Alternative to central/decentralized ventilation

Thank you very much for the lively exchange.
The Kfw 55 loan only needs to have been discussed with the bank before the start of construction. We have done that and it was documented by the bank, and now we have 3 months after the start of construction to submit the Kfw application. Therefore, nothing is happening there. But thanks for the hint.

What are your facts and measurements?

Humidity between 43 and 56%, drops only occur with a strong rapid temperature increase due to completely opening windows at higher outside temperatures. Source: Froggit weather station indoor sensor - CO2ppm measured only sporadically with a loan device, sleeping with closed window, still under 200ppm in the morning. Of course, it has a lot to do with the room volume, since CO2 is not bound by material like moisture is. In small rooms, it is better to sleep with air exchange through open windows or other ventilation.
Our solution excluded KfW funding, takes some work and is largely independent of a provider.
Many roads lead to Rome.

has decided after all. Glad that you have made progress. Think about a solution with an enthalpy heat exchanger for good humidity control.