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

Yes, it is a thermal bridge. As soon as you have dried out the wall, it should resolve.

I don't understand how this fits with your above very good post. If a thermal bridge is present, drying it out once only helps very temporarily. The thermal bridge must be eliminated.

If you fall back into the old behavior, it only helps in the short term. None of us knows how the thermal bridge was formed. Exterior corners, for example, have the notorious tendency to thermal bridges even with completely correct construction. There is nothing more that can be eliminated.

Yes, but who heats a bedroom above 19 degrees, please? Often it is much cooler there. I see no sensible solution in this!

You have recognized the problem... it is often even much colder in winter. This is not compatible with the current construction methods of houses.

Yes, but who heats a bedroom above 19 degrees, please? Often it is much cooler there. I don't see any sensible solution in that!

We... We have a constant 22 degrees throughout the house. With thin duvets and the door open for fresh air. Sleeping is wonderful. If needed, the window can be opened.

In summer, people often sleep at 21/22, sometimes even 23°.

I am always surprised that it suddenly becomes a big problem in winter.