Insulation in Florida.

Both, in that order, and they do different jobs. A radiant barrier reflects radiant heat from the hot roof deck back upward, before it can soak into your insulation, most effective during peak afternoon hours. Attic insulation resists conductive heat flow over time, including the heat that already made it past the radiant barrier. In FL's climate, the combination outperforms either one alone. The Score will tell you which you have, what's missing, and the order of operations.

Because the attic is where Florida's cooling load really lives. Sub-tropical sun heats the roof deck to 160°F+ on a July afternoon, and that heat radiates into the attic, then conducts down through the ceiling drywall into your living space. AC contractors call this "thermal load through the lid", and in many FL homes it's the single largest source of cooling load. Code-level attic insulation cuts it dramatically; a radiant barrier cuts it further.

Possibly, and that's a feature, not a bug. Most FL homes have AC systems that were sized for the old leaky envelope. Once the envelope tightens, the existing AC has more capacity than it needs, which means it cycles less often and removes humidity less effectively (short-cycling). When that AC eventually reaches end-of-life, it should be replaced with a smaller, properly-sized variable-speed unit, which costs less, runs more efficiently, and dehumidifies better. Insulate first, downsize the AC at replacement.

The insulation itself doesn't, but the air-sealing work that goes with it does. Most "insulation" projects in FL are really insulation plus air-sealing, and air-sealing the soffit, attic perimeter, and rim plate detail reduces wind-driven rain and air infiltration during a storm. It also helps maintain interior conditions if power goes out: a tight, well-insulated house holds AC temperature 6-12 hours longer than a leaky one. Worth knowing during peak storm season.

The Florida Building Code requires R-30 in zone 1A (south FL) and R-38 in zone 2A (central/north FL) for new construction. Existing homes often have R-19 or less. The diminishing-returns sweet spot in FL is around R-38 in the attic, beyond that, the marginal savings get small. Most cost-effective FL projects bring an existing R-19 attic up to R-38 and add a radiant barrier; the combination cuts cooling load far more than just adding more insulation.

Only when it's the wrong material in the wrong place. Fiberglass and cellulose work fine in vented attics, which is the standard FL roof assembly. The trap to avoid is sealing a previously-vented attic without converting it to a properly-designed unvented assembly with closed-cell spray foam at the roof deck. That's a more involved (and expensive) project that some FL homes benefit from, but it requires a contractor who understands the building science. Your installer will know which approach fits your house.