has two exterior walls. In the room, there is also an exhaust pipe for our controlled residential ventilation system; our house will be an Efficiency House 40.

So with an exhaust pipe, nothing regarding mold should happen there. The compensation loops of the underfloor heating should easily be enough to prevent cooling down.

In our storage room (about 4 m², but only a narrow exterior side, also with a window), we completely dispensed with underfloor heating because we also wanted to store food there. In addition, our freezer is located there. And what can I say: As long as we keep the door to the kitchen closed, the storage room becomes one of the warmest rooms in the house due to the waste heat from the freezer.

What temperature differences do you think would be generated if the chamber did not have underfloor heating, in a modern, insulated new building (Kfw 40!)? And which foods specifically benefit from it being 18 instead of 21 degrees in the room?

With proper installation of the exterior insulation in a KFW40 building, no condensation (and thus no risk of mold) should form on the interior surfaces of exterior components due to a long thermal transfer path. This also applies at higher indoor humidity levels (up to about 60%). Since there is also an exhaust pipe for the controlled residential ventilation system ensuring air circulation and thus air exchange, the risk is generally low (with properly installed exterior insulation). And the accidentally installed underfloor heating displaces any possible moisture from the lowest layers.