What I have:  recent construction house in CZ5, 2×6 basement “pony” walls where the foundation steps down to follow exterior grade.  Walls currently have 2×6 fiberglass batts with no vapor barrier/retarder (plus some signs of condensation on the interior of the sheathing).   I’m running a couple of dehumidifiers in the basement (RH around 35-40%).  Concrete walls have 1-1/2 rigid XPS NGX insulation.

I was originally thinking 5-1/2″ of mineral wool with Intello Plus on the interior (warm) side sealed to the framing with Contega HF, but due to some air leakage from the exterior I believe I should air seal within the stud bays of the pony walls first to do the best possible job.  The area involved is probably 200 square feet or so, so using 2″ of spray foam with 3-1/2″ mineral wool batts followed by the Intello Plus wouldn’t be a wallet-buster and would be in line with the ratio of “rigid” to “fluffy” insulation.

That said, what’s the “best” insulating approach for these 2×6 walls that doesn’t entail exterior insulation that will preclude future condensation buildup in the wall assembly AND deals with the air leakage?  If I go with a layer of interior foam does closed-cell (low GWP) or open-cell matter?

Thanks!