How much attic ventilation do I need?
Net free ventilating area must be at least 1/150 of the vented attic area (IRC R806.2).
The IRC requires the total net free ventilating area to be no less than 1/150 of the area of the space being ventilated. 'Net free area' is the actual open area of the vent, not its overall size, since screens and louvers reduce airflow.
This baseline 1/150 ratio can be reduced to 1/300 only when specific balancing or vapor-retarder conditions are met. Ventilation openings must be protected against the entry of rain, snow, and pests.
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Need this scoped to your jurisdiction?
Adopted edition and local amendments vary by city and county. Ask Donnie checks your jurisdiction and cites the exact adopted section.
Ask Donnie →General information based on the International Residential Code (a model code). This is AI assistance to verify — not legal advice. Confirm the adopted edition and any local amendments with your local building department (AHJ) and a licensed professional before you build.