I haven't lost any to heat, nor did I last year, when we were in a drought. I have metal coops, which have to be bad in this climate. I run a fan in each, 24/7, in hot weather. I change their water mid day, and hose down the coops. The coops are large and quite open air, and usually catch breezes well; it is no warmer in there than outdoors in the shade. Those in the new coop stay in the coop in the middle of the day for the shade; they are not free range but have a big yard with greenery growing. The few in my old coop are free range and go to the woods midday, or go under a shed next to the coop. I really think what saves them is that we get a good breeze here. We aren't far from a river, and seem to be in some sort of wind tunnel that it creates.
I'd set up a swamp cooler if our humidity weren't about as high as the temps.