All bathrooms need to be vented. Really vented. I’m not talking about cracking open a window. Nobody is going to do that during a cold Ottawa winter. When I do my home inspections, I check for good, reliable exhausts in the bathrooms.
The basic bathroom, the lone toilet or the two piece powder room with a toilet and sink, needs to be vented for odours. More importantly, the bigger bathroom, the one with a tub and shower needs to be vented too, not only for odours, but also for moisture and humidity.
In ‘olden’ days, before the 1980’s, it was thought that as long as there was a window, you did not need an exhaust fan. I’ve seen so many mouldy, soggy bathrooms that had windows. During a hot bath or shower, there is a great deal of moisture given off. So much moisture that it can’t be exhausted passively.
So, you need a fan to blow the humid air, and you need a duct to carry it out. That duct starts in the bathroom and ends on the outside. The best way for a fan to do its job is to keep the duct as straight, and smooth, and short as possible. The more bends there are, the bumpier the surface, or the longer the duct is, the harder the fan has to work to get the humility out. This certainly excludes the use of plastic, corrugated dryer ducts.
Generally, the ducts end outside either on an exterior wall, or through the roof, or out the roof overhang (called the soffit). Any of theses work as long as the exhaust cap is well sealed and firmly attached. Also, it is important that the duct itself is rigid metal and fully insulated. Again, no plastic dryer ducts.
If the ducting is done wrong, and the exhaust air leaks into the attic, the consequences can be devastating (see the picture). The bathroom humidity will discharge into the attic. Here the condensation will contribute to the growth of mould. The water from condensation will also rot the wood supports found in the attic.
The bathroom humidity may also condense in the duct and leak back into the room. You can get water dripping out of your exhaust fan and think you have a roof leak. In truth, you have a poor, cold duct that is not doing a good job getting the humidity out.
So, bottom line, you should have the bathroom exhaust fan venting through to the outside on an outside wall or in the roof soffit (not into an interior space, including the attic). The fan should use a unique exhaust duct that connects to a unique exhaust cap. And make sure the duct is rigid metal and fully insulated.