The cement board product referred to by Winterh is Durock (eg.
https://www.homedepot.com/p/DUROCK-N...2965/202263276). If you use that on the inside of the skirt, then you might consider cut/glue application of foam board (2-4") over the Durock for insulating, with beads of can foam at the edges and joints to seal them.
Some good reading on building an unvented crawl space can be found here:
http://www.greenbuildingadvisor.com/...ed-crawl-space. In addition to the info in that blog, there is a long "Related Articles" list of links to more info on the general subject.
When I mentioned the use of borate-treated cellulose, what I had in mind was dense-pack blowing of the cellulose into the joist cavities, using a sufficiently powerful blower, like the contractors would have. Sometimes the blowers available at the big box stores are ok for a loose blow into an attic, but not powerful enough for dense packing to upwards of 3 lb/cuft density, needed to counter settling of loose-blown cellulose, as in an attic. If you are redoing the floors inside anyway, then you have more options for insulating. If you have sufficient ceiling height to lose a couple of inches, you could lay down rigid board over the present floor, then a layer of T&G plywood, and finished flooring. But if you see this structure as being regularly occupied year round, then overall comfort might be better with an enclosed and heavily insulated crawl space, with no floor insulation at all, since the floor and crawl space would be reasonably close to interior temperature.