Garage doors are not always as well insulated as they should be. This fact has consequences on the temperature in the garage. If it's a stand alone garage, there's no problem other than you freezing when you want to get your car out of the garage. If the garage is a common part of your house and it's communicating with the other rooms through a door, then you're in trouble. Each time you open that door, you lose some of the heat you pay so much for.

This may not be like that from the very beginning. However, the normal wear due to time passing, rains, winds and countless moves of the door may weaken its insulation.

It's nice to buy new things, but sometimes this may be a unnecessary expense. In case of garage doors, if your old one is still solid and in good shape, there's no point to spend so much money to replace it. You can buy one of those garage door insulation kits that are available in home and garden stores and you'll see the results immediately, with minimum spending. With the saved money you'll be able to buy some garage floor paint or some metal shelves for keeping your tools in order.

I'm not a very technical person, but I understood that there is a measure of the insulating degree of various materials: this is called the R value. The bigger it is, the better. Maybe the insulating materials manufacturers grant a certain r-value in case you use their products. Anyway, you could ask your shop assistant about that.