All About Bathroom Fans  

The dangers of moisture
Too much moisture is not only a matter of comfort.  Without proper ventilation a build up of moisture can cause the paint on your wall to crack and peel and the wood of your doors and door and window frames to warp.  You also risk the development of mold in and around your tub or shower.

How they work
A properly working bathroom fan takes moisture from the inside to the outside.  On the other side of the fan is a duct, which leads the moist air to an outside vent.  Insulation around the duct will help to prevent condensation build up.  The duct should lay as horizontal as possible to prevent the condensation that does form from eventually dripping back into the house. 

Some houses will require a roof vent.  If possible, however, a soffit vent is sometimes a better way to go.  A soffit vent is hidden under the roof's overhang and it takes away the need to cut a hole directly into your roof.  Do not let your fan lead simply to your attic or crawl space.  This simply moves your moisture problem from one area of the house to another.

If you have a bathroom with an available exterior wall, you may be able to install a wall fan and vent.  These take the moist air directly from one side of the wall (inside) to the other (outside).  Wall fan vents look and work much like the average laundry dryer vent.

What you need
There are a variety of styles of bathroom fans to choose from.  You will want to take into consideration some of your other bathroom needs; many fans have the ability to multitask.  If you are low on lighting in your bathroom, look into the many styles of fans that have a built-in light.  Does your bathroom get a little too chilly?  There are fans that also have a heating lamp.

To figure out how powerful a fan you need, first calculate the volume of your bathroom.  This is done by multiplying the length, width, and height (l x w x h).  Then divide this number by 7.5.  This new number is your cubic feet per minute (CFM).  The CFM is the minimum airflow needed to achieve eight exchanges of air per minute.  All bathroom fans should have their CFM listed on the box.  Choose one with a number equal or higher than your rooms' CFM number.

More information on Bathroom Fans