How do you insulate exposed crawl space?
You fill the joist bays with fiberglass or mineral wool batts, then you cover the underside of the joists and batts with a few inches of rigid foam or rigid mineral wool boards, and then you protect that layer by covering it up with plywood or OSB. One layer has to be your air barrier. If you use rigid foam, make that your air barrier and caulk and tape the seams. If you use rigid mineral wool, the plywood will be your air barrier so caulk and tape the seams. I think mice would be hard pressed to chew through plywood from the bottom while crawling up piers, but if they can, protect the piers themselves with downward-facing anti-mouse cones made of metal or something.