After the four inch gravel layer, sealed poly sheeting and rigid EPS foam insulation we finish off our crawlspace with a three inch thick layer of concrete. Concrete not only protects the foam but also protects the the house from termites and other pests.
The concrete topping slab has a slight grade so that it will drain from front to back where there is a floor drain. This is great for floods and the occasional hose out.
After four layers of protective coatings and barriers, our house will be very well insulated and isolated from the detrimental effects of moisture, insects, rodents and soil gas for a very long time.