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; we evaluate during the estimate.
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. Our Cincinnati installers identify and route around existing equipment during the on-site assessment.