That's a good question. we too have our roofed run surrounds covered with plastic. The weather has been so variable, so we just have to wait until we see temps above 45 f.
I really wanted to reply to the thread discussing the person unable to get her 70 day old chickens back into the coop at night.
Regardless of the outside temperature the formula for ventilation of the coop is 1cubic liter of air flow per pound of chicken per minute. It's not the easiest...