I recently realized that my second floor bathroom fan simply sends the moist humid air from my bathroom straight into my attic. Would it work to run some rigid vent pipe to my soffits and vent it there? Should I even worry about it?
Yes, you should be concerned for a number of reasons. First, that warm moist air will condense on the underside of your wood roof sheathing. Once the wood gets to above 25% moist, decay organisms will “wake up” and go to work on the roof sheathing, turning it black, weakening it and a the worst-case scenario it will sprout mold spores.
Secondly, that moist air will saturate your insulation and render it a lot less efficient. As you may know, the effectiveness of insulation is measured by it’s “R-value.” For example, a six-inch fiberglass batt insulation has a value of R-19. However, if you add just TWO percent moisture to that insulation, the R-value goes down by ONE-THIRD!
Your goal should be to vent the bath exhaust fan to the outside. How you get there is up to you. Ridged pipe, or flex pipe, is a fine idea. I do, however, suggest that you terminate it outside and not anywhere inside the attic, including the soffit area. At the outside wall you can use a standard duct termination damper, similar to what is used for dryer ducts.