According to :
"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.

As I mention, that's merely wasteful... It gets vastly worse if you
have condensation dripping into the room.

I don't know whether putting a "trap" (S curve) into the vent (as
others have mentioned) will be sufficient to break the thermosyphon.
I would suspect not in most cases, but, at least it should prevent
condensation draining _through_ the fan... But, the "trap"
might get rather yucky after a few years.

Yeah, running the bathroom vent to the underside of a roof mounted fan would
be bad even in the summer too if you had AC.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.