Your result
This looks like a building fault
Score 19 of 2966%
Salty or crumbling plaster, marks low on the wall, or a problem that worsens after rain are not condensation. They point at water entering the building.
Ventilation will not fix this and redecorating will hide it for one season. Worth having someone identify the source before any repair — the cause is usually outside the room the mark is in.