I'm in Chicago, where we should have better than R 38 up in the attic.
Winters get very cold. Because of this, I don't EVER run the bathroom
fan, but instead open a window in the summer, or keep the bathroom door
open about 6" in the winter, and keep the HVAC fan on, instead.

someone I know suggested venting the bath fan into the plumbing stack.
I'm wondering if that would be a code violation or not... The stack
does vent out the roof! :-)