Cedar is a respiratory irritant which should not be used as animal bedding.
It's recommended to have 1 square foot of permanent ventilation per bird. Windows closed in the winter and the pop door opened at night are supplemental ventilation that doesn't count.
You want to arrange this permanent ventilation to provide ample, but draft-free air-flow. It's best to put it high over the birds' head. Many people accomplish this with open soffits (protected by wire), and either a ridge vent or vents at the top of the gable. I personally love the
monitor roof I have in my small coop.