It depends on the fiberglass you use.Originally Posted by LittleDragon
3-4 layers of 6oz FG would be enough for most sub boxes.
Really high powered systems would suffer under a lot of pressure.
I'm guessing you're putting 2 8" or 2 10" behind the seats.
I put 2 10" subs in sealed boxes behind my seats and the amp in the front trunk, but I used MDF for most of it.