Interior insulation Styrofoam mold formation

Hello,
our house built in 1965 was insulated on all interior exterior walls with Styrofoam (exterior walls 30 cm hollow brick - Styrofoam - 2-3 cm plaster - wallpaper). Unfortunately, I do not know exactly how thick the Styrofoam wall is. So far, the house has no exterior insulation. We did not know about the Styrofoam beforehand (discovered by chance).
After the previous owner moved out, we discovered mold in some corners. Only rooms where single-glazed windows were replaced with double-glazed windows are affected. In the rooms where the old windows are still installed, there is no mold.
At first, we did not want to insulate the house, but we assume there is a connection between the interior insulation and the mold formation. Does anyone have experience with this? Remove the Styrofoam insulation and then insulate from the outside?

Best regards

Initially, we did not want to insulate the house, but we assume that there is a connection between interior insulation and mold formation. Does anyone have experience with this? Remove the polystyrene insulation and then insulate from the outside after all?

Interior insulation is (almost) always a problem. Due to the new windows, the rooms have become more airtight. In the rooms with old windows, you probably have continuous ventilation through them – not really good for heating costs... The simplest solution is to ventilate and heat correctly. Always keep it warm and ventilate by shock ventilation.

Interior insulation is (almost) always a problem. The rooms became more airtight due to the new windows. In rooms with old windows, you probably have continuous ventilation through them - not really good for heating costs...
The simplest solution is to ventilate and heat correctly. Always keep it warm and ventilate by airing quickly.

Furthermore, the dew point with single glazing can usually only be on the window. With double or triple glazing and poorly executed interior insulation, the dew point shifts into the insulation / onto the wall surface.

That's basically the problem with interior insulation. The dew point always shifts at most to the inside of the actual exterior wall. The result is condensation between a very poorly vapor-permeable layer and thick masonry, which remains permanently damp as a consequence. That can only lead to mold. Exterior insulation, so that the dew point lies outside the masonry!

That is basically the problem with internal insulation. The dew point always shifts at most to the inside of the actual exterior wall. The result is condensation between a very low vapor-permeable layer and thick masonry, which is consequently permanently damp. That can only lead to mold. External insulation so that the dew point lies outside the masonry!

No. Internal insulation works, just not with Styrofoam. So removing old Styrofoam / EPS is already good. Then use capillary-active vapor-permeable insulating materials (hemp, jute, flax, wood fiber, calcium silicate) as internal insulation, do not seal the inside, i.e. apply lime or clay plaster with lime/clay/silicate paint and that's it. It works with 2 cm insulation thickness just as well as with 20 cm. The only "danger" is open joints on the weather side, because a lot of moisture can penetrate here, which no longer dries out thanks to the insulation. But this also applies to external insulation. So check the plaster or the brick joints accordingly. In an old building, adding an exhaust system in the bathroom for permanent minimal negative pressure makes the place completely dry. The Glaser method for determining the dew point, by the way, was not developed for internal insulation and is not supposed to be used there. Someone just started using it and since then it has become very dangerous. Similar to the comma error with iron in spinach, only worse. Whether one or three panes are in the window does not matter to the indoor air. Only with a single pane or two panes from before 1993 does humidity tend to condense on the window rather than on the old building wall. In all cases, the same amount of ventilation is required, only with old windows do you get warnings.

Thank you for the addition, yes, I assumed eps.