Revisions log for Answer #1193
This is the answer to the question Why is there dampness on a wall covered with drywall? How to fix it?
Revision 1
There are a lot of reasons, and the drywall has nothing to do with it. The house is one-story, there is poor waterproofing, dampness is coming from the basement.
What could be the problem:
The floor slab is not waterproofed. Problems with the roof, the wall gets damp at the top, treatment, repair the roof. Poor ventilation in the room, installed new double-glazed windows on the exit got a "thermos". House is not insulated, in the cold season, the dew point moves into the interior. There is a need for external insulation.
The only thing confusing, you write that behind the wall of another apartment, it is not clear whether only this wall adjacent to the apartment, or all the walls in the room gets damp.
Not enough heating, it is damp and cold in the room, this too can be a reason.
If in general and plasterboard is not waterproof, you need to replace it, dry the wall, mold deeply penetrates into the plaster (you wrote about the old), plaster may have to remove completely and treat the walls with products that destroy mold, for example not bad, used in living quarters "Quelyd".
And then to deal with the cause and eliminate it (see above, waterproofing, roofing, etc.)