Closed-cell (R-7 per inch) for crawl spaces, rim joists, below-grade, and where vapor barrier matters. Open-cell (R-3.7 per inch) for attics and interior walls where vapor permeability is OK. Both air-seal effectively.
More detail
Three properties differentiate the two foam types. (1) R-value per inch: closed-cell is R-7 vs open-cell R-3.7, so closed-cell delivers nearly twice the thermal performance per inch of cavity space. (2) Vapor permeability: closed-cell at 2+ inches thick is a vapor barrier; open-cell is vapor-permeable. In cold-and-humid climates like Cincinnati, vapor barriers belong on the warm side of the assembly (basement walls, rim joists, below-grade). (3) Air seal: both seal effectively at 1+ inches thickness. Decision rules. Below grade (basement walls, crawl spaces, rim joists): closed-cell. New conditioned-attic roof deck where vapor migration matters: closed-cell. Attic floors and interior walls where the assembly tolerates vapor permeability: open-cell. Most Cincinnati whole-home retrofits use both: closed-cell at the rim joist and crawl space, open-cell at the attic floor or interior walls. Our Cincinnati installers design the mix during the on-site estimate. Cincinnati hybrid-assembly context: most whole-home Cincinnati foam packages use both products in a single project. Closed-cell at the rim joist (vapor barrier matters), closed-cell on the basement walls and crawl space (below grade), open-cell on the attic floor or interior walls (vapor permeability OK). Our Cincinnati installers spec the mix during the on-site assessment and document each surface separately on the closeout package.