Lisa Marie, I am quite a bit north of you, I'm in Zone 4B where winter time temps go down to minus 20, and we often have weeks at a time when temp doesn't even get up to 0*F in the heat of the day! The general rule of thumb is this: You should have ventilation to at least = 10% of your coop foot print, OR 1 s.f. of ventilation per bird, which ever number is greater. You especially need ventilation during the winter. A closed coop is a moist coop which results in high ammonia levels, frost bite risk, and respiratory illness in the flock. BUT, your ventilation must be properly placed so that your birds are not in a draft as they roost at night. They need to be on a roost that is at least 18" below ceiling, and the upper level ventilation must be high enough above their roost height so it can draw mositure up and away from the birds without causing a draft on the birds. I have floor level vent, soffit vents, gable vents, and 3 windows. Other coop openings give me a total of 65 s.f. of possible ventilation in a 120 s.f. coop. At least 1 window is open year round unless we are having a blizzard. Of course the soffit and gable vents are always open. Some folks do block some of those with cardboard. When night time temps stay above 20*F, I open all 3 windows and they stay open to varying degrees. That's also when I open the floor level vent. The worst thing you can do is close up your coop at night or in the winter. Your bird's health depends on good ventilation.