I agree with Dodgeman about fibreglas as a good insulation for a garage door. I assume that the door is one that you already have, and are trying to insulate as a DIY project. If your door is one of the multipanel roll-up doors, you can contact a local sheet metal fabricator and buy some thin sheet metal rectangles to cover over the fibreglas insulation, and use sheet metal screws to fasten the rectangles in place. If you are concerned about moisture leaking into the insulation, you can go an extra step and envelope the fibreglas in a pocket of PE drop cloth, before putting the insulation into the panels in the door. The extra weight is not a great deal, and some manufacturers actually make such doors, with the insulation sealed between two layers of metal. I see no reason why a clever and thrifty guy can't do this as a DIY project, using an existing garager door. Good luck.