Insulation: If you are placing things in your car in a hot environment, there is no question about needing insulation. Extreme heat will break down just about anything you store there given enough time. Any insulation will help make the items last longer, the more insulation you use, the more time it gives you for viability of the supplies and materials store this way. But even if you are keeping a first aid kit in your car during the day, you need to insulate it. It may not suffer ill effects after just one day, but it will degrade faster without the insulation. If you are not removing the supplies at all, multiple layers will give you more time. Every layer will help, the better insulated the layer, the more it helps.

If you are using a car for long term storage, keeping the car out of direct sunlight will help tremendously. As everyone knows, the passenger compartment of a car in direct sunlight is like a greenhouse. If you can park in a covered garage or in the shade of a building it will reduce the temperature range that you must contend with. The trunk is better than the passenger compartment as it does not have windows that will allow direct solar radiation to penetrate. If you can't park in an area that is shaded all day, pay attention to cardinal directions and your location on the planet. In the northern hemisphere the North side of things will get much less sunlight, having shade for longer periods of the day.

Insulating your water also makes it much more palatable. I spent some time outdoors in the Southwest when the temperatures were 116 degrees, and I was in direct sunlight. My plastic Nalgene bottles allowed the sunlight to penetrate and my water was hot enough to make coffee or tea. My single wall stainless steel bottle stayed cool. An insulated bottle still had ice in it after walking for 3 hours. When you are thirsty in a hot environment you don't want to drink water that is hotter than the environment you are in.