My KU 3700 has double ply white on the inside and coyote brown on the outside. It's my understanding it's the same material as the pullouts, tarps, tipis, etc. just doubled up for durability. I have beat up this pack pretty good and there are still no tears. Was unsure when I first got it but it has proven to be tough. I think the double layer is the ticket but I'm no pack expert.