Romex in plenum?

[construct]

As has been said....if you have supply or return air passing through a crawl space without duct work and the crawl space is unvented, IMO that makes it a plenum. It may not be a compliant plenum, but a plenum none the less. According to the IRC, if you don't vent the crawl space, you must condition the space [supply and return air] or provide mechanical ventilation, along with insulating the crawl space walls and installing a 6 mil vapor barrier on the crawl space ground with lapped and sealed joints and edges. Although it is not necessary for the crawl space to become a plenum in order to condition the space. Just my 2 cents worth. :?

 

[RICK NAPIER]

According to my building inspector you can use a crawl space for a return but with a lot of conditions. This would make it "other space used for environmental air"