In the summer, I just leave the dirt. They scratch, and dustbathe in it just fine, and you can wet an area for them to cool down in. Wet hay gets moldy, so I don't use it in the summer. This year I did add some sand, because the coop needed more drainage. In the winter, I pile some hay in fairly large section of the coop for them to snuggle down in when it gets rainy, windy, and cold. I rake it out, and put fresh in every couple weeks, or sooner if needed. I also put up tarps on the sides, but leave a gap at the top because of moisture and frostbite, when the temps drop into the 40s. While chickens can withstand very cold temperatures, we don't have much of a fall, then steady cold like up north. We get cold snaps which usually involve rain and/or wind, so the chickens don't really have time to acclimate, and about the time they might begin to acclimate to the cold, it warms up. This is what I do, and works with my coop design. Try some of these things if you like, and modify to suit your situation.
