Incorporating some form of mechanical ventilation in bathroom design, commonly referred to as intermittent ventilation, is important. Regionally, it may be referred to as spot or local ventilation. The purpose of any type of bathroom ventilation is to remove moisture and odor from the bathroom before it can damage the immediate environment or contaminate adjacent rooms.

Successful design requires architects, designers and builders to understand the types of ventilation products available. Understanding where to position them and how to properly size them is also important. Remember the purpose of bathroom ventilation is to remove excess moisture, humidity, odors and other indoor pollutants such as aerosols and perfumes. Good ventilation also prevents the deterioration of interior wall coverings, structural framing and insulation.

Proper sizing

Fans are rated by the amount of air they move, measured in cubic feet per minute. Most codes require a 50 CFM intermittent fan or an operable window for bathroom ventilation. The Home Ventilating Institute and the National Kitchen and Bath Association recommend eight exchanges of air per hour in a bathroom. This recommendation will typically exceed minimum code requirements.

Determining the proper CFM for a bathroom is easy. The volume of the room is determined in cubic feet (LxWxH), and then multiplied by 8 (this is the number of exchanges of air per hour). The sum is then divided by 60 resulting in the required CFM for the room. An easier way to guesstimate is to allow 1 CFM per sq. ft. of room, or if there is an 8-ft. ceiling take the square footage of the room and multiply it by 1.07. If designing a large master suite or spa specify 50 CFM per fixture with 100 CFM per jetted tub or whirlpool.

Once you specify the size of the fan it’s time to determine the length and diameter of ductwork. Check the manufacturer’s installation guide to determine the proper length and diameter. Follow the manufacturer’s directions. If you reduce the diameter of the ductwork you will reduce the efficiency of the fan by creating static pressure. If you increase the length of the ductwork beyond the manufacturer’s recommendations or install too many elbows or transition fittings, you risk compromising the fan’s performance by upwards of 90 percent. In most cases a fan will need to be oversized in CFM to meet both the requirements for air exchange and length of ductwork. It does require careful planning on the front end for these installations to work.