Mold corner, ground floor, detached house, year of construction 1968

The corner looks as if a cupboard or something similar had been standing on the carpet. This greatly restricted air circulation behind it. The wall is probably built to the 1968 energy standard, so it is poor. Therefore, a high air exchange and higher room temperatures are necessary to prevent moisture from condensing there. The damp spot will then start to mold after a while (quite quickly). You can fix this by not placing cupboards in external corners and always leaving at least 5 cm of air behind furniture on exterior walls. If the roof is leaking, you have to address it from the outside, no matter how big the spot inside is. If you're lucky, it’s a leak at a penetration. But with 1968, I would simply plan to insulate and seal the entire roof anew.

Many many thanks! This is exactly something I as a layman can work with – to better understand the basics and connections and to derive something from it.