Crawl Space Insulation
There are three major environment situations that call for insulation in a crawl space. The first is your standard dirt floor (may have some plastic laid on the floor) crawl with open vents which is referred to as an open crawl space.
This environment is similar in temperature to the outside air in the winter which causes the homes floors to be cold and rooms farthest from the furnace to be drafty. Frozen water lines are common in this situation as is an odor in the warmer months. Most often these crawl spaces will have a lot of bugs and maybe even a rodent problem. This is the most common environment for a crawl space. Here you will find R-19 insulation (usually) in the floor joist cavities. This is required by code in most areas because of the open foundation vents and the problems they cause with energy loss.
When I say open vents, I am referring to any permanent foundation vent whether it mechanically opens and closes or not. On a side note, closing a mechanical vent does very little to keep the cold air out. It would be similar to a window in your home with metal louvers instead of double pane glass.
Crawl Space Insulation Process
The crawl space insulation is meant to keep the warm conditioned air in your home from being absorbed by the cold winter air in the crawl space. This however causes the problem with frozen water pipes because there is no heat source to protect them. The other problem with this design is most homes have their heat ducts running through the crawl space which are cooled by the winter air before they reach their destination. This causes the furnace to run more often and in longer cycles to keep the home at the desired temperature.
Some construction professionals will recommend stuffing some R-30 in the open vents for the winter to keep the winter at bay. This will work and is a very low cost solution to the threat of frozen water lines. However you may be causing a new problem by doing this. Yes, it will be warmer in the crawl space but that also means your moisture levels will raise inside the crawl space and home during the cold months. This will continue any moisture problems you may already have with the open crawl space design. You will also have to be quick to take the insulation out of the vents in the early spring to avoid worsening the moisture and odor problem.
In this environment anything you do to correct one problem will most likely cause a different problem to occur.