Pat is the BYC queen of ventilation and cold flock mangement. It gets pretty cold where she is too. If you know you are doing everything you can, then relax. Mainly you need to watch your birds closely. Their behavior will tell you volumes. Your nose will tell you a lot too. If you smell a bunch of ammonia then chances are you need more ventilation and/or a litter change. Deep litter that is allowed to compost in the coop will actually provide warmth for your flock. That only works well though with good ventilation--probably more than you think.