Height of Bath Fan Installation? – GreenBuildingAdvisor

We have a interior bathroom that will have a long bath fan exhaust pipe (~30′) to an exterior wall through the attic.  It’s a small full bath with an 80 CFM fan.  We are planning to use 6″ rigid pipe with a total ~8″ drop along the pipe to the wall exit (straight run).  This bath is right under our storage attic trusses (and the builder helpfully glued the attic flooring to the trusses so there is no getting to the area from above), and we have only about 18″ for insulation in the ceiling, and so it will need to be dense packed into the cavity around the fan and pipe. 

If I set the fan at ceiling height, I then have to have a flexible S shaped connector between the side exhaust port and up 8″ to connect to the rigid pipe. Alternately, would it would be okay to just build an 8″ tall chase (and air-seal and waterproof it) and put the fan at the top of the chase and then just attach the rigid pipe directly to the fan outlet for the run to the exterior wall?

I know some efficiency is gained by not having any elbows or curves in the exhaust line, but I also expect it’s a bit more efficient to have the fan right at ceiling height.  Am I missing something about why building a chase would be a bad idea (I’m specifically looking for why a chase would be okay or not, vs why the S-connector would be okay since I know that’s kinda standard)?   

Thanks!