Also, instead of the ceiling tile, which they normally will only sell in a package, not one at a time, you might consider the pink insulating foam that builders use to sheet the outside of houses. I use the 1/2" thick size and a 4x8' sheet is only like $8-$10.

http://www.lowes.com/lowes/lkn?actio...348&lpage=none

or

http://www.lowes.com/lowes/lkn?actio...503&lpage=none