how low can the top vent opening be?

[OregonChickenGal]

That's up to you, if it gets cold, you may want to get one that closes. I bought a decorative floor register for mine. It looks real nice, and it closes. That way I can regulate how much ventilation the coop gets. It gets cold here in the winter.

 

[Schultz]

Yes, especially in your climate the vent you choose should close or with some of the ones that don't close you could make a panel that slides in over the vent. You could also make one that hinges but the sliding one would allow you to vary the amount of ventalation vrs. just being fully open or fully closed.