I moved into my wife's apartment and she has really bad mold on the soffits, she says it's always like this.
The house is brick, first floor, corner room with two windows, 500mm walls on the soffits. Double-glazed windows, profile width of 60 mm.
While I think this:
1 Reinstall the window, thereby updating the joint.
2 Plastic linings or can I leave the plaster?
3 Put a plastic or wooden window sill.
What I would do:
1. Reinstall the window. I think the window is standing too slanted outward. Install the window 10-12 cm deep from the outside edge.
2. Remove the concrete window sill and put a stronger plastic one (something like Danke, Moller, Cristalit, etc.).
3. knock down the soffits and install sandwich panel soffits, if necessary. with insulation underneath.
Do not forget to cover the foam joint from the outside with soffits or at least seal it.
5. Since the window sill overlaps the heater, it is necessary to cut into it a ventilation grille so that the heat from the appliance would rise to the window glazing.
Check how the window is sealed. You may need to reseal it. Be sure to insulate the soffits. Plastic or stucco at your discretion. Put a plastic window sill and also insulate under it.
What about your ventilation and heating? Probably one of the reasons is insufficient heating. And because of the insufficient heating – not enough ventilation. And from that dampness and fungus.
For example, on our top floor the heating was very good, the windows were not closed, there were no problems with ventilation.
And as for windows, even with single-glazed packages – slopes and under window sills are insulated – there is no condensation, no dampness, no fungus.
So I strongly advise to insulate the slopes and under the window sills. And you have the first floor, I would advise to insulate the floor.