Few Questions on Knee Wall Insulation

Bear with me a bit, as this is my first house with a knee wall. Ok, we’ve been in this house for two Michigan winters, and each on we developed frost along parts of our interior walls at the point where it joins with the ceiling. These walls are exterior walls, and under a knee wall. This year I realized that none of the joints on the knee wall…ie, between the base plate and OSB it’s sitting on….are sealed and I’m ASSUMING that’s how cold air is slipping in causing the frost on the wall below it. All caulked and spray foamed up….however….

Our upstairs is FREEZING in the winter time. After reading a ton about knee walls and dealing with this frost issue, I’ve concluded that my knee walls are poorly insulated in terms of installation method. I know that you either have to properly insulate the knee walls themselves to block cold from the conditioned room, or block off the area behind the knee wall and insulate that area (allowing air to still flow over top of it). For cost and ease, I want to just super insulate the knee walls themselves and leave my storage space unconditioned air. Now the question:

1. My plan was to just slap hard foam insulation boards over what is pre-existing and seal the crap out of all the seams and cracks and joints. However, per the pictures below, the existing batts are installed with the kraft paper barrier touching the sheet rock and fiberglass exposed to the attic. If I put foam board over top of the insulation, won’t I be putting it between two vapor barriers? And isn’t that a bad thing? All the videos I see online show the batts installed with the kraft paper on the attic side….so putting foam over it still keeps it with vapor barrier just on one side which I couldn’t do if I’m reading it right.

2. So…if I’m right about the double vapor barrier, what do I do? Somehow rip out all the existing batts and start over? Or is there another product besides foam boards I can slap up there that won’t violate the vapor barrier rule (if that’s really a thing)?

Pictures for reference (sorry for the bad angles…I’m super tall so it was a tough photography session):

Front of house knee wall where I had the frost problem:

Top plate of front side knee wall from above picture. There is a foam baffle up farther you can’t see (is that how you do that?):

Rear "non-traditional" knee wall…bump outs are closets and bathroom…heat duct is also exposed to unconditioned air:

Thanks in advance for the help!


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