OMG, you're killing me! It's still 90 degrees outside, in Oregon. I would really welcome a blast of cool air that I didn't have to pay for right now.
If you could divide up the space, and give the chickens a smaller area for the cold months, that would help. Old plywood for walls and a lowered ceiling, or even recycled pallets. You can insulate that with used feed sacks, and a tarp or plastic around the outside of your *new* walls. Make sure you have good ventilation.
Be glad that you're not close to an electrical outlet -- the urge to use heat lamps is pretty high with new chicken owners, and they are very unsafe. Talk to some experienced chicken keepers in the cold climates. They have survived many winters with only good winterizing, and no external heating sources.
No need to start this now, of course, but you can begin gathering your materials. Good luck!