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It sounds like you are trying to avoid using foam, is that correct?
The typical approach to insulating and air sealing rim joists use either spray foam or rigid foam: https://www.greenbuildingadvisor.com/article/insulating-rim-joists
The comments in this thread about foam-free rim joist insulation might give you some places to start, especially the comment from Malcom Taylor: https://www.greenbuildingadvisor.com/question/insulating-rim-joist-without-foam
Using Visconn to air seal the rim joist seems like a fine approach to air sealing at the rim joist. I would expect the thickness to be specified in the manufacturer's installation instruction or specifications. Is it not? If you insulate with mineral wool, the challenge will be to create an air tight vapor retarder on the interior side of the mineral wool. That sounds tricky, which is why it is an uncommon approach. But where there's a will there's usually a way ;)
The air barrier needs to be continuous to have real affect, otherwise air will find away around and through assemblies.Sealing your rim joists is great, but the sealing system should tie into a continuous plane of air tightness.
If you have a continuous air barrier on the outside of the house, your interior vapor barrier/retarder does not need to be air tight. Think a sheet material stapled up but not taped or caulked at the edges or penetrations.
Am I understanding the situation correctly?
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I'm curious what you ultimately ended up doing with your rim joists. I have energy assessment clients who don't want spray foam, and I'm not a fan either. Foam board sprayed in place doesn't necessarily prevent all air infiltration because the corners behind the foam board aren't sealed. Been thinking a liquid applied WRB (Viscon or Prosoco) could solve for this. Not sure if you'd still want to do the foam board and then mineral wool or if just the liquid WRB and mineral wool would be okay.
gbshv,
It's not just the air-barrier that is a concern, it's also vapour movement. You need something impermeable located where it can stop moist interior air from getting to the cold rim-joist and condensing. That's what the foam board or spray foam is for.
The two links in Brendan's post #2 above and the ensuing comments cover things pretty comprehensively.
Thanks, Malcolm! Most of the weatherization crews around here go straight for the 2" closed cell spray foam approach. However, we've seen on many assessments where the foam is not dimensionally stable (or wasn't mixed properly in the first place) and has pulled away from the joists. There goes your air barrier! We prefer the foam board method but it was pointed out that even with 1-part foam sealed in place, there can still be air leakage around the backside of the foam board. Hence, the suggestion of the liquid applied WRB before the foam board goes on. Thanks for clarifying that you still need the foam board for vapor control vs. going right to mineral wool. I tend to forget that the liquid applied WRB is actually vapor permeable!
The same size sausage of Prosoco AirDam is cheaper than the same size sausage of Prosoco FastFlash by 10 bucks on Amazon.
AirDam is a vapor barrier.
Since "great stuff" can "closed cell" isn't loved here for long term stability, I have often wonder why it gets approval to seal rim joists.