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How To Ventilate A Bathroom Properly


A bathroom remodel can bring new life, new practicality and new functionality to a bathroom space. Whether a remodel involves changing fixtures, painting walls, installing new bathroom cabinets or completely gutting and reconstructing, a fresh bathroom look is a great way to update a home. However, issues below the glossy surface sometimes get overlooked in a bathroom remodel. One item that is important to consider is proper bathroom ventilation.


Bathrooms are warm, damp and humid most of the time they are in use. This combination of high heat and high moisture creates an ideal breeding ground for mold, mildew and rot. By ventilating a bathroom space properly, it’s possible to greatly reduce the damage caused by humidity and heat. Every design decision in a bathroom should consider airflow and venting.

Natural Light and Breeze

Bathrooms are such a small space and privacy is such a concern that there are often no windows in bathrooms. But consider installing windows for a brighter and a fresher space as well as for drying and ventilating the air. Would a clerestory, or high window, work in the space? Perhaps the space is large enough for a standard casement window that is out of the sightlines of the private areas of the bath. Window coverings can also solve the privacy issue, add color and spark to the décor, and still allow airflow through the bathroom for effective ventilation. While sky lights and solar tubes are great for adding natural light, they are of no use for ventilating.


Bathroom fans are usually standard in a remodel, but choosing the wrong fan means inadequate ventilation, leading to mildew and possibly warped bathroom cabinets. To choose the right fan, calculate how many cubic feet per minute or CFM a fan needs to move. The standard for bathrooms is moving the air eight complete times per hour. The homeowner or the contractor can calculate the needed CFMs using one of three methods.

The first method is an estimating method used for bathrooms under 100 square feet. Follow the Home Ventilating Institute’s guidelines of having one CFM per square foot of bathroom. So for a bathroom that is 75 square feet, get a fan with 75 or more CFM. The second method is used for bathrooms over 100 square feet, and makes a calculation based on the number of fixtures. For each standard toilet, shower or tub add 50 CFM, and for every whirlpool or jetted tub, add 100. So a bathroom with a toilet, a shower and a jetted tub needs 200 CFM. The third method uses a mathematical formula of square footage times ceiling height. Divide this by 60 and multiply by eight to get the number of CFMs that will give eight air changes per hour.


Using proper ducting is also crucial for effective ventilation. Although some contractors do it as a shortcut, make sure the fan is not vented into an attic space or a basement space. This just transfers humidity rather than removing it. Instead, vent the fan directly to the outside. Install the correctly sized exhaust fan as near an exterior wall as possible so that the duct run is shorter. The run should be no more than 6 feet for ideal fan operation. Use a 4-inch duct and make the run as straight as possible. Any turns, long duct runs or other factors that slow airflow will cause the fan to be inefficient. If the duct run is longer than six feet and/or has a turn in it, purchase a fan with higher CFMs to avoid humidity buildup or bathroom cabinet damage.

Replacing a Vent Fan

When remodeling a bathroom, sometimes the vent fan was at one time adequate for the space, but is now not working well or not working at all. While homeowners can replace the entire vent, finding a new fan that fits exactly in the space of the old fan is often a real challenge. One way to solve this dilemma is to write, call or make an online contact with the manufacturing company who made the fan and ask to purchase the motor only.

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