The Art and Science of Good Ventilation
What About the Bathroom?
Contemporary bathrooms are, generally speaking, larger, and many new master baths now include showers with multiple heads, whirlpool baths and steam rooms, all of which contribute additionally to the humidity load and create a need for increased ventilation. Larger rooms generally have more than one fan.
The most important thing to remember with regard to bathrooms is that showers are the greatest contributor to excessive humidity and that homeowners can save the house from excess moisture by running the fan for 20 minutes after each shower.
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Self-sensing humidity fans operate automatically when humidity rises rapidly and turn themselves off 20-30 minutes after humidity returns to where it was before.
There are two important things to understand about humidity sensing fans. First, the best models sense the CHANGE in humidity, not the humidity level itself, so climates that are naturally more humid will not trigger the fans to operate unless over-ridden by the occupant. And second, sensitivity levels and after-run times can be personally set by the home's occupants.
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Installing a wall timer switch allows the fan to remain on long enough to remove the excess moisture without having to return to switch it off.
Location of the bathroom fan is also a critical factor in ensuring proper performance. The first choice should be over the shower. If this is possible, select a fan UL-listed for tub or shower installation.
Where that is not practical, one should visualize a line from the bottom of the bathroom door and the heating/cooling register (which are the exhaust fan's air supplies) to the ventilator. Make certain that that line passes through the steam cloud generated by the shower. Air flowing through the room along that line will capture the excess moisture from the shower.
Since intermittent contaminants are produced in high concentrations, a high ventilation rate is required to keep them from dispersing throughout the house. This explains the relatively high airflow rate of typical intermittent kitchen and bathroom ventilation.
The Wauconda, Ill.-based Home Ventilating Institute (HVI) recommends eight air changes per hour for bathrooms. This recommendation is based on both years of experience and on calculated needs. Performance has been tested through research at Texas A&M, and the results show actual contaminant concentration follows theoretical dilution calculations with excellent reliability. Exceptions to the recommendation may be considered in unusual situations such as when a bathroom is unusually large. In the case of an extremely large bathroom, multiple fans may be required to effectively handle moisture and odor at their respective source.
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Bath fans must be sized to meet HVI's recommended 8 ACH. Calculate bathroom fan size in CFM in the following way:
- Take the height of the ceiling and multiply it by .1375. Take this figure and multiply by the square footage of the room. This will equal the recommended CFM's. (Example-10' x 12' room with 10' ceilings. 10' x .1375 = 1.37 x 120 square feet = 164 CFM's.)
- This will give you the minimum recommended CFM.
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