What, if anything, do you use to insulate the back and sides of your cages?

[whytepizza]

I use foam board on 3 sides of all of my glass enclosures. You can pick it up at any craft store or Wal-Mart. It works well for me. Without it my ambient temps were fluctuating a bit more than I wanted, but since putting it on all the tanks everything is peachy

So, what do you use to apply it to the glass?