It's late April and the daytime heat is starting to kick in, so I decided to go ahead with the insulation of the garage door.  I bought 3 4x8 sheets of 1" styrofoam from Home Depot.  They have a thin plastic sheet glued to each side which gives a smooth surface that won't allow the small pellets to break away over time.  I measured each of the 12 panel openings and custom cut each to size, then used spray adhesive lock them into place.  Last thing to do is to put weatherstripping around the frame of the door.