What is the minimum net free ventilating area required in relation to the area of the space ventilated?

The minimum net free ventilating area required for ventilated spaces is established to ensure adequate airflow and moisture control. In residential construction, particularly for attics and crawl spaces, proper ventilation helps to prevent issues such as mold growth, wood decay, and overheating.

The correct ratio of 1/150 indicates that for every square foot of attic or crawl space, there should be at least 150 square inches of net free ventilating area. This ratio takes into account that some of the ventilation area might be blocked or reduced due to obstructions, ensuring that sufficient airflow can occur to manage humidity and temperatures effectively.

Each of the incorrect ratios represents either over-ventilation (1/100, 1/50) or under-ventilation (1/200) relative to the recommended practices established in the code. Using a ratio of 1/150 strikes the right balance necessary for maintaining a healthy residential environment, making it the most commonly accepted and effective requirement.