Too high humidity in the apartment. 60-70% in winter

From my point of view, you also need to continue heating and ventilating. At low outside temperatures, it doesn't matter at all whether it is raining outside or not. The moisture is contained in floors and walls, doors and furniture. Based on your description, I also suspect incorrect behavior by the previous tenants. At 19 degrees in the apartment, you might still not get below 50%, which should not be problematic. We are happy if we don't fall below 40% in our new building, because that already becomes uncomfortable.

I aired out and the humidity dropped to 42.3% at 18.1 degrees and then rose again within 5 minutes to 57% and then continues to rise more slowly.

18.1 °C / 42.3% RH / 5.4 g/kg absolute moisture 18.1 °C / 57.0% RH / 7.4 g/kg absolute moisture If your measurements are correct, there was a rapid additional influx of moisture into the room air (showering, cooking, breathing, ..., wet surfaces). I agree with the prevailing advice: heat a bit warmer (21-23 °C), ventilate frequently with short bursts, keep observing. IMHO(!) you cannot soak walls through improper behavior. I would rather suspect a damage case.

The rapid decline and the quick rise are due to the fact that the sensor for moisture has a completely different response time than the thermometer, which is used for compensation, not the rapid entry of moisture.

We experts really have a hard time with you

Hi,

I wanted to get in touch again.

Meanwhile, it looks like the rooms fluctuate between 20-21 degrees. The humidity is at 50-55%, so it has improved a bit.

Nevertheless, even at these values, the windows have been damp on the inner edges in the mornings and evenings over the past few days, one even more extensively... Is this normal due to the cold outside temperatures? Can I somehow reduce this?

Best regards
bpe

Ventilate, ventilate, ventilate... wipe away.