No, the fans should not share the same terminal. This will reduce the performance of the fans, and will allow for one fan to blow some air back in to the other bathroom when both fans aren’t running at the same time. The fans also shouldn’t be venting through the soffit – you probably have soffit vents that would bring this moist air back in to the attic. If you have a gable end wall in the attic, that would be a good option. Otherwise, don’t worry about making a hole in your roof for the vent cap. It’s really not a big deal.

The Minnesota Mechanical Code requires bath fans to terminate at least three feet from doors, operable windows, and nonmechanical intake openings (Minnesota Mechanical Code section 401.5.2). As long as the wood at the exterior has a good coat of paint, you shouldn’t have to worry about it rotting.

I think that a ceiling mounted fan is preferable… but I really don’t know why. I have no good evidence to back up my preference. I’ve always placed ceiling fans just outside the shower area. I think that placing the fan inside the shower area would be best, but some manufacturers require the fan to be GFCI protected when this is done, so I’ve never bothered with that. The main goal of the fan is to get rid of moisture, so put the fan where it will be most effective at doing that.

I appreciate the information you have provided, thanks for sharing. I have a few questions that weren’t answered by your blog and am wondering if it’s something you can touch on. If not, can you recommend some resources?

Do you think exhausting the fan horizontally with a minor downward slope out an exterior wall is better than vertical through the attic and out the roof? Are there any draw backs? How far away from exterior windows should the exhaust vent be to prevent the warm, moist air from rotting the wood? What are you thoughts on a wall vs. ceiling mount fan? Is it better to place a ceiling mount fan above the shower, toilet or in the center of the bathroom?

Regards,
Janie Jurgens