Summer of 2011, here in Texas we had over 2 months of 100+ degree days. I watered down the dirt floor of the coop every evening so the girls could dig in the damp earth the next day. I have hardware cloth one foot high on all 4 sides at the bottom of the coop. I also have lots of hardware cloth panels for maximum air flow. When building my coop, I placed plywood on the walls where the sun would glare in on the girls. I also gave them frozen treats, watermelon rind, cantalope, and blocks of ice. None died. They laid fairly well, considering the heat.
This year, in addition to all the above, I laid a panel of concrete wire tilted toward the coop on the west side, where the afternoon sun shines, and planted green beans at the bottom. The vines gave more shade to the girls, but we got no beans. I guess all the nitrogen from the poo they have scratched out all around the outside of the coop. I had the same beans in the garden and got tons of them.