Radiant barriers reduce summer cooling load by reflecting infrared radiation from the roof deck; they do nothing for heating-season air leakage. Adding spray foam to the attic floor or roof deck addresses the air-leak and conduction losses that radiant barriers cannot. The two technologies are complementary, not redundant.
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Radiant barriers (typically perforated aluminized foil stapled to the underside of the roof deck or laid over the attic floor) work by reflecting infrared radiation. They reduce summer attic temperatures by 10-20 degrees and cut cooling load by 5-10 percent in southern climates. In Cincinnati Climate Zone 4A, the cooling-load benefit is modest (heating dominates the annual energy budget); the heating-load benefit is essentially zero because radiant barriers do nothing for conduction or air leakage. Spray foam addresses the dominant Cincinnati energy losses (conduction through under-insulated assemblies, air leakage during winter stack-effect, latent infiltration during summer humid air migration) directly. The two are complementary because radiant barrier reflectivity continues to provide its modest cooling benefit even when foam is added at the same assembly. Practical installation considerations. (1) Existing roof-deck-mounted radiant barrier with intended new roof-deck closed-cell foam: spray the foam directly over the radiant barrier. The foil cannot be reused or removed without disturbing the barrier substrate. Foam adhesion to the foil is adequate; some installers apply a thin tack-coat first to verify bond. (2) Existing roof-deck radiant barrier with new attic-floor open-cell foam: the radiant barrier stays in place and continues to provide its summer-cooling benefit; the attic-floor foam addresses the conduction and air-leakage losses. (3) New radiant barrier and new foam: install the radiant barrier first, foam over. Cincinnati cost-benefit summary: if a homeowner already has a radiant barrier and is considering foam, the foam is the higher-leverage upgrade by far. If a homeowner is considering both, foam first, radiant barrier only if budget allows after the foam scope is complete. The marginal benefit of adding radiant barrier to a well-foamed attic is small (1-3 percent additional cooling savings) versus the typical 25-40 percent total utility savings from the foam package.