Vestibule Pressurization

[rlafrance]

I’m fairly new to HVAC design and I have been told I need some air in my vestibules by the project engineer. I was always under the impression that the vestibules are unconditioned or possibly have a unit heater. Obviously you can’t condition the space fully because of the doors but is the goal just to pressurize the space to keep the unconditioned air out of the vestibule? Is there a standard or formula for amount of CFM to pressurize a vestibule? Is this the same for stairwells?