Internal insulation because of mold stains?

Install a decentralized ventilation system?

Too expensive (although tax-deductible). The tenant should rather ventilate more.
As an owner-occupier, of course, I would speak differently.

Too expensive (although tax-deductible). The tenant should rather ventilate more. As an owner-occupant, I would of course speak differently.

Funny enough, it is exactly the other way around for us now. We are installing the system in the rented apartment and not in ours. We have the problem under control with plenty of ventilation. The landlord will probably not be willing to ventilate so much that mold does not occur. And that only leads to trouble. That's why install a ventilation system and have peace.

And because the system is too loud for the tenant, it is turned off. What then?

It must be included in the rental agreement that the system has to run. The tenant knows this in advance and must accept it or not take a room in the apartment.

It should be included in the rental agreement that the system must be running.

Very thin ice. Who pays for the electricity? You are imposing additional costs on the tenant.
And if the system is running at "1," but that's not enough? Does it have to run 24 hours a day?

This should be agreed upon with the tenant. Forced measures are legally ineffective.

The tenant knows this in advance and must accept it or not take a room in the apartment.

There are many things the tenant says "yes sure" to but is not legally obliged to adhere to.

Ok, what to do then in such a case? I know if you don’t ventilate super consistently, you immediately get mold in the corners. As I said, we manage that well, but we also ventilate properly 2-3 times a day. I can’t assume that a tenant will do the same. The apartment is to be rented out as a shared flat. All fans run through a central control unit. I can attach that outside the apartment ^^