Bathroom Fans

Bathroom fans direct air away from a space in order to remove moisture or odors. They either exhaust air continually or intermittently and can be operated by a wall switch, timer, or automatically by a humidity sensor. Bath fans are a form of spot ventilation, removing contaminants from the air at their source before they have a chance to spread through the dwelling. These fans need to be appropriately sized for the space being ventilated. As bathroom fans change the dynamics of the ventilation of the entire house, integrating bathroom fans with the whole house ventilation system can better balance exhaust and intake air supplies.

Not everyone will turn on a bathroom fan during a shower. In instances where there is a low probability of regular occupant use of the fan, it is recommend that a humidistat be installed to automatically run the fan.

See the Notes on Use section to determine appropriate fan sizing.

Environmental Context
A good bathroom fan should provide adequate ventilation while using energy efficiently. Energy use increases as the amount of ventilated air increases, so it is important to correctly size a bathroom fan. Too little ventilation may result in condensation, which in turn can lead to mold, mildew and fungus, and structural damage. An oversized fan may needlessly use energy and pull excessive amounts of conditioned air to the exterior.

The negative environmental impacts of energy use are well documented; a well functioning and efficient ventilation system can mitigate these by lowering energy consumption. Efficient models use less energy to provide the same amount of ventilation. Choosing an ENERGY STAR rated model is an easy way to ensure efficiency.

Notes on Use
The Home Ventilating Institute (HVI) has some basic recommendations for the sizing of airflow for ventilating fans. They suggest that the fan be sized for 1 cfm (cubic foot per minute of airflow) for every square foot (sf) of bathroom space, up to 100 sf. For bathrooms over 100 sf, the HVI suggests that the fan be sized according to the number of fixtures in the bathroom.1

Other standards, such as ASHRAE 62.2 Low Rise Building Standard, consider whole house ventilation as well as individual room ventilation. ASHRAE 62.2 2004 requires that bathrooms fans vent at least 20 cfm continually or 50 cfm.2

Bathroom Ventilation Needed3

Bathroom Size Calculation Formula Ventilation Rate Required
Less than 100 sq. ft. 1 cfm per square foot minimum 50 cfm
More than 100 sq. ft. Add the cfm requirement for each fixture Toilet: 50 cfm
Shower: 50 cfm
Bathtub: 50 cfm

Bathroom fans can be installed vertically on walls or horizontally in the ceiling, depending on the model. Bathroom fans should be ducted directly to the exterior whenever possible. Ductwork running through attic spaces should be insulated, particularly in cold climates, due to a risk of condensation on the interior of the duct. A bathroom fan should never be allowed to exhaust into attic space. Some recommend that in cold climates, bathroom fans be vented down through the wall and out by the edge of a foundation.4 This method reduces the likelihood of interior moisture drip.

Bathroom exhaust fans can be used with heat recovery ventilators (HRV's), but may not function as well with energy recovery ventilators (ERV's), since these devices recover the humidity that the bathroom fan is trying to eliminate. ERV's can also leak some air between the incoming and outgoing flows (up to 10%).5 The ASHRAE standard 62, appendix Y, clarifies the cross leakage permitted in certain energy recovery systems.

Options and Analysis

Fan Types
There are two main types of bathroom exhaust fans: blade and squirrel cage. Generally the squirrel cage type (a cylinder with blades around the side) is quieter and more efficient than the blade type, but is more expensive.

Some bathroom exhaust systems allow for the installation of the fan in line with the duct and away from the room, resulting in a quieter system. Referred to as remote mounting, this approach can be taken when noise is of particular concern and when the bathroom fan will run continuously as part of a whole house ventilation strategy.

Many bathroom exhaust units operate only as a fan, though some models may include a light and or a small heater. These combination units can add functionality without requiring much additional wiring or mounting difficulty.

Light and Fan Units
An integrated light can reduce the number of fixtures and switches in the bathroom. Integrated lighting should be fluorescent or solid state (LEDs) to minimize energy use. To ensure the use of fluorescents, some models use 2-pin compact fluorescent bulbs instead of a conventional socket.

Heated Units
A heating element integrated into the bathroom fan can provide quick supplementary heating. If retrofitting an existing bathroom, these units can add warmth and comfort to a sometimes cold space. In newer, better insulation, there should be less need for additional heat sources, though if the central heating of a building is set to an energy-saving low temperature, supplemental heating in the bathroom may be desired.

Operating Energy
The US EPA ENERGY STAR program rates bathroom fans and other appliances for energy efficiency. Using an ENERGY STAR rated bathroom fan will guarantee a minimum performance level, including:

  • Maximum sound level depending on fan size (from 2.0 to 3.0 sones)
  • Minimum efficiency of 2.8 cfm/Watt
  • Minimum light efficiency (uses fluorescent bulbs)
  • Minimum rated airflow6

The initial cost of energy efficient bathroom fan models is slightly higher than that of their inefficient counterparts. However, they can pay for themselves with lowered energy costs. The US EPA estimates that a high-efficiency fan can save as much as $120 over the life of the fan.7 Quieter, low-sone fans may also be more expensive than cheaper, noisier models. It is worth noting that decreased fan noise is associated with higher use rates, which can improve overall air quality.

Bathroom fans vary in their longevity: higher quality fans will often outlast cheaper, lower quality models. The average life for bathroom fans is around 10 years, though some companies claim longer lifespans. When a bathroom fan wears out or breaks, replacement is generally simple, using the same sized opening in the ceiling or wall.

ENERGY STAR bathroom fans are widely available. Their small size makes shipping a reasonable delivery option if necessary. Whenever possible, however, local sourcing can lower the total environmental impact.

Indoor Air Quality
A properly installed and operating bathroom exhaust fan improves indoor air quality. In addition to the immediate benefits of removing moisture and odor with the fan, exhausting moisture prevents mold growth. Reducing mold growth lowers the levels of indoor contaminants and improves indoor air quality.

Timers and delay off switches can provide a low cost way to ensure proper ventilation is maintained. Some fan models are set to run continually at a lower rate in order to avoid moisture buildup and operator error.

Sound volume is measured in sones, which relate to how people perceive sound. Low-sone (low-noise) bathroom fans are often preferred because they have only a fraction of the typical fan's sound level. A quiet fan is under 2 sones; less than 1.2 sones is very quiet. A fan over 4 sones may become a nuisance and discourage use.

Other Resources


1 "Bathroom Ventilation Guidelines" HVI Web site. 2005. Home Ventilating Institute. 1 Aug 2008.
2 Raymer, Paul H. "Residential Ventilation Systems and ASHRAE 62.2". 1 Aug. 2006. U.S. Department of Energy Efficiency and Renewable Energy. Building Codes Department. 1 Aug 08.
3 "Bathroom Ventilation Guidelines" HVI Web site. 2005. Home Ventilating Institute. 1 Aug 2008.
4 Cold Climate Housing Research Center Web site. 2007. Cold Climate Housing Research Center. 1 Aug 2008.
5 ASHRAE Standard 62-2001-Ventilation for Acceptable Indoor Air Quality. ASHRAE, 2001. Addendum Y, Part 5.x.2.2.
6 "Ventilating Fans Key Product Criteria." ENERGY STAR Web site. U.S. Environmental Protection Agency. 1 Aug. 2008.
7 ibid.