Insulation, Radiant and Vapor Barriers - floor insulation

I had to install a post and beam floor in my 22'x30' addition. The beams at 4' centers. What is the best way for me to install insulation since I don't have floor joists to staple to?

Any help would be great..

I would use fiberglass rigid board insulation that comes in 4' x 8' sheets one inch thick. Then tack 1" x 2" wood slats perpendicular to the beams every 4 feet.

Then cut the rigid board to the width between the beams. The rule here, measure twice, cut once, clearly applies here. Slide the insulation between the beams over the slats. Continue to do so until the entire cavity between the beams is covered with insulation, then tape the seams. Then start another layer of insulation by sliding the insulation under the insulation you installed until the cavity is covered again and tape the seams. It is good to stagger the seams in this application. Usually starting at one end with one layer and then starting at the other end accomplishes this. Continue adding layers until you reach your desired thickness. Once you have your desired thickness cut a 1" x 2" wood slat the distance between the beams, insert it up against the insulation pushing the insulation until it is flush against the floor and nail it perpendicular inbetween the beams sides. Insert a slat every 2 to 3 feet.

Since you are going to cut so many rigid boards, you might want to create a jig. If you are interested, I will tell you how to make one.