Alan,
I believe that George is right about the joist space as being "Other space used for enviromental air."
The joist space is not a duct or plenum.
The definition of Plenum in the NEC says "A compartment or chamber to which one or more air ducts are connected and that forms part of the air distribution system."
To me the cold air return is not part of the air distribution system.
JMO, Chris
