I have a second floor bathroom that has a ventilation fan which exits to the roof. Whenever I take a shower there is an incredible amount of freezing air (midwest winter) that is coming in through the bathroom fan (not running because the bathroom door is open). How can I prevent this? The simple solution would seem like just turning the fan on but there is still a really cold draft that whirls about the shower area (enclosed shower stall with open top).
Your fan is equipped with a backflow prevention flap. Check that it's installed correctly and not gunked up with so much lint that it doesn't function.
It's usually a simple task to remove the cover, unplug the fan and light, and drop the motor panel. This usually doesn't require work on the wiring connections. From there you have open access to the flap.