Roost-level ventilation should be closed for the winter in cold climates like yours. The chickens shouldn't have wind blowing directly on them as they roost. Temperature isn't the issue - they can tolerate very cold temperatures - it's wind that's the problem, when it can ruffle and open up their feathers, breaking their natural insulation. Snow can blow in through those open windows, too. You should have 1 square foot per bird of permanent ventilation open in all seasons, but it should be high up, above their heads. Do you have any ventilation other than the windows? And how big is your coop? Your coop is extremely cute, but sadly from what I can see in your pictures it looks too small for the number of birds you have, and insufficiently ventilated for winter. They need 4 square feet of floor space per bird in the coop, 10 in the run, as a minimum. So with 6 GLW's you'd need at least 24 square feet inside (not counting nesting boxes), or a 6x4 coop. Your run doesn't look covered either, and even if it is, it's very small and very open, so it will be buried in snow in the winter. Chickens hate snow, they'll crowd inside the coop and get into mischief when cramped and bored (pecking, bullying, etc.) Look into adding ventilation high up in the coop, and putting plastic sheets, tarps or something over the sides of your run to give them a wind/snow barrier so they can spend more time outside. If the run isn't covered on top, find a way to cover it in a way that won't weigh it down and collapse it if snow piles up on top. Or if you can't do that, prepare to be out there shoveling so the chickens can get out of the coop as soon as they can.