I want to insulate under my house, I have a crawl space with dirt on the floor and my floors are wood. What can I use to keep out the cold air from coming up through my wood floors during the winter months?

Use batt insulation between the floor joists.

Lay a visqueen plastic vapor barrier on the dirt floor and then the batt insulation between the joists like amricca suggests. Be sure the vapor barrier on the insulation is up againist the underside of the floor. Your crawl space should also be vented.

Excellent answer from ballengerb1, all I can add is to use r 30 for floors.