If you're in Illinois, in our current weather, and have only one coop window open, I'd say it is almost guaranteed your coop is too hot
Seriously.
But, you can put a thermometer out there. Then there are sort of two different answers to your question:
1) If your coop's indoor temperature is no (or barely) hotter than the outdoor in-the-shade temperature, particularly in the evening, then your coop is adequately ventilated and there is nothing you can really DO to cool your coop down more ANYhow. If the coop is distinctly hotter than the outdoors, it is time to get out that sawzall (or whatever) and make a lot more biiiiiiiig windows/vents/openings in the coop.
Big. Little holes and heat-register-size vents dont' do much of anything.
And, 2) if your coop temperatures are near 90 F, your chickens are likely to be at least *somewhat* heat-stressed; over 95 you are looking at the potential for serious problems or death. (In desertlike low humidities, they can take somewhat more heat than that, especially breeds that are adapted well to heat, but not *much* more). If your outdoor air temp is near 90 *too*, obviously there is not a whole lot (short of installing an air conditioner
) that you can do to cool the coop per se, but you can use other things like jugs of ice, or fans to create air movement, or (in dry climates, which is probably NOT Illinois right now) swamp cooler/ misting fan type setups, to make the chickens a bit more comfortable and give you a bit more margin for error in hot temperatures.
Good luck, have fun,
Pat
