How big is the greenhouse (in feet) and how many chickens are you talking about? The concern is overcrowding, which applies any time of the year.
You have two issues in winter. One is a cold wind hitting them with them having no way to get out of the wind. That should not be a problem in a greenhouse. The other issue is moisture. If it gets too humid in there and it gets below freezing they can get frostbite. Do not worry about how cold it will get, they are wearing a down coat. But high moisture when it is winter in Ohio can cause frostbite so can you vent that greenhouse?