Insulation of unheated stairwell - Mold problem?

Hello everyone,
I have already searched in the forum but found nothing suitable on the topic.

We are currently planning the energy-efficient renovation of our house from 1980. One of the measures will be the insulation of the exterior wall. 10 cm of insulation (mineral insulation materials) will be applied.

Now we received information from an acquaintance that he has a mold problem in his relatively small stairwell. In his opinion, this is because the installed underfloor heating cannot properly warm the room.

In our case, there is no heating at all in the stairwell. The stairwell is exterior, i.e., outside the actual apartments and open over 3 floors.
We are now considering installing, for example, a wall heater in the stairwell to have the option to possibly heat the stairwell.

Do you think this is sensible, or is it not necessary in a stairwell?
Or do you see a risk of mold in an unheated stairwell?

Regards

Is your stairwell inside or outside the thermal envelope?
So how are the doors and walls to the stairwell designed?

It makes little sense to heat a stairwell that is outside and not insulated on the outside.
Basically, you have two options:
1. Integrate the stairwell into the thermal envelope (insulate the exterior wall - insulate the windows) and heat it.
2. Keep the stairwell outside the thermal envelope (insulate the interior wall, front doors between the stairwell and apartment(s)) and ensure that no warm and moist air from the apartment enters the stairwell.

You also don’t have to heat a garage to keep it free of mold, but no condensation occurs there due to warm air from the living space.

For us, option 1 would apply (1. Integrate the stairwell into the thermal envelope (insulate the exterior wall - insulate windows) and heat it)

Doors to the stairwell are "normal" apartment doors, i.e. not specially insulated. The exterior wall of the stairwell is insulated just like the rest of the house. The front door will be renewed, as well as existing glass blocks being replaced. Only that there is no separate heating in the stairwell and the room would only be warmed by air from the living area...

In our case, option 1 would apply (1. Include the stairwell in the thermal envelope (insulate exterior wall - insulate windows) and heat it)

The doors to the stairwell are "normal" apartment doors, i.e. not specially insulated.
The exterior wall of the stairwell is insulated just like the rest of the house. The front door will be replaced and existing glass blocks will also be replaced.
Only that there is no separate heating in the stairwell and the room would only be warmed by the air from the living area...

this will predictably go wrong because the combination of 1 + 2 as described by is not "intended" by physics. However, in such a renovation I do see potential for funding and thus the sensible involvement of an energy consultant. They would then also show you the corresponding possibilities and risks

this will predictably go wrong because the combination of 1 + 2 as described by is not "intended" by physics

I don't understand the comment? As I said, with us it will be variant 1, i.e. the stairwell is insulated just like the rest of the house.
The whole thing is being carried out as an energy renovation in cooperation with an energy consultant. So far, he has not explicitly identified the stairwell as a risk or problem. But I will talk to him about it again...

I don’t understand the note? As I said, we will use option 1, meaning the stairwell is insulated just like the rest of the house…

only with the difference of the missing heating … (storage capacity for water from colder air is simply different than from warm air) So in my opinion, condensation from the warm apartment air will settle in the insulated but unheated stairwell and lead to mold. Alternatively, moist air from outside also causes this. But mold doesn’t care about that… Maybe your accompanying energy consultant still has a trump card up their sleeve? (ventilation system with humidity sensor?) And as always: layman’s opinion