Very high humidity in all rooms

I don't understand what these leading questions are supposed to achieve. It has already been said that manual ventilation is not sufficient with new windows. Unless you consider ventilation a hobby. And if you start ventilating, then not only in the bathroom but also in the living rooms. Ventilation alone – whether manual or automatic – will not solve the problem. Old building walls simply insulate poorly, and you can only avoid the dew point with very dry air. In the long run, external insulation will have to be installed

There are many more EFM (partially renovated old buildings) with new windows without ventilation systems, without external insulation, ... that do not cause problems given appropriate user behavior. Ventilation systems are a good thing if you want to have them/pay for them. But reality is that many don't want to/do not do that...

There are many more EFM (partially renovated old buildings) with new windows without ventilation systems, without external insulation, ... which do not cause problems with appropriate user behavior. Ventilation systems are a good thing if you want/can afford to have them. But reality is that many do not want/do not do that...

Of course, you can also manage without insulation and without ventilation. To make the physics work here, you simply heat to 23 degrees, ventilate 6-10 times a day to get rid of the heat, and have very dry air. A recipe for saving money definitely looks different, and so does comfort...

One more time the question to the OP (everyone else please don’t answer ahead :))
Do you think that airing once after showering and then leaving the house... is enough?
Do you think that hanging wet towels outside after showering and quickly going over the shower/wet surfaces once with a squeegee helps?
Would you be willing to install a simple humidity-controlled electric bathroom fan?
Are you (you all?) able to ventilate more often and more thoroughly?

I know what you’re getting at. A fan for all rooms wouldn’t be a bad idea, I think, since you’re not always at home.

I’m trying to gather ideas here and prioritize them by cost-effectiveness and benefit in order to plan a possible implementation.

Then all houses from the 60s would have to be moldy. Since this is not the case, there must be (still) other problems here. A decentralized ventilation system never hurts? We hadn't opened the windows for 3 days and even in the bedroom currently 22.5°C and 57% rh. In the living room with 42% already almost too little humidity.

I also wondered why the previous owners made a simple (thin) interior insulation. My crystal ball says there were reasons for that...

Previous owners did not have the same airtightness as we do now. Previously, you could look outside through the roller shutter box. If I put plastic on the wall, the wall would still remain cold and moisture would condense between the wall and the Styrofoam. (Screenshot) As I see it, there are only the following options, right? 1. Install ventilation. 2. Blow-in insulation. 3. Climate panels on the north wall. External insulation is certainly much better, but I have to check whether the brick facade can support it. That means I need not only insulation but also a new facade. The roof must be pulled out because otherwise the insulation would protrude. Cost: 60-70k. Wouldn't it be enough if the U-value is below 1, so that moisture condenses on the window with a value of 1?

I know what you're getting at. I think a fan for all rooms wouldn't be wrong, since you're not always at home. I'm trying to gather ideas here and arrange them according to cost-effectiveness and benefit to plan a possible implementation.

In partially renovated old buildings... you can find airtightness ranging from very leaky to super tight. In many buildings, a permanently running/time.../humidity.../ or otherwise controlled electric fan in the bathroom is enough to solve problems because the building gets enough air... as long as the user behavior and the building fit together. I'm not a fan of standard recipes... it always depends on the individual case what I need, what I want, what I can/want to afford. This basically also applies to insulation and heating...