I am trying to Calculate the required Pressure Drop adjustment for a lab facility, using Addenda "p" from ASHREA 90.1-2007. The credit for "Fully ducted return and/or exhaust air systems is rasied to 2.15 in. w.c. for lab facilities. Does this number already include required filtration in the exhaust, and exhaust airflow control devices, or are those separatly added on to that 2.15 number?