Under the rule of thumb for attic ventilation, one square foot of net free vent opening is allowed for every how many square feet of horizontal attic surface when there is no vapor barrier?

• A. 100 sq ft
• B. Each 150 sq ft
• C. 200 sq ft
• D. 250 sq ft

Answer: (B)

A simple rule of thumb for attic ventilation is that, without a vapor barrier, you need one square foot of net free vent opening for every 150 square feet of horizontal attic surface. This ratio ensures enough air can move through the attic to remove heat and moisture. Net free vent opening is the actual air-flow area of the vent after accounting for screens, louvers, and other obstructions. So if your attic floor area is 150 square feet, you’d aim for about 1 square foot of vent opening; 300 square feet would need about 2 square feet, and so on. If a vapor barrier were present, the guideline would typically shift to a smaller vent area (1:300). The other numeric options would reflect different ratios and don’t match the no-vapor-barrier rule.