VAV Fume Hood Velocity in Proposed vs. Baseline Case

We have a University Science Building that will house research and teaching labs with multiple VAV fume hoods in several spaces. Our design operational procedure for the VAV fume hoods is to maintain 80 feet per minute face velocity across the hood under normal operation and a reduced face velocity of 60 feet per minute when the space is unoccupied or nothing is present near the hood. A zone presence sensor is installed on every fume hood to determine when something or someone is in the vicinity of the hood and increase the airflow to the 80 fpm setpoint. A typical fume hood would be constant volume or have a constant face velocity whether it be 80 fpm, 100 fpm, or possibly greater. The airflow requirements for the fume hoods are driving the room ventilation requirements. Since ASHRAE 90.1‐2004 Appendix G paragraph G3.1.2.5 requires that minimum outdoor air ventilation rates be the same for the proposed and baseline buildings, are we able to model the energy savings from reducing the airflow thru the fume hoods during unoccupied periods and if so, what are we to base it against? Can this be modeled thru scheduling or can it be a process load?