I live in Houston, TX (Very Hot, Very Humid). In the summertime, my dryer is dripping with water, when not in use. I do not have overly excessive drying times.
My dryer is vented via an unfortunate run of over 25 feet with two elbows. It exhausts through the roof. Dryer is in the center of the first floor, so the vent goes up to the second floor, runs through the ceiling/floor, comes out in an attic and through the roof. The attic is unfinished, with temps well over 120F on an average summer day in Houston.
My guess is that I have hot moist air pouring down my dryer duct, hitting my nice cool air conditioned dryer and condensating.
Initial google searches have pointed me at the Fantech DBF110 which is a booster fan that you install on your duct run. I'm not certain this is going to solve my problem, as it doesn't run constantly, only when the dryer is running.
Would I get more bang for my buck by insulating the duct and trying to seal it up? Any recommendations on materials if that's the case (thermal blanket, just wrap fiberglass, other)?
As Eric suspected, I didn't have a working flap on my dryer vent. So I bought one and gave it a test run and I wasn't happy with how little the flap opened from the pressure on the dryer. So, I installed the booster fan as suggested by Doresoom as well. We had a few hot humid days after that, dryer stayed dry. My clothes drying times are pretty low too, 30-45 mins.
Fan pic is before I cleaned up the electrical, looks a little neater now. The ideal install for that fan is with the bracket on a stud, unfortunately there was no good way to attach a stud and avoid cutting a new hole in the roof. Luckily the fan is pretty light.