On a July afternoon in Houston or Laredo, the air in an unconditioned attic can be fifty or sixty degrees hotter than the room below it. Everything separating those two spaces is a few inches of insulation and a layer of drywall with holes in it — and the holes are the part most people never think about.
R-value is measured on a lab sample with no gaps and no air movement. A real attic has recessed light cans, plumbing stacks, bath fan housings, wiring penetrations, and open top plates where interior walls meet the ceiling. Air moves through all of them, and moving air carries far more heat than conduction through the insulation does.
That is why air sealing comes before adding depth. Sealing the penetrations and top plates first turns the insulation into a thermal layer that behaves something like its label. Adding a foot of blown-in fiberglass over an unsealed ceiling plane is paying for performance you will not get.
Depth matters too, and consistency matters more than the maximum. Most attics have been walked on, had HVAC service done, and had storage put down, and the insulation is compressed to nothing in exactly the places where somebody stepped. A consistent, uncompressed depth across the whole plane beats a deep pile in the middle and bare drywall at the eaves.
The two details that get skipped: baffles at the soffit vents so the attic can still breathe, and an insulated, weatherstripped attic hatch. An uninsulated pull-down stair is a hole straight into the hottest part of the house, and it takes an hour to fix.
None of this is glamorous work and none of it shows up in a photograph of your kitchen. It is also, on most Texas homes, the fastest-paying money you can spend on the building.