Certain types of windshield sun-reflectors can make an already damaged windshield worse. Drivers use sun-reflectors for several reasons. The two most common reasons to use a reflector are:
1. to prevent the interior of the car from heating up when the car is parked in direct sunlight
2. To avoid extreme heat and sunlight from causing damage to the dashboard and upholstery.
The windshield is meant to form a solid barrier between you and the outside. The windshield must also have a degree of flexibility so that it is not too rigid, or it would break easily when driving over bumps in the road. All glass expands and contracts in the heat and cold. When you have a rock chip this expansion and contraction can cause the chip to crack, sometimes clear across the windshield, resulting in the need for a replacement.
Using the type of sun-reflector that has a shiny surface does keep the interior of the vehicle from getting too hot. Unfortunately, using this type of reflector also heats the windshield to a temperature sometimes well above 120 degrees, enough to cause the windshield to expand, and the rock chip to crack. The sun hits the reflector and because the reflector is behind the windshield on the inside, it is the windshield that heats up instead of the interior of the vehicle.
The best sun-reflectors are made from light absorbing materials rather than light reflecting materials. There are many on the market. Light absorbing materials are not shiny, they are opaque, usually in a medium to light gray-tone. Here is a sample of one:
Notice the dull quality to the material. These reflectors do not have to cost a lot of money or be custom fitted to be effective. I use one that cost $9.99 and it has lasted for over 5 years. It folds up neatly into a circle with a small strap to keep it together when not in use. It stores in the back seat pouch for easy access.