alternative to PT plywood and ground contact insulation

[cporten]

Am pricing out a 3 season sunroom addition (with year around in the future) and have been forced to go the route of using a pressure treated joist and plywood floor system, Is there another product out there which would be structurily somewhat equivalant, as thick, and give me a finished floor look? I.E.: T&G composite decking. The floor system will be very close to the ground (sitting on Sonotubes set to below frost 4') with no venting and I will have to build the walls off of it. Also, what is available for insulating between the joist,with ground contact,insect resistant with moisture guard?

[Rich]

I believe Trex makes a T&G decking type product. It may not be Trex but search on google for plastic lumber or plastic decking.
Is there a possibility of pouring a 3" concrete slab? If so i would strongly suggest doing that. If you can't I would put down some sand and then heavy plastic and some more sand on top of that. Then just use R-11 foil faced batts for insulation.