Nice looking coop but yes you need more ventilation than those tiny vents in the back can provide. And light as well - you have those nice windows but they can't provide light if they're covered up.
Did you actually check if the windows are letting in draft, or did you close up the windows because you feel it's cold? If there's little to no draft there's no reason to close up or cover the windows, even in winter. My top roost sits directly between two windows but I very rarely need to close them, as there's usually no issue with drafts as they don't face predominant wind directions.