What you're looking at

A roof section cut through the ridge, showing the air-flow path that defines a *vented* attic. Cool outdoor air enters at the soffit vents, rises through the attic, and exits at the ridge -- carrying heat away before it conducts through the ceiling into the conditioned space below.

Why the radiant barrier matters

The reflective film on the underside of the roof deck reflects ~95% of the incoming infrared radiation back at the sky. In hot inland zones the deck itself can hit 160F+; the radiant barrier blocks most of that heat from ever reaching the insulation, which means the R-38 cavity stays cooler and the cooling load drops.

Where this assembly is NOT prescriptive

- CZ1, CZ16: no radiant barrier (cool-coastal CZ1; alpine CZ16). - CZ3, CZ5, CZ6, CZ7: ceiling R-30 instead of R-38 (mild zones don't need the extra cavity insulation). - CZ4, CZ8–15: R-19 below-deck cavity insulation replaces the radiant barrier (Title 24 2025 requires the below-deck approach in steep-slope shingled roofs in these zones).

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