if ur car is brand new which it is from the manufact date then close cell foam shouldnt really be necessary. i started deadedning my cobalt new as well and most panel rattling came from pulling the panels multiple times. So my main suggestion to you would be to figure out exactly where ur goal is and do it all at once. run the wires sound deaden the car ect all in one sweep and ull be good to go.
replace any broken clips if u have them right away. the rediculus 5 bucks per clip or whatever bullshit the dealership charges is worth having all those clips in
lastly if ur really looking for a majorly quiet ride LLP all around wherever it fits. just remember that its hard as hell to get it to stick on a vertical surface... so use contact cement, try ur best to get it one and done stuck, and get something to hold pressure on it for a few hours to dry