We all know that venting of the bath fans at the soffit vents in the attic is bad. Is it code? Anyone have a code # so I can give the builder who is telling me its ok? Thanks in advance.
International Mechanical Code: 401.5.2 Exhaust openings.
Outside exhaust openings shall be located so as not to create a nuisance. Exhaust air shall not be directed onto walkways.
This might help although not Code related.
**Duct the fan to the outside and seal the line **
Retrofitting a bathroom with an exhaust fan can be a pain in the neck.
You’ll have to cut a hole in the ceiling for the fan, install a vent through the roof or a gable-end sidewall and connect the two with a duct.
Taking shortcuts, like running the duct to a soffit vent or the attic and calling it good enough, may be tempting but ultimately aren’t worth it.
Some points to keep in mind:
- Locate the fan close to the shower.
- Keep the run of duct as short and straight as possible.
- Seal the ducts so moisture can’t escape, and seal the fan housing to the ceiling with a bead of caulk.
If you don’t have any access to the ceiling from above the job will be all that much harder. You’ll be forced to tear out part of the ceiling to run the duct. Or use a through-the-wall fan. They’re not quite as efficient as a ceiling unit, but they’re a much better option than ignoring the moisture problem.
Follow the Manufactures listed Instructions on the Installation.
Code only provides you with ventilation minimum requirements where a window that opens is good enough. Sometimes Codes are not the best options because it is the bear minimum requirements in the Building Industry.
Marcel 
You have that right. Anything less would be illegal.
I don’t like the bath vent terminating in the soffit, but some people can get away with it. The actual code text is a little fuzzy:
IRC 2003 M1506
MECHANICAL VENTILATION
M1506.1 General.
Where toilet rooms and bathrooms are mechanically ventilated, the ventilation equipment shall be installed in accordance with this section.
M1506.2 Recirculation of air.
Exhaust air from bathrooms and toilet rooms shall not be recirculated within a residence or to another dwelling unit.
I personally consider a bath vent that terminates in the soffit to still be “within a residence”, but I’d be hard pressed to tell you how it might recirculate.
Clothes dryers are much more cut and dried, but bath fans; not so much. Best I got…
Pete
It’s been my experience though not in the codes most local officials don’t allow it. I’d make a quick call to the local building dept. and ask. Couldn’t hurt and it’d be good fyi.