Insulating your garage door is truly subjective in Los Angeles. In Los Angeles, people insulate for the summer heat, not winter cold. If your garage door faces south or west, perhaps insulation will stop that hot summer heat. North or easterly facing garages don’t have to ward off too much direct sunlight, so insulation in these circumstances is for other reasons (noise and appearance).
One thing insulation does NOT do, but is touted by others as doing, is make the door stronger. The insulation is foam, how can foam make steel stronger?! It absolutely does not. When you have a polyurethane foam with a steel back, it’s the steel back that makes the whole door stronger, not the foam.
California building code is such that if you have a water heater in your garage, chances are very good that you have 4 vents in the side walls of the garage that are designed to vent the gas from the heater. These vents mitigate a lot of the insulation value (R value) as air from the outside mixes freely with air from the inside. In this case, if insulation is important to you, don’t buy the most expensive, highest R value, get a mid-range polystyrene insulated door.
WHY do we recommend insulation? When the sun is hot and your garage is insulated. Then, insulating the door as well as you can makes sense. Also, when you don’t like the look of a hollow panel door. They look very metallic and softening the inside with some insulation makes sense: it looks good from the inside and stops the vibration of the door closing and opening. It makes the door quieter and it looks great. Another great reason to get insulation is the sound reduction. The noisy door is quieted by the insulation in it. Either polystyrene or polyurethane have the same effect.
Consider that when the sun is hot and the roof of the garage is visible and uninsulated, insulation won’t make a difference in the internal temperature of the garage. Every garage door company wants to sell you insulation that you don’t need. With us, it’s a practical matter. When the door is installed with a water heater, the lightest insulation is best. When the garage is fully insulated (including the roof), higher insulation is valuable. Insulation can also be used to protect the items in the garage from extreme heat. Insulation in California is not mandatory but it can be very, very nice.