[Speedslug]

I have seen 2 by 4s framing inch and a half foam sheets with half inch (or thicker ) plywood over that.

I have also seen 2 by 4s on edge framing double thicknesses of foam for more insulation.

If you use bat fiberglass under the floor joists you might want to nail quarter inch 'hardware cloth" under that to keep the critters from making a home in your nice warm insulation.

I have even seen foam sheets laid down under sheet plastic under concrete pours of 4 inches thick. The plastic prevents moisture from wicking up into the concrete.