You can get a better effect with the phase change materials. CaCl2 hydrate has a transition temperature of 29 C = 84 F. That's good for keeping warm in the wint

You can get a better effect with the phase change materials. CaCl2 hydrate has a transition temperature of 29 C = 84 F. That's good for keeping warm in the winter, but not so great for the summer. On the other side, acetic acid has a transition temperature of 17 C = 62 F. But that will be below the dew point sometimes, so you need a clever way to distribute the cold and stay dry.