I need to install 2 new vents for my bath fans that currently run through the attic and are stapled beneath a vent in the attic. Is it best to run then along the joist, through the siding or run them straight up to the roof. I do live in Chicago.
Wall or gable end.
The less holes in the roof the better.
The wall isnt really an option since that would be the the soffit. That leaves me with 32' across the attic joists to the gable ot 8' up through the roof.
In my experience venting bathroom fans at soffits is an invitation to problems, for example unless everything is very carefully installed and well-connected there is a good chance of eventual separation of the duct from the exhaust termination at the soffits or damage to the duct where it bends down to meet the termination .
Once that happens, you are pumping warm, moist air into the soffit, with a high likelihood of damage to at the eaves and roof above and also the possibility of extensive mold growth:
and unfortunately (especially if it's low pitched roof) it's often very difficult to access the eaves to perform the careful work needed to create a durable installation.
Also, unless you are using solid metal pipe properly connected at the joints and properly supported to maintain a constant appropriate pitch, long runs of duct are prone to dips where condensed moisture collects as liquid water and eventually drips down to the ceiling below, especially if a long length of flexible duct is passing through an unconditioned space such as an attic where the low temperatures occur during the winter.
For these reasons at my own rehab properties where bathroom exhaust must pass through an attic below a shingled roof with a pitch greater than 4/12 I prefer insulated hard exhaust pipe routed to a gable or a roof jack - if the roof jack is properly installed (and properly re-installed at each re-roof) it is unlikely to leak, and a vertical pipe to the underside of the roof is him less prone to mechanical damage and less prone to create condensation problems both because it is more appropriately pitched and because it is often a shorter run and easier to insulate.
Finally, though I know you won't do it this way, flexible duct seems to bring out the absolute worst in hack installers, for example what I call "toothpaste tube" bathroom fan duct installations, here's two examples I have observed at home inspections this year here in Chicago, and I've seen him many, many others:
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