Foggy windows despite ventilation system

Yeah, either the Netatmo is showing garbage or the window is simply completely leaky = almost no insulating effect between the panes.

Yeah, either the Netatmo is showing garbage or the window is simply completely drafty = almost no insulating effect between the panes.

So in the meantime I measured with a conventional thermostat. Here too, the values regarding humidity were almost identical.
How can you check if the window actually has a problem?

About the windows:
- Wooden window, double-leaf: 1.51x2.51m
- Uw-value: 1.3 W/m2 K
- Insulating glazing
- In front of the window door, the air supply of the ventilation system is centrally located in each room

I would then also recommend a heat exchanger and not a preheater... because the latter requires a lot more extra electricity.

Just measure the temperature on the window. The glass must be freezing cold if water is condensing like that. If you don’t have a suitable thermometer, just do it by hand. As a reference, you can use objects from known temperature ranges, e.g., fridge, cellar, bedroom. If the insulation works, the inner window pane should only be a few degrees cooler than the room temperature.

Just measure the temperature on the window. The glass has to be freezing cold if water is condensing like that. If you don't have a suitable thermometer, just do it by hand. As a reference, you can use objects from known temperature ranges, e.g. fridge, cellar, bedroom. If the insulation works, the inner window pane should be only a few degrees cooler than room temperature.

So the glass is ice cold at the bottom – clearly colder than the room temperature. So the glass or the window is the problem?

How do the other windows behave? Colder, warmer?