If vents are not present in your crawl space insulate the walls instead of the joist bays.
Insulation crawl space floor joists.
Floor insulation for moderate climes popular reads.
Department of energy recommends r 11 3 insulation under floors in warm climates and an r 25 6 to 8 in cold climates.
If the crawl space is ventilated which is ideal because it aids in the elimination of moisture then fiberglass insulation can be easily installed under the subfloor between the floor joists.
It is important to secure insulation and cover it with a vapor barrier in order to prevent moisture and the mold that often follows close behind.
Cover the walls with insulation from the sill plate to the plastic covered dirt floor.
In moderate or dry climates without the threat of sustained subfreezing temperatures insulation between floor joists makes sense.
Fiberglass insulation batts or rolls are the most economical and easiest diy choice for insulating between the floor joist in a crawl space.
Joist bays the open spaces between floor joists are prime hosts for insulation under specific circumstances.
Where winter temperatures are extreme opt for insulating the walls and sealing off the crawl space entirely.