Residential Building Inspector B1Roof-Ceiling ConstructionMedium
An inspector is evaluating the attic ventilation system for a single-family dwelling. The attic has a total area of 1,500 square feet and is provided with both soffit and ridge vents. What is the minimum net free ventilating area required for this attic?
- A5.00 square feet
- B10.00 square feet
- C7.50 square feet
- D3.75 square feet
Show answer & explanationAnswer & explanation
Correct answer: A. 5.00 square feet
According to IRC R806.2, the minimum net free ventilating area for attics is 1/300 of the attic floor area, provided a balanced ventilation system (e.g., soffit and ridge vents) is used and certain conditions are met. Calculation: 1,500 sq ft / 300 = 5.00 sq ft.
Why the other options are wrong
- B. 10.00 sq ft is double the requirement for a balanced system.
- C. 7.50 sq ft would be for a 1/200 ratio, which is required for unbalanced systems or specific conditions.
- D. 3.75 sq ft would be for a 1/400 ratio, which is not standard for balanced systems.
Attic Ventilation Ratio
The ratio of the total net free ventilating area to the attic floor area, used to ensure adequate airflow for moisture and heat control.
- Typically 1:150 for unbalanced systems.
- Can be 1:300 for balanced systems with specific conditions.
- Half of the ventilation should be high (exhaust) and half low (intake).
Memory trick: Balance gives you double the space, unbalanced needs more air.