blueboard, and if that's the case it is R5 per 1 inch. You can get the stuff in 1" and 2" thickness, comes in 2x8 foot panels. I used the same stuff when I rebuilt my box, works great, easy to work with, you can get nice tight seams and I caulked all my seams, then used GreatStuff in any small voids that I couldn't fill with the blueboard. Cheers, John