Mold inside a 3-year-old house wall/ceiling corner

Too little ventilation. Well, and necessarily also heating...

But at 19.3 degrees, you really can't say there is too little heating...

Something is wrong, is this the basement? During the heating period, the humidity is normally around 40 percent. That probably means that water is actually seeping in from outside somewhere. In other words, a leak!

It is simply more likely that there is too little ventilation. If there is no [Kontrollierte-Wohnraumlüftung] or purely mechanical solution like [Fensterfalzlüftung], airing out once a day is by far not enough... All houses nowadays are airtight. In the heating season, no less humidity accumulates indoors than in summer,... If the house is occupied without regular ventilation, the humidity also rises...

2. I just measured the temperature on the wall. It is 2 degrees colder (17.2 compared to 19.3) than in other places. How a thermal bridge should form there I wonder, but I am not an expert.

I had the same problem just now in an apartment (not inhabited by me). The corner was simply colder and therefore there was mold. You can ventilate, but it doesn’t help. The insulation (for whatever reasons) doesn’t fit = thermal bridge. Possible solution: Insulate the inner corner with calcium silicate boards. Then nothing should mold anymore.

I also feel that 19.3 degrees is too low. We always ventilated well in the apartment but last year tried to postpone the heating season to the beginning of November. And suddenly, due to heating too late, mold/mildew or whatever formed on the roller shutter box. Heating too little is just as bad as ventilating too little. After we removed the stains and heated properly, everything was fine again.