For an uninhabited attic of 800 square feet exhausting by a mechanical system, what is the minimum outdoor air rate in CFM?

• A. 8 CFM
• B. 16 CFM
• C. 32 CFM
• D. 64 CFM

Answer: (B)

Ventilation requirements for spaces with mechanical exhaust are often based on the space area when the space is unoccupied. For an uninhabited attic being ventilated by a mechanical exhaust, the minimum outdoor air rate is determined by a per-square-foot rate. In this case, the standard uses 0.02 CFM per square foot. Multiply by the attic area: 800 ft² × 0.02 CFM/ft² = 16 CFM. So, the minimum outdoor air rate is 16 CFM.

If you were thinking in terms of occupants, that would be zero since there are no people in the attic, but the area-based requirement still applies. The 0.02 CFM/ft² factor leads to the 16 CFM result.