Bathroom exhaust fan general requirements Every bathroom without an openable window (min 1.5sq. ft) requires an exhaust fan. The bathroom ceiling exhaust (or wall) should be discharging to the house exterior, not into the ceiling / wall frame cavity, or another room, crawlspace, attic, etc. This is extremely important if your bathroom contains any significant humidity source like a shower... 

Many of my clients ask me about the big square box in the center section of the house’s upper floor ceiling behind a set of self-closing louvers. It’s a whole house fan and it can either benefit home owners or make living conditions worse. Where and when it makes sense to use the whole house fan Preferably mild climates with cooler and less humid nights Use the whole house fan only if the temperature...