I have a completely above-grade single-story house with exterior walls made out of 6″ CMU (hollow/unfilled) in Climate Zone 2, no inside framing, you go inside and you see the same CMU blocks that you saw from the outside.

Well, that is until I installed 3/4″ polyiso on the inside a few years ago.  Now I’d like to install more insulation, so I planned to install 1.5″ to 2″ of polyiso + furring (rainscreen) + siding on the outside, but now I’m wondering if insulating the exterior and making a CMU sandwich between two impermeable polyiso layers is a mistake since I read somewhere that masonry is supposed to breathe to dry out.  I don’t know how the wall is gonna get wet when it’s completely covered, and I don’t know why wet masonry would even be a problem because it can’t rot like wood, but anyway, is this a problem, and if so, what’s the worst that can happen?