You didn't say if your flock free ranges all day and only sleeps in the coop at night or if the chickens stay in the coop during the day some to. If they only roost in the coop I'd think it would be ok. If not, personally I would want more ventilation. You can always cut out another square hole in the coop and add hardware cloth. For winter use I'd use a 1/4 plywood to make a 'door' on hinges that will shut blocking out the cold weather.