Also found this:
Mineral Wool is spun from mineral mining waste into felts, blankets and batts. Post-industrial recycled content averages 75% in keeping with the Environmental Protection Agency’s (EPA) guidelines. Its rating is commonly R19 to R26 and costs about 50˘ per sq.ft.
I don't think the price per foot is current. 2"x 24"x 48" at McMaster-Carr runs $10.67 for non-backed. Pretty rigid stuff. I'm using it in my 3x3x6 footer as well. Going to test it today..
