Mold underneath interior paint but NOT inside the wall (exterior wall) – GreenBuildingAdvisor

See attached photos. While preparing to repaint, I noticed mold when I took off baseboards. My question is about finding the cause, as I’m already planning to rip out affected areas (unless someone strongly advises against).

I used concrobium to wipe away what I thought was surface mold, but was likely just old crusty dust. What stayed behind is the mold you see in photos. It’s underneath the paint layers, on the drywall surface.

Pinless moisture meter reads totally dry, no matter where I sample.

The mold is firmly “on” the paper, no surface texture to it, and can’t be wiped away.

I cut a large sample (sample-1.jpg) and found no mold inside the wall. I’ll know more once I tear out large sections, but so far (even running a borescope into the wall) I don’t think I’m seeing any mold on studs.

It’s a late 1960’s home, and has some kind of product in the wall — I’m a new homeowner and don’t know what that aluminum looking stuff (see sample-1.jpg) that says “2.25 medium” is. Insulation? Vapor barrier?

This room is at the back side of the house. The walls appear to have siding right on the studs, with no sheathing. If it’s also insulated (that aluminum looking stuff), my research says that’s a recipe for bad things overall, without air-sealed sheathing, then siding over top as is the modern approach.

I cut two other samples (sample-2.jpg, sample-3.jpg) and found no mold inside the wall, where it was moldy under the inside painted surface.

My thoughts on the cause and the fix:

Cause: It’s an old house with leaky walls, and condensation caused by air leaks is creeping up behind baseboards.

• Replace moldy drywall sections at inside-bottom of exterior walls

• Caulk the joint at the drywall and the bottom plate/floor. Air flows in that gap right now, it’s not sealed.

• (Longer term task) take off old siding, sheath the house with ZIP and tape. Insulate inside walls at this time. Liquid flash sheathing at sill plate/foundation and also at top plate. Prepare siding with drainage etc etc

• Seal up joist bays from inside the crawlspace with canned spray foam.

Further doubts/questions

Does my diagnosis seem ok? I haven’t checked other rooms yet — that would solidify the cause even more if found at all exterior walls like this.

But if my moisture meter reads totally dry, and the inside of the wall isn’t moldy, could it be the previous owner had some moisture-causing thing in this room? Again, I need to test other rooms. Can’t just go exposing mold willy-nilly though so that will take some time.

But why would mold only be underneath the paint like this? And not inside the wall? I suppose the drywall paper is better “food” than paint. Still, as a new homeowner it seems quite odd.