1. Select the correct size bath fan. Many have a square footage chart included at time of purchase, but there are tables available online as well, such as this one from HVI:

http://www.hvi.org/publications/HowMuchVent....

2. Ensure adequate make up air can reach the fan. As pictured, allow a minimum of 3/4" clearance under the door, and even more if possible. Use caution, however, that hollow doors only have a small solid rail at the bottom which could limit the cut off.

3. Always terminate the ductwork to the home exterior. In the photo, the duct is attached to a pre-cut galvanized sheet, and connected with a 4" starter collar. It is attached directly under a roof static vent which was installed solely for this purpose.

4. How long to allow a fan to run post-showering? The Home Ventilating Institute recommends that a fan should be left on for 20 minutes after a shower to thoroughly clear humidity and to ensure moisture and condensation in the fan body or ducting is minimized. A preset timer, such as illustrated in the attached photo, helps achive this.

5. Not sure if your fan is drawing air? As pictured, you can give it a quick (albeit not scientific) test using a sheet of paper. if your fan is blocked by insulation, the paper likely will not stick.