It's better if the duct in the unconditioned attic space is insulated, it'll help the evacuated air stay warmer and help prevent held moisture from condensing on the walls of the pipe and running back down into the fan housing during cool seasons. Again, condensation may not be a problem in your locale, with cold attics condensation is more of an issue. There really aren't any code restrictions on locating a bathroom vent termination hood. Except that they can't vent into an attic or into a crawlspace. Plumbing vents, yes, there are restrictions. Pot/attic vent outlets, there are guidelines for those too. I'm in New England, with snowfall and roof penetrations in mind, personally my first choice is to vent out an attic gable wall. If you're going out the soffit, do not let the duct be a blind termination behind perforated soffit material. You can go through the soffit, there are termination caps designed for this. One caveat with soffit venting is that it can sometimes create a mold/mildew build-up on the sidewall of the house and on the underside of the soffit, and the evacuated air can be pulled back in to the attic if you have soffit-to-ridge vents. That's not as big of a deal with bathroom vents as it is with clothes dryer venting, but I thought I'd toss it out based on the stuff I've seen over the years. If you need to go out the roof, that's a common location, no worries there. Your bath vent has a certain CFM air movement rating, it's based upon a certain amount of resistance as the air moves through the exhaust duct. If you use ribbed flex duct, if you have too long a length, or too many turns in the dict, the resistance increases and your CFM decreases. So short and straight works best.

You can get a low profile fan that will fit within the space. I have a powder room on the first floor and chose the Panasonic WhisperFit Lite to go there. It fits nicely and is ducted right out the nearby side wall. The WhisperFit is designed with the motor on the inside of the fan, rather than sticking out the top. In my situation the fan was a replacement for an old, LOUD f-fan. I used rigid duct wrapped in foil-faced fiberglass insulation. The Panasonic fans (and maybe other brands) are designed to allow you to mount the body and secure it before the fan itself is inserted into the body. This gives you more room to work. The fan I chose is 60 CFM, if I remember right. In my small powder room, this is enough ventilation that dust will roll across the floor when the door is closed. I installed a timer instead of a switch, which keeps me from forgetting that it's on. If you don't need one with a light, just look for the WhisperFit series. They can be found on eBay. I would assume that you could branch the electrical circuit from the existing fan to this new one for the simplest hookup. Then both would run at the same time.

Babka: ha. they (fixtures) are vented to the space above, (which is under the stairs), not to the outside - which is against the code, I know that much. I guess they (whoever added this bath) ran into the same problem with venting fixtures as we are facing trying to figure how to vent the exhaust fan. and maybe that's why there is no exhaust fan to start with. The bathroom is under the staircase whichi s in the middle of the house, and there is space directly above it because the ceiling is short. But then the ceiling height changes as you go to any neighboring room (the rest of the house around has much higher ceilings). Basically, you can put the duct vertically through the ceiling above the bathroom, but then as you turn it 90 degrees towards in the direction of any outside wall, it would have to come out through the wall of any room around, not in the ceiling above the rooms. And if you try to go higher with the duct so that the turn is at the level of the over-the-ceiling space for these rooms, then the duct would have to come out through the staircase! Hard to explain, but I hope you go the picture. Also, even if we could somehow get the duct to the space over the ceiling of any adjacent wall, then we still can't open up that high ceiling to run the duct further because it has some fancy beams and moldings (house is over 100years old) so we cannot touch the ceiling. That's why I wrote "absolutely no way" for the ceiling route Unless, as somebody above suggested, we place the fan in the ceiling, but run the duct through the wall down to under the house (actually through the under-the-stailrcase part as the stairs go down). But as far as I understand that comment, we will have to install some inline fans in the duct to keep the air moving down...

It was probably the soffitt vent. I would never connect my fans to anything other than a roof vent where it escapes outside my house. Our GC connected all of ours to soffitt vents. That is the bare minimum to pass code (at least 14 years ago). Our new Panasonic instructions state clearly they are to be exhausted out the roof. We had our Solatube installer fix them.