Depending on the crawl space you are referring to.


CLOSED CRAWL SPACE
A foundation without wall vents that uses air sealed walls, ground and foundation moisture control,
and mechanical drying potential to control crawl space moisture. Insulation may be located at the
floor level or at the exterior walls.







CONDITIONED CRAWL SPACE
A conditioned crawl space is a foundation without wall vents that encloses an intentionally heated
and/or cooled space. Insulation is located at the exterior walls.

WALL VENTED CRAWL SPACE
A foundation that uses foundation wall vents as a primary means to control space moisture.
Insulation is located at the floor level.

All building codes are local, and some locations in the US have no insulation requirements whatsoever.

Many US building codes are based on the ICC/IBC/IRC, which is revised & re-issued on a regular basis. The insulation requirements in the IRC vary by US climate zone. In the US many states have updated their codes along the lines of the IRC 2009, but some are moving up to IRC 2012, while others are based on the IRC 2006.

The insulation minimums (including crawlspaces) for codes based on the IRC 2009 can be found in Table N1102.1.1, p3 of this document:

http://media.iccsafe.org/geo/docs/2...3Sep09.pdf

In  IRC 2012 attic-R got bumped up a bit for most zones, crawlspace R got bumped only in colder zones:

http://publicecodes.cyberregs.com/i...sec002.htm

If you live in the US if you want to share your ZIP we can probably dig up the relevant chapter and verse code for your location, but if you insulate to IRC 2012 you'd be at code or better for 99.9% of the US. 

Take special note of the "Floor R value"  column if you don't insulate at the crawlspace walls, but in general the peformance is better if one air-seals and insulates at the crawlspace wall than under the floor.