Cincinnati sits on the dividing line between Climate Zones 4A and 5A per the 2021 IECC. Climate Zone 4A means moist mixed-humid: 4,500-4,800 heating-degree-days, dew points above 65F for weeks of summer, and a code-required R-49 attic + R-13 to R-21 wall target. Spray foam delivers those targets in less depth than fiberglass and includes the air seal.
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IECC (International Energy Conservation Code) divides the US into 8 climate zones based on heating-degree-days plus a moisture classification (A=moist, B=dry, C=marine). Cincinnati falls into Zone 4A, the same zone as Indianapolis, Louisville, and Pittsburgh. The cutoff line between Zone 4 and Zone 5 runs roughly along the OH-MI border; the cutoff between A and B runs west of the Mississippi. What 4A means in practice for residential envelope design. (1) Heating-degree-days at 4,500-4,800 annually means heating-season air leakage is expensive; cold air infiltration during a typical Cincinnati January adds significantly to gas heating costs. (2) Summer dew points sitting in the high 60s to low 70s for weeks at a time means cooling-season latent load (humid air the AC has to dehumidify) is high; closed-cell foam at the rim joist and crawl-space walls reduces both sensible and latent cooling costs. (3) IECC 2021 attic insulation target is R-49 minimum for retrofit and R-60 for new construction and major additions. Wall cavity target is R-13 to R-21 depending on assembly type. Floor over unconditioned space: R-19 to R-30. Basement walls: R-13 to R-19. Closed-cell foam at 7-8 inches delivers R-49 in an attic; open-cell at 13-14 inches delivers the same. The Cincinnati-specific design wrinkles. First, the freeze-thaw cycle produces ice damming on under-insulated roofs; conditioned-attic roof-deck foam eliminates ice damming by keeping the underside of the deck warm. Second, mid-century housing stock with R-9 to R-18 effective attic insulation falls 30 to 40 R below current code; bringing those homes to R-49 with foam produces immediate measurable winter heating savings. The 4A boundary classification also means homes in northern Cincinnati suburbs (Mason, West Chester, Liberty Township) actually sit very close to the Zone 5 cutoff line; some local builders spec to Zone 5 targets (R-49 to R-60 attic, R-19 walls) for resale flexibility.