Columbus sits in Climate Zone 5A per the 2021 IECC, a full zone colder than Cincinnati. Zone 5A means cold mixed-humid: 5,500-5,800 heating-degree-days, a long heating season, and a code-required R-49 to R-60 attic plus R-20 or R-13+5 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). Columbus falls into Zone 5A, the same zone as Cleveland, Pittsburgh, Chicago, and Denver. The cutoff line between Zone 4 and Zone 5 runs across central Ohio, below the Columbus metro; the cutoff between A and B runs west of the Mississippi. What 5A means in practice for residential envelope design. (1) Heating-degree-days at 5,500-5,800 annually means heating-season air leakage is the dominant energy penalty; cold air infiltration during a typical Columbus January adds substantially to gas heating costs across a long season. (2) Summer dew points sitting in the mid-to-high 60s for stretches means cooling-season latent load (humid air the AC has to dehumidify) is meaningful though secondary; closed-cell foam at the rim joist and crawl-space walls reduces both sensible and latent cooling costs. (3) The 2021 IECC attic insulation target for Zone 5 is R-49 minimum for retrofit and R-60 for new construction and major additions, as adopted by the Ohio Residential Code. Wall cavity target is R-20, or R-13 cavity plus R-5 continuous exterior insulation. Floor over unconditioned space: R-30. Basement walls: R-15 continuous or R-19 cavity. Closed-cell foam at 7 inches delivers R-49 in an attic (8.5 to 9 inches for R-60); open-cell at 13-14 inches delivers R-49 (16-17 inches for R-60). The Columbus-specific design wrinkles. First, the freeze-thaw cycle plus reliable seasonal snow cover 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 50 R below current code; bringing those homes to R-49 to R-60 with foam produces immediate measurable winter heating savings. Because Columbus is solidly inside Zone 5 (not on a boundary), local builders spec consistently to Zone 5 targets, and the deeper attic requirement is the main number that distinguishes a Columbus foam job from one farther south.