The core of our popular garage door models, the 9700 and 9600 provide an insulation R-value of 10, more than twice as much as common polystyrene insulation board the same thickness. It's bonded to each steel section, creating a composite structure with higher strength, greater rigidity, improved dent-resistance and better thermal protection.
To understand how R-value and heat flow are related, it is easier to think of your garage not as a single entity, but as a part of a system with the external environment. As with most everything in nature, heat will want to redistribute itself in an effort to reach a state of equilibrium, or equal temperatures on both the inside and the outside of your garage. To do this, heat must pass through the garage door to reach the other part of the system.
Door quality can impact the thermal efficiency of your home. Your garage door is typically the largest opening in your home. In the summer, heat is produced outside of your garage. Thus, it is naturally hotter outside of your garage than inside and heat will try to "get in" to even out the system. If a garage door has an R-value of 3, 1/3 of the heat will get through the door and into your garage. This does not mean that the internal temperature will be 1/3 of the external temperature, though. It just means that as more heat is produced externally, 1/3 of it will enter your garage. Insulating your garage door can help keep the heat out.
The opposite is true in the winter. Heat is produced inside your garage in the winter, either by heating or by the effects of the sun. As it is cooler on the outside of the door, heat will try to escape to the outside through the door and other parts of your garage to even out the system. If keeping your garage warm in the winter is a priority, then choose a door with a higher R-value.