Greenhouse insulation

My wife and I are building a 4 season 21x36 Polytec greenhouse with 2 wall polycarbonate sheeting in East Tennessee. One of the many problem (subject of another post) encountered so far is how to insulate the base. The base consists of 6x6 treated posts with treated 2x10s between. One side of the house is on a down slope that leaves about 3 feet of exposed base. My wife wants to have "raised beds" on the low side and tables sitting on gravel on the high side. I'd planned to put styrofoam to insulate and black plastic to keep water away from the wall. I'm not sure what order to do this. I'd planned to have, from outside to inside, facia (probably metal), 2x10 walls, insulation, black plastic, gravel/soil.
Any constructive comments would be appreciated.
Thanks, Harry
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