Jack that's entirely up to you. Around here you can't double like that (potential for inadequate ventilation leading to mold issues) but I have heard of others doing that. I like the idea of it being insulated personally. If it's going to be exposed long term maybe you can look into a rigid foam insulation. It's like the blue or Pink foam you see at HD or Lowes but it can be had in much thicker sheets than that and for less money than just stacking a buch of sheets together. Here in the mountains we are required to have foam sheathing even under the slab so we have to be "creative" at times like this.
Allen
