Air sealing is the largest factor. Fiberglass batts slow conductive heat but leave air pathways open. Spray foam expands to fill every gap, seam, and penetration, cutting the infiltration load that keeps your AC cycling.
Attic heat gain drops significantly. Applied to the underside of the roof deck, spray foam keeps a superheated attic from baking your ceilings and ductwork all day. That one application tends to have the single largest impact on your summer bill.
Humidity load goes down. Closed-cell foam acts as a vapor retarder. In a Gulf Coast climate, controlling moisture infiltration reduces the latent load your AC handles alongside the heat, which matters more here than in dry climates.
Attic ductwork stops sweating and losing cooling. Ducts in an unsealed, 130-degree attic bleed conditioned air before it reaches your vents. Bringing the attic inside the conditioned envelope means the air you pay to cool actually arrives cold.
Room temperatures even out. Infiltration and radiant heat create hot spots that no thermostat setting can fix. A properly sealed home stays more consistent throughout, not just cooler on average.
Performance stays consistent over time. Fiberglass settles and compresses over years. Spray foam does not shift, degrade, or absorb moisture, so the air seal and R-value you get on day one are roughly what you keep.