I've never found a pair of cool boots. I've worn all leather and the mesh types as well, can't tell the difference myself. In the summer my feet, in my full leather boots. do get hot but I've never experienced it as an oppressive heat, I just accept it as my norm. I'm lucky I think because I do not suffer from sweaty feet, after my first real year of hiking and suffering from so many blisters, I now am fastidious about my feet hygiene. I do however, in the hotter months, when I stop to rest, I always take my socks off and expose them and my feet to the air. I wiggle my toes, stretch, and splash some water on them if I can, dry them and then just walk on. I personally find that the key to comfortable feet is to also change my socks every day and during the day at stops, first I wear them the right way around and at the next stop turn them inside out and wear them again. Also, when actually hiking I clip my socks to the back of my rucksack to first dry (I wash a pair every day) and then leave them to continue to take up the air. I do not tend to keep them packed away in the stale air of my rucksack.