ICF is stronger than reinforced block & foam, but it's also a lot higher-R, arguably much higher than needed in the keys. An IRC 2018 code-minimum reinforced block (or poured concrete) with foam on the exterior side could be R3 continuous foam on the exterior (, say 3/4" EPS covered with an EIFS finish), whereas even a 2" + 2" minimalist ICF is R17-ish. From a financial rationality point of view, take a peek at the zone-1 (top) row "whole assembly" values in Table 2, p.10 of this document: https://buildingscience.com/sites/default/files/migrate/pdf/BA-1005_High%20R-Value_Walls_Case_Study.pdf They're suggesting R10 "whole-wall" might be the limit of financial rationality. The interior & exterior air films add about R1 to the stackup, and 1x furring with another air gap to provide something to hang wallboard on adds another R1, and a 8" CMU block with conrcrete filled cores at least another R0.5, adding up to R2.5. So with as little as R7.5-R8 of continuous insulation you'd be "there". That could be 2" of EPS glued & cap-screwed to the exterior, or 1.5" of exterior EPS and using 3/4" foil faced polyiso on the interior between any furring used to mount the wallboard. Foam insulation between steel studs is largely wasted, due the severe thermal bridging of the studs. You're better off using continuous sheet foam between the CMU and studs, and thinner studs (or steel U-channel furring). If you're looking for something that's flood tolerant with all the insulation on the interior, use 1.5-2" of EPS up against the CMU, taping the seams and setting the steel studs tight to the foam. That would outperform 2" of closed cell polyurethane or 4" of open cell foam between studs, at a much lower price.
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