Problems of humidity in old buildings

Hello everyone,

I have a problem with humidity in the apartment.

All rooms have between 55 and 70% humidity despite regular airing.

We had mold remediation done a few months ago and now want to solve the problem, as we also took over this condominium a few months ago.
The mold remediation was paid for by the previous owner.

We have a 100mm hole in each room for air exchange.
There is a fan in the bathroom that is supposed to create negative pressure in the apartment.
I believe this nonsense simply doesn't work properly.

Now my consideration would be:
I want to install a decentralized ventilation system in each of the 100mm pipes, which would be set to certain humidity values, and then I would have peace.

However, I am unsure if the 100mm pipe is sufficient?
Enlarging it is difficult because I need the approval of the other owners. And that takes time...

Would that work?

Thanks in advance for your opinions

The mold remediation was still paid for by the previous owner.

There you can already sense the beginning: What happened there? What was remediated and why?

We have a 100mm hole for air exchange in every room.

A hole in the wall to the outside??? Like looking out and hoping no critters come in and there’s a draft? In summer, you bring in all the humidity through that, in winter the walls cool down as if there were no tomorrow. Then the walls get so cold that moisture condenses there (Google "dew point") and you end up growing mold. The idea of decentralized ventilation is good and 100mm pipes (or 105) are enough for that. Normally you need a slight slope in the borehole so that condensate water runs outside. Get a company that knows about this and implements it for you...

All rooms have a humidity level between 55 and 70% despite regular ventilation.

So? Those are normal values. You have exactly those right now with a decentralized ventilation system.

Currently, I have 92% relative humidity at only 12 degrees (outside) ==> that is about 52% relative humidity at 21 degrees. You will never be able to change that regardless of the methods you use (except for explicit dehumidifiers).

Otherwise, I can only agree with my predecessors. A ventilation system with heat recovery provides carefree ventilation - but it cannot work miracles.

There is the beginning to guess: What was going on there? What was renovated and why?

We had mold on two exterior walls. A mold expert checked all the rooms here and presumably, due to cold exterior walls and little air exchange (bed and couch were against the walls), this caused the mold.

The entire plaster was removed here and subsequently calcium silicate boards were installed.

A hole in the wall to the outside??? Like looking out and hoping no creatures come in and there’s no draft?

We have a 100 mm hole in every room with a plastic flap in front of it.
Inside there is only a grid that prevents insects from entering the apartment.

But no fans etc.