I use to store several patched tubes (700 x 23 butyl) in my garage high on a shelf. Then I discovered that they were developing big holes that looked like chemical deterioration. Nothing had come in contact with the tubes. Since then, I've had the exact type of tubes stored inside the house for many years with no problems. With 3 cars and extra gasoline stored in the garage I've suspected that higher ozone levels is the problem. Also our city is known for high ozone during the summer months.