Treated wood or Not?

I'm building the floor system for a screened in porch. Rectangular in shape. CMU support walls all around. I'd like to use a wood floor system with treated 3/4" plywood, then Durock, and finish it with a tile of some type.. The ledger boards are treated, of course, but I'm wondering if the 2x joists need to be as well. I mean, this is really no different than any other room. or is it? The joists here shouldn't be any more susceptible to attack than the rest of the house.
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In a previous post snipped-for-privacy@yahoo.com wrote...

If the porch is not subject to moisture then PT joists are probably not required. However, if it is a small area you might want to consider using PT material just to be safe. The additional cost will be nominal if the area is small.
--
Bob Morrison, PE, SE
R L Morrison Engineering Co
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Also, how close are they to the dirt? If you don't have the code required 18" or whatever it is, then I would play it safe and use treated. Like Bob says, the cost of treated is minor--why not play it safe?
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