[bpape]

Before you concentrate all that material on the ceiling, I'd take a very close look at any specific issues you're having and address those appropriately.

Most height related frequency response issues are high enough in frequency that 4" with a couple inch gap is sufficient to deal with them. Decay time is a different matter. Just trying to address the most things in the room for the money. Almost always, you're better off spreading things around a bit for best efficiency rather than concentrating them in one place.

Bryan