I store my bikes in a temperature and humidity controlled environment at 55 F. and 67.7% humidity held constant so as to duplicate as closely as possible the conditions in the Gascony caves where are housed the finest preserved thirty thousand year old cave paintings of the first known bi-modal wheeled transport still extant in the Western Hemisphere. On occasion I have to move them to the garage (attached, un-airconditioned) to make room for ebay sourced case lots of vintage Tangerino and Wild Irish Rose. The tires, after three years and several thousand miles, show no signs of degradation despite the well known hot Atlanta summers and the buildup of engine heat prompted by parking two hot cars in the same ticky tacky garage.

I have had tires whose sidewalls oxidized and cracked. But, as sheldonbrown.com points out on the subject, that bike was stored for four years in a closed room adjacent to an ozone emitting gas furnace.