XPS (extruded polystyrene) is the blue or pink board insulation
you find in Home Depot and handles moisture much better than polyisocyanurate. Beaded polystyrene - white beaded foam boards that I always think of as styrofoam - is a different animal and I don't believe it handles moisture nearly as well.
For my fridge insulation
, I used polyiso on a vertical wall where I had limited room but the rest of the insulation was done in XPS. The polyiso boards come wrapped in a foil. After cutting and fitting, I sealed off all the edges with foil tape to keep moisture out of the foam. I would especially do the bottom of the fridge in XPS if you have any chance of condensation
seeping down into the insulation or into the fridge cabinet from some other source.