"I seem to recall, either in Fine Homebuilding or a CMHC publication
that
under certain circumstances in cold climates venting up through the
roof
can lead to thermosyphoning hot air continuously when the fan is off.
"

Veeery interesting! I had never thought about that. I suspected a
similar and worse effect occurs when people run a bath vent to the
underside of a roof mounted fan. When the roof fan runs in summer, it
will suck cool airconditioned air from the bathroom, even if the
bathroom fan is off. I would think as you pointed out, that just
having a straight up run from the bathroom to a regular roof vent would
provide and ideal path for convection.