Bathroom fans can be installed to vent out the side of the house or through the roof. It is generally preferable to vent out of the side as that is usually the most direct route. If the bathroom has never had a fan before then the duct will have to be run to the outside.

Some common bathroom fan installation mistakes:

• Venting a fan directly into an attic or behind a wall. The entire point of a bathroom fan is to vent moisture out of the house to avoid mold and mildew problems. If a fan is expelling moisture into another part of the house. This can often times be much worse because any problems that start due to water damage can become major problems before being noticed due to the fact that the moisture is going to an area that is rarely seen by a homeowner.
• Venting too far away from the shower. It is preferable for a vent to be as close to a shower as possible. This is sometimes not feasible due to the length of ductwork needed to run the vent or beams and joists in the ceiling that could be in the way.
• Having too long of a run for the vent or too many bends. The less distance and turns in the vent the better for a fan as that will ensure moisture escapes quickly from the home before it has a chance to form condensation.
• Installing a fan on a circuit that cannot handle the additional electrical load. Sometimes a new circuit will have to be installed in order for the fan to operate without tripping any existing circuits.

There is almost always a trade off between the most efficient vent-run and the best location to install the fan. A bathroom vent fan should ALWAYS terminate outside of the house.