In moderate or dry climates where the temperature rarely drops below freezing, install insulation between floor joists. In climates with frequent and sustained subfreezing temperatures, insulate the walls and seal off the entire crawl space from the elements.
In moderate climates, install six-inch thick, R-19 fiberglass batts between floor joists. Keep the batts unfaced and install them so they touch the underside of the subfloor. Support batts with wire webbing or wood laths placed approximately every 18 inches. Do not use tension rods, as they compress fiberglass, lessening its insulating properties. Seal electrical wiring or plumbing holes, and be sure to insulate plumbing pipes to prevent heat loss, freezing and condensation.
In both moderate and cold climates, moisture control and mold prevention are crucial to effective insulation. In moderate climates, hire an insulation contractor to determine the appropriate amount of ventilation for the crawlspace and install vents as needed.
In cold climates, a professional contractor can install rigid insulation on all walls of the crawlspace and seal all air leaks. You can do this job yourself, but it is challenging and time-consuming. In order to adequately insulate the crawlspace against freezing temperatures, remove all vents and seal any remaining holes, insulate the rim joist and foundation, and add a plastic vapor barrier to the floor. Reduce moisture by installing a dehumidifier or a sump pump.