In article , Speedy Jim says...

wrote:

The 2nd floor bathroom fan vents into the attic. (WRONG, I know.)
I've been in the attic, and I see that I could either:

vent to an outside wall
or
vent out the roof

SNIP

Vent out the existing roof vent as the best choice,
all things considered. Maybe it can do double duty
and still vent the attic as well.

Venting direct to the attic space can cause mold growth
on the sheathing and rafters.

And brings in warm air where it should stay cold during the winter, leading to
problems like ice dam formation. If you live in a cold climate.

Banty