Bathroom Ventilation: Fact and Fiction

Is that bathroom fan really doing what it was designed to do? Research performed by the CMHC (Canada Mortgage and Housing Corporation) has found that many homes have bathroom exhaust fans that are noisy, move very little air and are not energy efficient.  Many homeowners are not even aware that their current bathroom fans may be contributing to damage in their homes.

Bathroom exhaust fans are an important part of a home’s ventilation system. They eliminate odors, improve indoor air quality, and remove moisture and humidity that can lead to structural damage or mildew and mold growth. Unless a bathroom is properly ventilated, the moisture from a shower has no place to go and can penetrate into drywall, attic insulation and structural joists. If a mirror is steamed after a shower, or there is a build-up of condensation on bathroom walls, it may be time to service or upgrade the bathroom fan.

CHMC’s research shows that many bathroom fans across Canada should be replaced or serviced due to inadequate airflow, inability to overcome static pressure, high leakage rates and generally poor condition. Inadequate airflow was cited as the most common problem, with improper fan selection being among the causes.

There are several factors to be considered when selecting a new or replacement bathroom fan: airflow rate, sound levels, energy efficiency, and aesthetics and fan control.

Airflow rate

Of the factors listed above, the airflow rate is the most important. The Home Ventilating Institute (HVI) has provided guidelines for the proper ventilation of a bathroom. For bathrooms under 100 square feet, the basic rule is to exhaust a minimum of one cfm (cubic foot per minute) for every square foot of bathroom area. So an 8’ x 10’ bathroom, 80 square feet, would have a minimum airflow requirement of 80 cfm. For bathrooms over 100 square feet, the calculations are based not on square footage, but on the number and type of fixtures in the bathroom. An allowance of 50 cfm should be made for each standard toilet, bathtub and shower. Allow 100 cfm for whirlpools and hot tubs. For example, a bathroom with a hot tub, shower stall and toilet would require a minimum of 200 cfm (100 + 50 + 50).

Once the required airflow in cfm has been determined, it is necessary to establish the amount of static pressure (SP), or resistance to airflow, in the exhaust duct system. This has a significant influence on the amount of airflow the fan will deliver.  Many bathroom exhaust systems have a static pressure of 0.3” to 0.5”, which must be taken into consideration when selecting a bathroom fan.  When static pressure increases in an exhaust system, the amount of airflow, or cfm, decreases.

Most bathroom fans sold today are marketed with cfm levels based on open-air conditions, or 0” SP.  A fan rated for 100 cfm at 0” SP may not provide the necessary airflow for a bathroom when it is installed, because of high system resistance.  It has been determined that some bathroom fans with an initial rating of 90 cfm at 0” SP provide no airflow at all when installed in a typical bathroom exhaust system having 0.5” of static pressure. To avoid this problem, it is important to know manufacturers’ published airflow rates at varying static pressures to allow for proper fan selection.

Air make up is an important factor to consider when sizing a bathroom fan. Fans will only remove air from an area at the rate that the air can be replenished, regardless of the correct sizing or rated air flow. In many cases, the space between the bottom of the door and the floor is adequate, but in some cases an additional air make-up source must be installed to allow the fan to operate at peak performance, such as an additional grille installed in the bathroom door.

Sound level

The second issue is the sound level of the bathroom fan. Various studies have shown that many people do not turn on their bathroom fans because they are too noisy. If the fans do not operate, they serve no useful purpose. When considering fan sound levels, it is important to check for the HVI rating. If a bathroom fan does not carry an HVI rating, there is a very good chance that the fan will be noisy. Recently there have been several proposed changes to local codes indicating that a bathroom fan installed should not have a sound rating higher than 1.5 sones (a sone is a scientific unit of perceived loudness).

A quiet alternative to traditional bathroom fans is the remote mounted in-line ventilation fan. These fans are mounted in the attic, thereby removing the motor and fan assembly from within the bathroom itself, and provide quiet and effective exhaust ventilation to deal with most airflow requirements. By removing the fan assembly from the bathroom space and mounting it remotely, the possibility of the ultimate in quiet operation, virtually 0 sones, can be achieved (depending on how far away the fan is mounted).

Energy efficiency

With energy savings being on everyone’s mind, choosing an Energy Star fan is, without question, the best choice. More than half of the fans operating in Canada today are not energy efficient models; in fact, some fans in operation today are using more than 180 watts of power. A retrofit of the bathroom fan can lead to savings in the electrical bill and increased performance of the bathroom fan.

Aesthetics

Once a fan has been found that is quiet, energy efficient and delivers the required airflow, it is important to look at the esthetics of the fan, or how the fan will fit into the decor of the bathroom.  This is strictly a personal choice.  Two different styles of bathroom fans are worth considering: traditional ceiling mounted fans and remote mounted in-line fans.

Traditional ceiling mounted fans are most commonly seen in bathrooms today.  This type of fan has a fan and motor assembly mounted in the ceiling, housed in a box with a fixed air intake grille covering the mechanics of the fan.  Although many of the newer designs have improved sound levels and airflow rates, they still have the motor and fan assembly mounted in the bathroom area, tend to be noisy, and are not designed to overcome the higher static pressures found in many exhaust systems.

Remote mounted in-line ventilation fans provide a much quieter operation, and are designed to overcome higher static pressures typically found in bathroom exhaust systems.  In-line bathroom fans are normally mounted in the attic space, removing the mechanical aspects of the fan from the bathroom.  The only visible portion of the exhaust system in the bathroom is an attractive exhaust grille.  This provides a more esthetically pleasing look to the bathroom, with the ability to adjust airflow as required.

Another feature of remote mounted fans is that the air intake grille can be mounted directly over a shower or hot tub.  In addition, the ability exists for one fan to exhaust air through multiple grilles.  Remote mounted fans can provide up to 80% of initial fan rating, even with 0.5” of static pressure present in an exhaust system.  This may result in the reduction in the size of fan required to meet airflow requirements.

Fan control

The last point to consider when upgrading a bathroom ventilation system is how to control the fan.  This is an important aspect of the ventilation process, and is something that should be strongly considered.  CMHC and HVI both suggest that a bathroom fan run for a minimum of 20 minutes after a shower, to allow for the removal of excess humidity and moisture.  Many bathroom installations have the fan controlled by the same switch as the light, resulting in the fan being turned off as soon as the occupant leaves the room.  It is recommended that a separate switch control the fan or, better yet, a timer that allows the fan to run for a pre-determined amount of time after showering.

There are many aspects to properly selecting the correct bathroom exhaust fan.  Much of the information available today can be confusing.  Whatever choice is made, it is important that the selection is based on fact, and not fiction.

 

*Source: canadiancontractor.ca