USA First, when considering insulation, consider R Value first, then type of material. In the northern half of the country, a minimum R30 would be required at the floor. If using fiberglass with a facer, or any insulation that has a vapor barrier, place the vapor barrier up against the floor. For better results, also install a vapor barrier on the ground in the crawl space. A heavy (10 mil thick) plastic, completely sealing the area wall to wall will do fine. The building codes all refer to R Value, no matter what type insulation. However, any insulation is better than none at all. 2" thick 4' x 8' sheets of extruded (not expanded) polystyrene or, better yet, polyisocyanurate nailed to the bottom of the house with no gaps at all will help, even though the R Value is only about 10. Do not listen to anybody who does not talk in terms of R Value and vapor barriers.