Most people don't pay a whole lot of attention to their crawlspaces, Why?
Well, they are dirty and are usually inhabited by a lot of spiders adn other creepy crawlies.
Having a crawlspace with a dirt floor can cause problems, mostly from humidity. There is always moisture in soil, soil is porous, it also contains bugs, which can crawl in to your home.
Another problem that you might find is odor, especially in the fall and spring when there is more moisture in the soil.
The natural air movement in your home is always upwards, due to the stack effect. Cold air pushes in from the bottom of your house through openings and cracks and forces warm air upwards.
The best way to insulate would be closed cell spray foam which acts as a vapour barrier and insulation as well as a crawlspace liner for the dirt floor.
Fiberglass batt insulation is the WRONG material to use when insulating a crawlspace. It absorbs moisture. loses R-value, attracts mold, promotes wood rot and falls out of place.
How you insulate a crawlspace depends on whether its ventilated or unventilated. A properly sealed, moisture-protected and insulated crawlspace will increase comfort, save on energy costsm improve the duribility of the home and reduce enrty of moisture and other potential irritants or pollutants into the home.