Whether/how to cover exposed insulation

[Flyover]

Over in the Painting board I asked about painting some exposed/unfinished ceilings. Along exterior walls in those rooms, some of the stud cavities have exposed insulation. (Note: this is inside the conditioned space.)



Not every stud cavity is like this; some have a section of joist material installed sideways (e.g. in the right-hand side of the picture), presumably with insulation behind that. But about 80% of them are like this.

1. Is the exposed insulation a problem in of itself? Should I buy a joist and cut it up to size and install the pieces in the stud cavities over the insulation?
2. If the exposed insulation isn't a problem, what should I do when it's time to paint? If I'm spraying, can I spray right over it?
3. What's up with this? Why are only some of the stud cavities covered?

 

[Sparky617]

In basements insulation is installed with the vapor barrier facing the heated space above. The facing is flammable and needs to be covered in a living space with drywall. For your situation where you want to paint the bottom of the subfloor and the joists I'd probably install some kind of blocking at the wall to cover the insulation and give you a cleaner look. You could install a 2x8 or 2x10 to match the existing joists. The one with the duct is going to be a bit more of a challenge but I'd probably cut a U shape out of the 2x to enclose the bottom and side of the opening with the part above the duct open, and largely not visible. This would all be for aesthetics.

I can't tell from the photo what's going on in bays that don't have visible insulation. what is on the other side of this wall? Is it exterior and you're looking at the rim joist on the bay to the far right. You want your rim joist insulated. You could cut some Poly Iso foam board to insulate the rim joist. This could just be painted as poly iso has a pretty good fire rating.

 

[Flyover]

In the photo, the bay to the far right is not at a corner of the house. There are more bays further to the right along that same wall that have the exposed insulation. I don't know why some are covered and some aren't. I don't know what's behind the piece of 2x8 covering that bay. (I assume there's insulation behind it, but that's a hopeful assumption.)

Looking at it more closely, the insulation does appear to be laying "flat" with the vapor barriers facing the space above. For whatever it's worth, this room is on the lower level of a split; standing in it, the ground outside is about waist-high. But I also have this same situation in a room in my "true" basement where the ground is close to level with the ceiling of that room.