ᐅ Remove old polystyrene interior insulation? Alternative insulating plaster?

Hello!

In brief:

• House from the 1950s (24cm (9.5 inches) exterior walls)
• Renovated in 1999 with 14cm (5.5 inches) exterior Styrofoam insulation
• Partial interior insulation with 4mm (0.16 inches) Styrofoam boards under wallpaper
• and partially painted 1cm (0.4 inches) Styrofoam boards

There is practically no wall without this annoying Styrofoam on it.
Now I am worried about possible mold growth because the average humidity is around 65%. I opened up a corner, but at least there was no mold to be found there.

What would you do? Remove the Styrofoam or keep it?
I am thinking about applying a mineral-based roll-on plaster, but of course that would mean giving up the (albeit slight) interior insulation.
Is there mineral insulating plaster for interior use that can be applied with a roller?

Is there already any sign of mold? Or why would mold suddenly appear after 18 years?

Also, when combined with external insulation, internal insulation is usually unproblematic. Without insulation on the outside, the dew point shifts into the wall.
However, this issue does not occur with properly installed external insulation.
That said, the centimeter (cm) is rather insignificant. So you can handle it like a roofer.

Whether there is actually any mold behind the polystyrene, I cannot say; in any case, there is no visible sign of it on the outside. Even the one corner I exposed is clean.

I left it as it was at my place. I also have both exterior and interior insulation. If you want to be 100% sure, remove it. Otherwise, the only effective method is consistent ventilation to keep the indoor humidity low.

Thanks for your feedback. I think I will open another wall to be sure... just not sure where.