Attic ventilation calculator

Estimates required net free area from IRC R806.2, then translates it into quantities of generic vent types (or a specific brand's real products). This is a starting point to verify, not a spec — go measure.

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Only check this if you've confirmed the exception's conditions apply — see the ventilation ratio entry for what those are and how they vary by code edition. Defaults to the conservative 1/150 baseline.