The effective R value for crawl spaces can be difficult to estimate.
Even if the crawl space is vented to the outside, it will run warmer than the outside ambient temperature. This will reduce the heat loss through the floor. I handle this by using an R value that is 50% more than the actual R value. For example, if the floor is insulated with R19 bats, I would use (19)(1.5) = R 28. The 50% is just a rough estimate -- it is basically the same as saying that the temperature inside the crawl space is runs somewhat closer to the outside ambient temperature than it does to the living space temperature.
Gary April 17, 2008