I am remodeling a shed and would like to insulate the floor. Its a 12×16 shed with metal joists that are spread 24in apart and are 6 inches in height. I could insulate from the inside but could not go more than 1/2 inch in thickness. My plan would be to go on the outside as shed sits about 8 to 12 inches above ground. To make my installation easier my plan is to use 1×4 treated lumber to raise the insulation up to the OSB flooring. The insulation will be cut 24 in wide and will have an 8 ft on the center with 2ft long pieces on the edges. That way I can reach the seam and tape it.

I will use a sheet that is 1/2in and then a 2 in sheet below it so the 1×4 keeps the foam tight to the floor. The metal joists have a lip on the bottom that will hold the 1×4 during installation and I can eventually screw the 1×4 to the joist to make a more solid installation

My question is where to use Owens Corning NGX which is polystyrene or use Rmax Polyiso with aluminum facers. I prefer the Rmax but getting it on 2 inches thick is very difficult. I only need 6 sheets or 4×8 insulation. I live in Austin, Texas so our winters are pretty mild and go around low 30’s sometime and high 30’s most of the time.

Which way should I go polystryrene or polyiso?