Yes, you want to vent a bathroom fan to the outside. You don't want the moisture in the attic. All of ours are vented through the roof.

Contractor wants to run the new exhaust fan ducting up through attic to the existing roof ridge vent. Is this a valid exit location? Allowed by code? I can't seem to find anything suggesting this as an option. How would the ducting be attached to the vent?

I don’t think the contractor is saying he’s going to use the ridge vent as the terminus for the ductwork, rather he’s planning to connect the ductwork to the existing (old fan’s) roof vent, which is located at the roof ridge. Does that make sense? I mean, I can’t imagine in this day and age when we know better that your contractor would just run the ducting up to the ridge vent and call it good. If that is indeed the case, time to find a new contractor!

Will moist air even move (straight) up and out?

Yes.

Edited to add a link to this article: Venting a bathroom exhaust fan

Ridge vent is for venting the attic.

It should be vented in a way that NO, ZERO, air can move from the fan exhaust duct into the attic. Connecting a duct to the top of the inside of the roof where the ridge vent is, if that's what he's suggesting, is a poor idea and likely not to code.

He needs to poke a hole through the roof and put a vent stack/metal cap on it. The type of roof material may dictate what's possible. Through the roof and into the sunshine is what's needed. If you're in a snow area, the caps can be tall enough to not be covered. Look at this.

Do you mean a ridge vent or a gable vent, located at the top of the gable near the ridge?

Thank you for responding so quickly. The contractor specifically said ridge vent. The link and the video are quite helpful and will help me ensure this gets completed the appropriate way. Makes me want to hire an HVAC specialist! I don't know how or even if the current fan is vented, but I do know it's not to the ridge vent, since this roof is fairly new and it did not have a ridge vent before. The gable ends seem too far away to be logical, plus I see no cap. How farcan the duct be run to the terminus/exit? This bath is located on the front center of the second floor between two dormers, so a vent cap in the roof would be unsightly (so the duct would need to be run to the rear roof slope, or over to a gable wall, and both options are not that close by.) Thank you again for sharing your expertise. I can now speak intelligently with the contractor.

It should be vented in a way that NO, ZERO, air can move from the fan exhaust duct into the attic.

This really sums it up for me. Thank you!

Happy to help. I've had a bit of this work done myself, in two different houses. In one, I had a Fantech BFRK 100 installed. There was an existing bathroom exhaust fan that worked poorly but I didn't want to put in a new grill. With the BFRK, you take out the guts of the old fan, keep the housing in place, and then connect ducting that runs first to the Fantech unit located a few feet away up in the attic and then from it to the roof passthrough. I had that done by an HVAC contractor who was on site replacing HVAC equipment. I'm not sure they would have done the work otherwise, it was like a favor to me.

The second one was the replacement of a 20 year old exhaust fan with light in a WC closet that was noisy and ineffective. I had that done by an electrician who also was doing other work. He needed to go into the attic to remove the old unit, connect the new one to the framing, put in a new duct and connect the power. It made a huge difference. That one also vents through a roof pass-through, important because we use it to vent the bathroom of moisture from showers and baths.

Good luck and keep your BS detector handy. If other exhaust vents terminate in the attic, it's time to fix those too. Each duct run needs its own roof passthrough unless you're using a Y setup with one fan at the base of the Y for two locations. Be careful if so, those can be tricky if the duct runs from the rooms to the base of the Y aren't of equal length.

As far as duct run distances go, the unit will specify the maximum distance. For my most recent job, the ceiling unit with light that was 110 CFM, I want to say the spec for the duct run was something like 60 feet. I wrestled with putting a wall vent on a gable end but decided to stick with the existing through-roof vent.

I had an appt for an estimate on bath remodel. Licensed contractor said he'd "just vent through the soffit or to the ridge vent." I had been doing research and knew those options were wrong. How is it that I know more than a licensed contractor??

40 years ago, building code said exhaust need to just above ceiling insulation in the attic, 20 years ago, it changed to gable, ridge or soffit vent, now, required to the outside of attic. So, they all work (at least 20 years), it is up to you and how much you want to pay. Myself, don't like a pipe go through my roof, one more possibility of leaks in a few years if they did not do it right. (remember, people do the bathroom work are not roofer)

Actually, I have no problem with a soffit vent for a bathroom fan. they make vent 'caps' for this.

Had appt w another contractor who said they'd go through the roof. "Broan makes a kit for that."