A dwelling unit bathroom requires minimum intermittent exhaust of at least how many CFM?

• A. 30 CFM
• B. 50 CFM
• C. 60 CFM
• D. 80 CFM

Answer: (B)

Ventilation rate is set to ensure moisture and odors are effectively removed from the bathroom. For a dwelling unit bathroom, the minimum intermittent exhaust is fifty cubic feet per minute. This level is chosen because it provides enough air exchange during use (and for a short period afterward) to keep humidity from building up and to reduce mold risk. Lower rates, like thirty CFM, generally don’t remove enough moisture in typical showers, while higher rates such as sixty or eighty CFM exceed the minimum but aren’t required unless the bathroom is unusually large or the design calls for it. Intermittent means the fan runs during use and for a short period afterward, rather than being on continuously.