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Why is CFM important when it comes to ventilation fans?

Bathroom Ventilation Guidelines

Bathrooms are prone to moisture problems due to high levels of heat and humidity from showers and bathtubs. Heat and moisture accumulates on walls, ceilings, and woodwork. The remedy is ventilation. According to the Home Ventilation Institute (HVI), bathrooms should be intermittently or continuously ventilated with an exhaust ventilation fan at a rate of one cubic foot per minute (CFM). Here are some measurements:

An 8x5 bathroom requires exhaust at 40 CFM.
Bathrooms over 100 square feet can be ventilated according to fixture count (50 CFM for each fixture. A tub, shower, and toilet require 150 CFM).
Jetted tubs need an additional 100 CFM.

Use a high-capacity fan, or install more than one fan in strategic locations. Consider using an in-line fan system to run fans at each port. For the most efficient ventilation, buy fans with the right CFM rating for your room size and number of fixtures. Make sure to look for an industrial exhaust fan for commercial use. Failure to exhaust hot and humid bathroom air can lead to mold and mildew, and it can damage the structural integrity of a building or home.

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