OK, back on.....
I just got this from Ethan.
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Corner bass traps are out of the way of direct reflections, so they should have the paper/foil facing toward the room. This absorbs more bass, and avoids sucking out all the mids and highs. This article compares faced and unfaced rigid fiberglass at various densities:
Density Report:
http://www.ethanwiner.com/density.htmlIn all cases the facing is toward the room.
Regardless, you do not want plywood behind corner-mounted panels.
--Ethan