No such thing as "too much ventilation" in a chicken coop. Though there is certainly "too much draft". High heat environments favor open air coops - imagine replacing the entire carage door with chain link and hardware cloth, so the entire "door" is open space, and you still have a less well ventilated coop than many make in your conditions. and unless that opening faces right into your prevailing winter wind, that's how I would go about converting an unused garage.no just old garage nothing in it has plenty of ventilation probably to much
Good luck with your build.