There was no existing fan, so I installed it from the attic.
After a bit more research, it appears I have a builder's grade nutone fan, so I ordered this "upgrade" kit to add 10 CFM to the throughput and slightly decrease the loudness:
http://www.nutone.com/products/product/088cb36d-e2eb-41da-8187-f79a099bb9e1
The best part is, I get to just swap out the fan part and not worry about crawling in the attic more.
Yesterday I popped my head up there and found that the exhaust vent was just laying in my insulation with water resting in the low point (condensation?), which has definitely been causing issues. Some day I will have to get that shitty implementation replaced with an insulated exhaust pipe going all the way to the outside... for now I've pointed the existing exhaust at an attic air vent.
I made a similar discovery. Mine didn’t have any vent pipe at all. The fan was just pumping warm moisture straight into the attic, begging for some mold to grow. At any rate, it was a fun weekend project to fix:
A few key points gleaned from my research that I implemented: (I’m sure you can have a working system without following these rules, but I liked the theory behind them – not looking to start an argument)
1) Use 4” PVC (My fan doesn't have a 4" exhaust, but I wanted to future-proof my vent since most fans nowadays are 4"). Compared to flexible duct, rigid smooth wall pipe provides much better airflow (quieter) and doesn’t have low spots where condensation can collect.
2) The pipe has a slight rise from the fan, then slopes down to an exit through a gable. Any condensation flows out through the side of the house. I haven’t (yet) seen the need to insulate the pipe. After one winter, there hasn’t been a single drip come back through the fan.
3) Don’t terminate at a soffit – those are for air intake. Don’t terminate through an existing roof vent – a lot of the moist air won’t escape and it blocks normal airflow.
4) Exiting through a gable, if you have one, is a good choice in order to prevent adding another roof penetration that has the potential to leak.
Lastly, I would just note that it’s important to get the right-size fan for the size of your bathroom. Too weak is an obvious problem. But don’t just get the fan with the most CFM you can afford. Too powerful of a fan is an inefficiency - that conditioned air that is being removed from your house needs to be replaced somewhere from outside air, which will have to be heated or cooled depending on the season.
Now, I gotta change out this buried knob and tube wiring and add a ton of insulation up there.