Work out net free vent area, split between eaves and ridge, from the size of your attic.
How it works
Attic floor area × 144 ÷ the code ratio gives square inches, shared between intake and exhaust.
Before you order
- Do not block soffit vents with insulation. Use baffles.
- Mixing ridge vents with powered fans can short-circuit airflow.
- Check the net free area on the product, because it varies.
Common questions
Is the rule 1/150 or 1/300?
The code starts at 1/150 and allows 1/300 under specific conditions that vary by edition.
What is net free area?
The actual open area of a vent after screens and louvers.