Cincinnati averages about 4,900 heating degree days a year (NOAA 1991-2020 normals at CVG), under zone 4's 5,400 ceiling, and is wet enough to be classed moist (A). For foam, 4A sets the attic target (R-49 under Ohio's code) and the closed-cell depth unvented roofs need.
More detail
The label comes from the thermal climate zone table in the International Energy Conservation Code. Zone 4 covers places with more than 3,600 and no more than 5,400 heating degree days (base 65 degrees F) a year, and a location counts as moist (A) unless it qualifies as dry or marine. NOAA's 1991-2020 normals for the Cincinnati/Northern Kentucky airport show about 4,892 heating degree days and 1,160 cooling degree days, well inside zone 4, and the IECC county tables have long put Hamilton and Clermont counties in 4A along with every county in Kentucky. Two local wrinkles matter for a foam job. First, Ohio's 2019 residential code, which still governs new one- to three-family homes here, uses the older county map that places Butler and Warren counties in 5A, even though the 2021 IECC map moved both into 4A, so a Mason or West Chester house is still checked against 5A rules for floors over unheated space (R-30 rather than R-19) and for unvented roofs. Second, those unvented roof rules are where the zone number reaches the foam spec. When closed-cell is sprayed against the roof deck with air-permeable insulation added below it, Ohio's code calls for at least R-15 of the closed-cell layer in zone 4 and R-20 in zone 5 so the sheathing stays warm enough to avoid winter condensation, and in zone 5 the foam against the deck must also work as a Class II vapor retarder, which closed-cell does at typical depths and open-cell generally needs a coating to do. The rest of Ohio's prescriptive table is the same in both zones: R-49 ceilings, R-20 walls or R-13 plus R-5 continuous, and R-10 continuous or R-13 on basement and crawl space walls. Kentucky's residential code still uses 2009-level energy values, starting at R-38 for ceilings. The component-by-component numbers for both states, and what they mean for an existing house, are on our climate zone and R-value table; a plain-English walkthrough is in climate zone 4A explained, and where the vapor-retarder property earns its cost is covered on closed-cell spray foam.