I live in a relatively cold area (Columbus, Ohio) and have R45 insulation in my attic. I installed an in-line bathroom vent fan (Fantech PB100) using 4" insulated flex-duct per Fantech's instructions. The insulated flex duct runs straight up through the attic insulation and then passes through the attic cavity to a roof cap. (The total run of the flex duct is approx. 10' with 8' of it being in the non-insulated attic cavity.) The fan runs on a timer with up to 60 minutes of run time. When it's cold out (20F), the warm, moist air pumped through the cold flex duct above the attic insulation creates condensation inside the flex duct. This condensation runs back down through the flex duct and drips onto the person showering.
I've talked with Fantech tech support, but they've been unable to provide a solution. The well insulated attic causes the flex duct above the insulation to become unusually cold (esp. during the night) resulting in condensation when warm, moist air from the shower passes through it.