I was going to go with two in the ceiling, but is one enough?

I have a 5/8 DD/GG ceiling.

i did one layer in mine, and I am happy w the results.

I also used Kinetic Isomax clips and hat channelling in the ceiling too....

http://www.pjbj.ca/labels/basement.html

I would be interested to see the log graph of the benefit of additional volume.

But I'm a nerd.

You can find that data on the NRC website.

http://www.nrc-cnrc.gc.ca/

I guess my tax dollars do go to something useful.

I pay taxes on both sides of the border. :(

Generally 6" is about the practical max thickness.

http://www.avsforum.com/avs-vb/showthread.php?t=1045780&highlight=insulation