The attic ventilation rule
The International Residential Code (R806) asks for 1 square foot of net free vent area for every 150 square feet of attic floor. You can use 1 per 300 when 40% to 50% of the vent area is in the upper part of the attic and the rest is intake at the eaves.
Split about 50% intake, 50% exhaust
Example: a 1,500 sq ft attic needs 720 sq in of NFA at 1/300, so 360 sq in intake and 360 sq in exhaust. That is 20 ft of ridge vent and 40 ft of continuous soffit vent.
Typical net free area by vent
| Vent | Typical NFA |
|---|---|
| Ridge vent | 16 to 18 sq in per ft |
| Box / static vent | 50 to 60 sq in each |
| 12 in turbine | about 112 sq in |
| Continuous soffit strip | 9 to 10 sq in per ft |
| 16x8 soffit vent | about 56 sq in |
| Vented vinyl soffit panel | 4 to 7 sq in per sq ft |
Want a pro to confirm these numbers?
Numbers look right? A licensed roofer can measure on site and give you an exact price.
Why ventilation matters
- Hot attics cook shingles from below and void some warranties.
- In winter, warm moist air condenses on cold decking and causes mold and rot.
- Poor ventilation is a leading cause of ice dams.
Common mistakes
- More exhaust than intake. The exhaust then pulls air from the house.
- Mixing ridge vents with gable vents, which short-circuits airflow.
- Insulation blocking soffit vents. Use baffles.
Planning new soffit? Use the soffit calculator.
Sources and further reading
- 2021 International Residential Code, Chapter 8: Roof-Ceiling Construction
- U.S. Department of Energy: Energy Saver
- Asphalt Roofing Manufacturers Association (ARMA)
Frequently asked questions
How much attic ventilation do I need?
1 sq ft of net free area per 300 sq ft of attic floor with balanced intake and exhaust, or 1 per 150 otherwise.
How many feet of ridge vent do I need?
Divide the exhaust NFA by the vent's NFA per foot, usually 18. A 1,500 sq ft attic needs about 20 ft.
Can you have too much attic ventilation?
Too much exhaust without enough intake can pull conditioned air out of the house. Keep intake equal to or greater than exhaust.
Do I need soffit vents with a ridge vent?
Yes. A ridge vent only works with intake at the eaves.