R-19 floor-system-insulation is one option, shown below in yellow, to insulate a crawlspace. Fiberglass crawlspace insulation batts in the floor system offer zero air-sealing benefit, only cover about 70% of the floor system at best, gravity works against it, and the bent wires rust, vibrate, weaken over time… the whole strategy is bad. (We have done floor insulation with batts properly, and it wasn’t worth it in the end.)

The second method is perimeter-wall-insulation. Foam board insulation was the original method for moving the thermal boundary to the perimeter of the foundation, and it is superior to the above method, but provides very little air-sealing benefit, without the use of atleast two other products, orange expanding foam, and tape: (This is a home built by Stetten Realty Inc shown below, it also shows the proper way to insulate a “pony-wall”, with a continuous layer of heavy house-wrap that covers the above average thermal bridging with a thermal break. Pony-walls can as much as 40% framing / thermal-bridging)

This is all closed-cell spray-foam ^. One monolithic piece all the way around R10(or more) and separated pieces in the band-joist R15(or more).

R30 with Open Cell Foam
similar price w/
R15 with Closed Cell Foam

We are actively avoiding seams, others apply a “skirt” to the bottom of the wall, then foam board, w/ nails, shots, washers, tape, foam, sawdust… and 3 joints (leakage sites) minimum all the way around.