Ok thanks!!
But won't the heat just go out the windows?
Yes, but it will take the moisture and ammonia that the the birds' respiration and poop create with it. You want some air exchange and the movement of warm air up and out will give it to you. A well-ventilated coop that is dry and draft-free (different from ventilation) will allow the birds to keep themselves warm.
If you don't mind my asking, why did your first coop burn down?

