Windows with insulating double-glazing are many times tighter than preceding windows. Your apartment thus lacks permanent controlled ventilation. You now have to adapt the ventilation process to these new conditions.
Water vapours from the breathing and sweating of living beings, as well as the water vapour from cooking, washing, and watering of plants negatively influence the relative humidity in the apartment. A person exhales 1 to 2 litres of liquids a night merely while sleeping. Regular ventilation of interiors reduces air humidity and prevents the occurrence of mildews.
Ventilation should be short and intensive, by opening all windows and ideally by draught. Up to 5 minutes is usually enough, depending on the outdoor temperature. Ventilate 3 to 4 times a day as required. During ventilation, windows should be fully open and heating off. Permanent ventilation through semi-opened windows is not so efficient. External air must always be used for ventilation because it contains less humidity than the air in the room; never use air from another living space. Once the windows are closed, the fresh cold air is warmed very quickly.
A dewy window is a signal that ventilation is required! It means the air humidity in this space is too high.
Tip for correct ventilation:
As soon as you open the window, the external glass pane immediately becomes dewy. When this deposit of moisture disappears and the pane is transparent again, you can close the window. The required amount of air is exchanged during this period while the walls and room equipment have failed to get colder.
Dew point – precipitated water
The glass or frames and other parts of windows may "sweat" under certain climatic conditions.
Similar phenomena based on identical physical principles occur in common life. For example, when you take a jar out of the fridge, it quickly gets dewy. Eye-glasses immediately get dewy when entering a heated room from the cold outside. This phenomenon can be explained by the fact that hot air absorbs more moisture than cold air. The hot air gets cold upon contact with the cold eye-glass surface and the excessive amount of moisture that cannot be absorbed is precipitated into water.
This phenomenon especially occurs when the hot air in a room containing a lot of moisture encounters eye-glasses chilled by the cold air outside.
In practice, this phenomenon takes place in bathrooms, kitchens and bedrooms as well as in rooms that hold numerous indoor plants. The precipitated water mainly forms at the bottom edge of glass panes where the hot air radiated from a heating element cannot flow around the bottom edge of a window due to a prominent sill, or when sun blinds are lowered and inhibit the flow of air around the window glass. The only prerequisite to prevent this phenomenon is correct ventilation!