Colbyt and Jack Hammer did not answer your question. While it is a
code violation and likely dangerous to do so, it would likely work
provided that the exhaust fan is running 24/7/365.25. ;-) Don't do it.
Stupid idea for all the reasons mentioned, plus when it comes time to
sell, OP will have to have it done over the correct way if the new buyer
has an inspection done. Methinks OP also has not considered what a pain
old cast iron is to work with in an attic. Putting another vent through
the roof however many feet away they have to be, is a lot less work. If
OP has fancy shingles or tile or something, that they are not sure how
to make a clean hole in, they should look at soffit venting.
The exhaust fan in a bathroom is used to vent the moisture from a shower
also, not just the smell, so you'd be throwing vapors into an enclosed space
where it could turn into mold and mildew creating other issues. Stick with
Phish's idea of direct-to-outside venting.
The exhaust fans in my house vent an inch or two below the ridge vent
at the peak of the roof. I suppose it could have even been even with
the bottom of the vent if they had tried.
The propulsion of the air by the fan would cause it to continue up
into the ridge vent and out the holes. And the heat of the air vented
from the bathroom would cause it to rise into the ridge vent and out
And I guess when there is any breeze, or a hot air differential, the
exhaust air would get carried along by the general air current from
the soffits to the ridge rail.
It doesn't matter for me, because I only use the exhaust fan after I
clean the bathtub, and there is no extra humidity, but the other
hundred houses are probably built the same. But I spend very little
time talking with neighbors about their houses so if this has caused a
problem, I woudn't know.
(Every ten years I need to remove a layer of milkweed from tthe bottom
of the soffett vents.)
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