Our LEED project building has a large three floor basement parking (74900 ft2) with small enclosed lobbies (with self-closing doors) connecting occupants to the office floors above. The three floors are connected with two large ramps each taking up half of each floor space.
The two bottom basement floors are under ground and the top basement floor is on ground level with large openings to the outdoors, this is where the whole basement gets it ventilation. The top basement floor has offices with a self-closing door.
We have exhaust systems placed on the two bottom basement floors which provide an exhaust rate for the whole three floor basement that will exceed the LEED requirement of 0.50 cfm/ft2.
The issue that we have is how to prove or demonstrate a negative pressure with the small lobbies and offices on the top floor, as this basement space can almost be seen as a large volume that is not totally enclosed. The reference guide states “sufficiently exhaust each space where hazardous gases or chemical may be present or used to create negative pressure with respect to adjacent spaces when the doors to the room are closed”, except this basement garage isn’t a room and technically we can’t close the doors to this space.

Would anyone know how we would deal with this situation, or how we would show the differential pressure of this space if we want to target the IEQc5 credit?