Venting Bathroom Exhaust: Exhaust diverted into attic can cause mold and reduce insulation efficiency

Tom Kraeutler asks,

I need some tips on venting bathroom exhaust. 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 ridged vent pipe to my soffits and vent it there? Should I even worry about it?

Our Answer

Yes, you should be concerned about this bathroom exhaust situation 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 percent moist, decay organisms will wake up and go to work on the roof sheathing, turning it black, weakening it and, in the worst-case scenario, allowing mold spores to sprout.
Secondly, that moist bathroom exhaust air will saturate your insulation and render it a lot less efficient. As you may know, the effectiveness of insulation is measured by its 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.