The dart in each layer makes like a scoop or cupped shape in the IX stopping it from laying completely flat. If the darts are staggered like Scott shows then the cupped shapes keep space between the layers. This should give the dead air space needed.
Now this is all in theory. I haven't made one for myself so I have no real clue.
Yosemite Sam: Are you trying to make me look a fool?
Bugs: You don't need me to make you look like a fool.
Yosemite Sam: Yer deerrrnnn right I don't!