I installed a bathroom vent fan in a similar situation. The duct runs through the joist space about 5' then elbows up into an attic area (its a 1 1/2 story home where the second floor has dormered ceilings, so thats why there is attic space on the second floor). Once it elbows up and over the joists it runs over a couple feet to the exterior wall. Seems to be working well. I'm not exactly what you mean by condensation, or what would cause that. I would think the CFMs from the fan would keep the moist air flowing out before it condensed on the duct.