If a roof has 900 square feet of horizontal attic surface and no vapor barrier, how many net free vent square feet are required?

• A. 4
• B. 5
• C. 6
• D. 7

Answer: (C)

Attic ventilation is based on a simple ratio: when there is no vapor barrier, you need net free vent area equal to 1/150 of the attic floor area. The horizontal attic surface here is 900 square feet, so 900 ÷ 150 equals 6. That means you must provide six square feet of net free vent area.

If there were a vapor barrier, the requirement would be 1/300 of the attic area, which would be 900 ÷ 300 = 3 square feet—but with no vapor barrier, the correct amount is six square feet.