I am actually out in the country and periodically we do experience high winds sometimes with driving rain.
All galvanized pipe
I had to learn that the hard way...
This is just my opinion but if you tie both fans into one vent you should use a "Y" piece of duct-work with (2) back-flow preventing inserts. Also, when venting through the soffit take into consideration that the space between where the roof rafters come down (on an angle) and where the inside wall below meets the roof rafters is somewhat narrow. You'll have to compress the hose in order to get it into the soffit. If it's new work - no problem but if it's a remodel you'll have a tough time getting into that area if you're a big guy (like me). Personally speaking, and again IMHO, I wouldn't just dump moist air into the soffit. I would use a soffit vent kit like this one :
True. The exhausted air won't be buoyant in the summertime and won't rise out through the ridge vent, but it probably won't matter then, either. (depending on where you live -- Nevada and New Jersey have different needs for dealing with moisture)
In summertime all the air in the attic is buoyuant enough to cause natural convection to occur as long as you have sufficient openings to allow air being drawn in to flow freely. Typical situation of drawing in through the overhang - that air is much cooler than what is in attic even when it is over 100F outside.
Regardless of how the air may behave, dumping the air into the attic ist verboten.
I get that but try telling that to a client when their bathroom is cold because wind is pushing cold air in there via the vent, or when it is raining from the vent because the ice that built up in it is thawing, especially in a place where nobody is going to enforce those rules.
In which case rigid sheet metal is the way to go. But seal any joint that ends up at a low point such that it is waterproof.
This is why you put louvers on the outside of the fan discharge, like for a dryer, and why you pitch the exhaust down to the outside.
I can tell you from experience that one reason for the damper not closing properly is a build-up of lint. I had to go out onto the roof to clear the vent maybe twice a year.
I don't typically have problems with wall caps, they are essentially same thing as a dryer vent. It is the roof caps that I have had trouble with.
I have had this trouble in first winter after installation on brand new homes. Customer doesn't think to highly of you for installing something that will do this sometimes, especially when the carpenter tells them it should have just been vented to the attic or into the soffit. After having this trouble half dozen times or so, I pretty much quit venting them out the roof anymore.
