Look for the foam insulation building panels, they come in 4x8 sheets. these can be glued to the door panels, but you must consider clearances as the door moves up and down. Also, you will be changing the weight of the door, so your counter springs will need to be adjusted. If your springs are perpendicular to the door on pulleys with cables, you can do this yourself with the door in the open position. If the spring is a torsion spring above the door on the door header, DON'T TOUCH IT! Have a professional garage door installer come out an tune your door for you.