ᐅ Is that mold? What is the black substance between the wall and ceiling?

Hello dear community,

this is not about any defects or warranty cases, but simply because I can’t quite classify it.

A few weeks ago, black spots started appearing sporadically on the exterior wall between the wall and ceiling of our walk-in closet. Upon closer inspection and scraping off the paint, it turned out to be "black growth."

The interesting part is that the black growth is exactly where, shortly before moving in, the joint between the wall plaster and ceiling plaster was filled and smoothed with painter’s acrylic to create a seamless look between ceiling and wall. I believe this procedure is generally well known. This was done on all joints between ceilings and walls (by the painter).

Has anyone ever seen anything like this?

Best regards

I’ll take a guess and say that this is the wall vapor barrier that wasn’t cut...

I also suspect the vapor barrier. What kind of texture does the "black growth" have?

In my opinion, it’s definitely not acrylic. To me, it looks more like some kind of vapor barrier. But what is a vapor barrier doing on the ceiling of a living space? That doesn’t really make any sense.

That makes sense; it belongs between the wall and ceiling.

Ok, I don’t have a new build, and I haven’t seen this done like that before. I only know of it being done once towards the basement.