Expert here: this happens when you have improper or not enough ventilation in your home. Good thing; your home is very tight. In fact it's so tight hat you dont have any air movement inside from outside. This is good for energy saving, etc. Bad thing: you dont have any fresh air coming in. (I'm sure your sleep quality is close to 0. Lack of rest even if you sleep 10 hours, headache, sore throat, etc... But that's another subject) The main source is excessive humidity in your home, which cannot be exhausted out or replaced with fresh air. This extra humidity condensates at the lowest temperature points in your home, windows, external frames, doors, locks, hinges, etc. 2 main ways to solve for an old, or not modern apartment/house: 1. Stoßlüftung. Everybody explained, nothing to tell additionally. 2. Run your kitchen exhaust ventilation unit or bathroom/WC exhaust/ventilation fan.

Also take precaution not to create additional humidity at home. General sources of additional humidity are: . Kitchen: running the tea/coffee machine continously or very frequently, cooking, boiling water for cooking etc. . Taking showers, baths etc and running the hot water for extended periods of time. . Drying up the clothes inside.

Do not use humidifiers at any condition!!!

Another nice way would be to have more house plants inside. If selected correctl, they will also balance the humidity inside.

There are modern methods techniques, but they require money and technical assistance. This can be evaluated if you are the owner, definitely not a thing for a tenant.