Insulate crawl space by installing fiberglass batts between the floor joists or insulating the walls of the crawl space and completely sealing it off. Choose floor insulation in locations that experience moderate winter temperatures, and seal off the crawl space if winter temperatures regularly dip below freezing.
In moderate climates, install thick fiberglass batts between floor joists, ensuring contact with the subfloor. Place batt supports every 18 inches, and make sure they don’t compress the insulation, since doing so reduces its effectiveness. Seal wire and plumbing holes with spray foam insulation to prevent cold air leaking through the floor. Cover pipes to prevent freezing, and wrap heating ducts to minimize heat loss. As an alternative to fiberglass batts, use closed-cell spray foam insulation, but avoid open-cell as it retains too much moisture.
In locations that experience colder temperatures, it is more efficient to seal the crawl space completely. Insulate the inside of the foundation walls with a double layer of rigid foam insulation board, and cover seams with waterproof tape. Remove and seal any vents in the foundation walls, and use closed-cell foam to insulate the perimeter joists. Cover the floor of the crawl space with a vapor barrier, and install a dehumidifier or sump pump to deal with any condensation and prevent mold formation.