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Isn't that the million dollar question?

Infiltration is always a tough cookie, it's really more of an art than a science unless you can actually measure stuff (eg: blower door for enveloppe infiltration in a closed state), and even measuring can be challenging especially for larger spaces, and is usually something you do at one point in time rather than on the full range of operating conditions...

What follows below is just my quick(ish) two cents, so take it with a grain of salt.

In the case of your auditorium, you have to consider two different infiltration mechanisms:

• Regular envelope infiltration: no wall, even without exterior windows or doors, is perfect, it's always permeable. You can find some default infiltration values in the ASHRAE Handbook of Fundamentals, which is generally the source for the infiltration values you'll find in the OpenStudio Library Space Types, eg for 90.1-2013 Secondary School Auditorium, that'd be 0.0446 CFM/ft², though that probably includes some windows/doors factored in. If you don't know better, you can always use this type of default value. Otherwise you can estimate and enter an equivalent Air Change per Hour (ACH) number. Note that there is also interaction between infiltration mechanisms and HVAC operation (eg: if your space is positively pressurized by your HVAC system you should expect (much) less infiltration as a result).

• Doors opened before and after the event.

  • When both the interior and exterior doors are opened
    • You could just estimate a higher ACH (eg: 2.0 ACH) for both the main auditorium and the vestibule.
    • Or if you want to be more accurate, you can perhaps consider that the flow is caused by Wind Only. ASHRAE Handbook of Fundamentals 2013 (or 2009, or 2013-4*n more generally) has a chapter 16 "Ventilation and Infiltration", in which you'll find equation (37) to calculate flow through an opening as a function of wind speed and with an "effectiveness of openings" coefficient you can calculate: the weather file (EPW) you use has both wind speed and wind direction. (There is also equation (38) for stack effect related infiltration, but I don't think it applies much to your vestibule...). Here's equation (37) in IP units:

$$Q = 88.0 \cdot C_v \cdot A \cdot U$$

where;

• $Q$ = airflow rate, cfm
• $C_v$ = effectiveness of openings ($C_v$ is assumed to be 0.5 to 0.6 for perpendicular winds and 0.25 to 0.35 for diagonal winds)
• $A$ = free area of inlet openings, ft²
• $U$ = wind speed, mph
• $88.0$ = unit conversion factor

(Note that the chapter has lots of other interesting info, such as infiltration from automatic doors, through an air curtain, etc, as well as examples.)

• When only the exterior door are opened: I think it's probably fine to just use the regular envelope infiltration above for the main auditorium, but for your vestibule you should jack up the infiltration rate at this point by calculating it from equation (37) or estimating it as a higher ACH.