So, I finally got a quote on properly insulating my beach house crawlspace and would like to run the final plan past the experts here.

The sand floor will be covered with 15 mil plastic which will run up the walls about 6" then sealed on the edge with foam. The fiberglass will be removed from the underfloor above and 3" of closed cell soy based foam will be applied with 2" on the joists. All a/c ductwork will be sealed with mastic and returns and registers sealed within the foam insulation. The space will be vented with unlouvered Smartvents which will be kept closed (we are in a flood zone and vents are required by code).

I could save money by not covering the joists with foam but am worried that they may be susceptible to rot even if I run my Sanidry dehumidifier. I can't treat the space as open pier construction because half of it is below grade. I do have two large openings in the above grade walls, front and back in lieu of flood vents but am closing these. They are about 2 x 4 ft. Would leaving them open and adding a few more open vents be enough ventilation to leave the joists uncovered? Also, I was told by another contractor (different house, different problem) that using foam under the floor was a bad idea because of possible water entrapment from leaks from kitchen, bath etc. The floors above are limestone and prefinished hardwood. Is this true?

I'm getting ready to get started on this project. If anyone sees any glaring errors in our approach please let me know. Basically the space will be unvented and unconditioned. Is this a bad thing? Should we leave the vents open and not dehumidify? The last thing I want to do is spend a ton of money and cause damage to my house. Thanks so much for you help.