Adding material to a garage door increases the overall weight of the door and can throw off the balance of the springs. If you add weight to the door then you should also change the springs to compensate for the additional weight. I have found that the cost of buying these insulation kits and re balancing a garage door is not very cost effective. Adding foam to a hollow door will make it quieter but is not enough to make a big difference in weather sealing. A three layer insulated garage door with properly installed weather seal trim and a good fit to the floor is the best way to keep out the cold and hot weather in your garage.