A switched exhaust fan that is rated at a minimum of 50 cfm (cubic feet per minute) or a continuous fan that produces 20 cfm or more is necessary to meet International Residential Code (IRC R303.4, M1507.4) requirements. Although the continuous fan is an option, we never see it in residences. 

    Also, the code requires that the fan must terminate outdoors. Because the purpose of the fan is to get bathroom odors and the humid air from bathing out of the home, the fan cannot dump air into the attic, which would only move the high-humidity air to a new problem area. It should not be ducted into the attic or under the floor. Also, duct termination at the edge of the attic near a soffit vent does not count.

    As an alternative, a window with a minimum opening of 1.5 square feet is also acceptable by code (IRC R303.3). The requirement refers to openable area, not the total area of the glass. A single-hung window, for example, will have an openable area of slightly less that half of the total surface of the window.

   Opening a window was the way bathrooms in older homes were ventilated, but it is not acceptable to most homebuyers today. The average family lives in a closed, air-conditioned environment, and opening the bathroom window—especially when it’s 95º outside in August or 20º in February—just doesn’t happen.

  Wall switches are available for exhaust fans with timers, like the one shown below, so you can set the fan to run for several minutes after you leave. 

    Also, a newer generation of wall switches uses a humidity and temperature sensor that leaves the fan on until the humidity and temperature has dropped to an acceptable level. An example on one made by Panasonic® is shown below.

    Also see our blog post My bathroom is stinky and humid even though it has an exhaust fan. What can I do to fix it?

• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •

  To learn more about heating and air conditioning systems, see these other blog posts:

• How can I find out the SEER of my air conditioner? 

• My air conditioner won't turn on. What's wrong? 

• How can I find out the size of my air conditioner? 

• How can I find out the age of my air conditioner or furnace?

• How can I tell whether the condenser (outdoor unit) is an air conditioner or heat pump? 

• Where is the air filter for my central air conditioner and furnace? I can’t find it? 

• Does an old air conditioner use more electricity as it ages? 

• How did homes stay cool in Florida before air conditioning?

• How can I find out the size of my air conditioner? 

• How can I find out the age of my air conditioner or furnace? 

• What is wrong with an air conditioner when the air flow out of the vents is low?

• Why has the thermostat screen gone blank? 

• Why does it take so long to cool a house when an air conditioner has been off for a while? 

• Why is my air conditioner not cooling enough? 

• What are the most common problems with wall/window air conditioners?  

• Will closing doors reduce my heating and cooling costs? 

    Visit our HEATING AND AIR CONDITIONING page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.