Plaster Wall Moisture Problem
You're looking at a plaster wall moisture problem that
has been soaking up water for quite some time. Whenever you see a wall
in this condition, there's a good chance that the wall framing, is also
suffering from water damage.
I have actually seen water that was sitting on the floor, from a broken water bed, actually worked its way up a plaster wall, about 36 inches. It's hard to imagine, but I have seen it happen to interior and exterior cement and plaster-based walls.
Home Repair Tip: I recommend removing the damaged wallboard and replacing it. I have seen other contractors and I even done it myself, let the walls dry out, then scrape off the damaged plaster and then re-applying a layer of joint compound, over the damaged surface, until it was smooth.
Only later to find out that the plaster repair job, that looked so good, when I was done. Was now separating from the damaged plaster and falling off in small sections.
I hope you understand, what I am talking about, when I say, that you need to remove the damaged material and replace it. If you let something like this set for a long period of time, there's a good chance that mold and fungus will start growing behind the damage plaster.