If your HVAC is in the attic: spray the roof deck (creates a conditioned attic). If HVAC is in conditioned space: spray the attic floor. The wrong choice can create moisture problems; the on-site estimate confirms which is right for your home.
More detail
The decision rule is where the conditioned envelope ends. (1) HVAC equipment in the attic: spray the roof deck. This brings the attic inside the conditioned envelope. The HVAC equipment is no longer operating in 130°F summer heat and 30°F winter cold. Energy savings 30-45% on cooling load alone in many Cincinnati homes. Required: closed-cell foam in Cincinnati climate to maintain vapor performance. (2) HVAC equipment in conditioned space (basement or interior closet): spray the attic floor. The attic stays vented and unconditioned; the floor air-seals and insulates the boundary. Open-cell foam works fine on attic floors with proper soffit ventilation. (3) Mixed configuration (HVAC partly in attic, partly elsewhere): Cincinnati-area installer evaluates and typically recommends roof-deck foam for the cleaner result. Either way, the full scope is detailed on our attic spray foam in Cincinnati page. Wrong choice consequences: floor-foam with HVAC in attic leaves equipment exposed to extreme temps, wasting most of the savings. Roof-deck foam without ventilation strategy can trap moisture in unvented assemblies. Cincinnati pre-existing-HVAC consideration: many older Hyde Park, Norwood, and Mariemont homes have HVAC equipment in the attic from 1990s-2000s upgrades that simplified ductwork. Roof-deck foam in those homes typically delivers the highest energy savings because it brings the equipment into the conditioned envelope. Existing equipment gets identified and routed around during the on-site assessment.