Climate zone target. DOE sets R-49 as the recommended ceiling R-value for Climate Zone 2. R-38 is the accepted floor when adding to existing insulation. Starting from zero, R-60 is achievable and worth considering on a long-build horizon.
Foam type determines the thickness you need. Closed-cell delivers R-6 to R-7 per inch. Open-cell runs R-3.5 to R-3.7 per inch. Hitting R-49 with closed-cell takes roughly 7 to 8 inches. Open-cell needs 13 to 14 inches for the same result.
Roof deck vs. attic floor is a different calculation. Spraying the underside of the roof deck (an unvented attic assembly) carries a separate compliance path in Louisiana's energy code. That path can allow R-20 at the deck in Climate Zone 2, because the assembly changes how heat moves through the structure. A qualified installer will know which path applies to your job.
What you already have counts toward the total. If your attic floor already has R-11 batt or blown insulation, you only need to add R-25 to R-38 to reach R-49. A site visit confirms what is there and what is missing before any quote is written.
Moisture control and R-value work together in SWLA. Southwest Louisiana averages over 90 days per year above 90 degrees with high humidity. Closed-cell foam on the roof deck adds a Class II vapor retarder, which limits moisture intrusion and reduces mold risk. That function sits alongside R-value, not instead of it.
New construction and retrofit requirements differ. Louisiana follows IECC 2021 for new builds, which mandates R-49 for Climate Zone 2 ceiling assemblies. Retrofit jobs carry more flexibility, but most SWLA contractors recommend meeting the new-construction floor on any significant job.