I am renovating both my bathrooms due to water leaks. I have ripped everything up and all that’s left are the joists. My have is approximately 2 feet off the ground and I live in South Louisiana, so pretty wet, humid climate. My question is, because there is the possibility of moisture below my house, should I use 3/4" PT plywood as my subfloor?
While most seathing is rated for short term moisture exposure with water resistant glues, of your budget can support the use of pressure treated or marine grade plywood it would be a good idea with a damp crawlspace. Another long term solution would be to look at a combination of perimeter drainage to reduce water entering the crawlspace and a heavy mil vapor barrier on the soil in the crawlspace to reduce ground moisture migration.
You need to use a vapor barrier between the open ground and the sheathing. Similar to what you would do for placing carpet directly on concrete.