Exhaust Ventilation

Exhaust Ventilation Systems Are Simple and Dependable

UPDATED March 1, 2014

Bird's eye view

Exhaust fans are the easiest way to provide mechanical ventilation for a house

Bathroom fans and kitchen range hoods are usually already in place; if these fans are controlled by timers, they can easily provide adequate ventilation.

Exhaust-only systems are the most common ventilation system choice for cold-climate builders. They are significantly less expensive than balanced ventilationMechanical ventilation system in which separate, balanced fans exhaust stale indoor air and bring in fresh outdoor air in equal amounts; often includes heat recovery or heat and moisture recovery (see heat-recovery ventilator and energy-recovery ventilator). systems.

See below for:

Fan Types

Fans come in two varieties: single-port and multi-port.

Single-port exhaust fans are rated for noisiness using the sone scale. One sone is equivalent to the noise that a quiet refrigerator makes in a quiet kitchen. The sone scale is linear, so a 2-sone fan makes twice as much noise as a 1-sone fan. At least two manufacturers — Broan NuTone and Panasonic — make fans that are rated at 0.3 sones, making them nearly inaudible. Sone ratings are not used to rate multi-port fans.

Exhaust fan air flow (or capacity) is measured in cubic feet per minute (CFM). Fans are also rated on the basis of their “efficacy,” which is the capacity in CFM divided by its power consumption in watts. To receive an Energy StarLabeling system sponsored by the Environmental Protection Agency and the US Department of Energy for labeling the most energy-efficient products on the market; applies to a wide range of products, from computers and office equipment to refrigerators and air conditioners. label, bathroom fans rated up to 75 CFM must achieve 1.4 CFM/watt. Larger bathroom fans must achieve 2.8 CFM/watt. For noise levels, Energy Star fans for small bathrooms must be rated at 2 sones or less; large fans must have sound levels of 1.5 sones or less.

To learn more about multi-port fans, see "More About Exhaust Ventilation," below.


Design Notes

Get a fan of the right capacity

The Home Ventilating Institute, a trade group representing manufacturers of ventilation equipment, recommends that a bathroom of up to 100 sq. ft. be equipped with an exhaust fan able to remove 1 cu. ft. of air per minute (CFM) per square foot of floor area. For example, a bathroom measuring 7 ft. by 10 ft. with an 8-ft. ceiling should have an exhaust fan rated at 70 CFM. That’s enough to change the air in the room eight times per hour.

For larger bathrooms, fan capacity should follow these recommendations, according to HVI:

  • Toilets: 50 CFM.
  • Shower: 50 CFM.
  • Bathtub: 50 CFM.
  • Jetted tub: 100 CFM.
  • Enclosed toilet rooms should have an operable window or a fan.

    For example, a bathroom with a shower, jetted tub and toilet would need ventilation of 200 CFM, either with a single fan that serves the whole room or separate fans for each fixture.

    Builder Tips

    Switches offering more than on and off

    Bathroom fans are typically controlled by a simple on-off switch, making homeowners responsible for deciding when the room should be ventilated and for how long. If the main purpose of an exhaust fan is spot removal of bathroom humidity, it should be left running for at least 20 minutes after a shower to adequately address moisture. Manufacturers offer a number of improvements over simple wall switches to control bath fan operation:

  • Delay timers. Switches with built-in timers allow fans to run long enough after a shower to be effective even when occupants aren’t around. They are relatively inexpensive.
  • Humidistats. These controls monitor humidity levels and turn on the fan as required. Like timing switches, they permit fans to run long enough to be effective. An override switch allows the fan to be turned on regardless of humidity. The main drawback of a humidistat is that it requires twice yearly adjustment by a knowledgeable homeowner, since the normal range of indoor relative humidity differs from winter to summer.
  • Programmable 24-hour timers. If a bath fan is the main fan in a home's mechanical ventilation system, it should be controlled by a 24-hour timer programmed to operate the fan so that the ventilation rate meets ASHRAE 62.2A standard for residential mechanical ventilation systems established by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers. Among other requirements, the standard requires a home to have a mechanical ventilation system capable of ventilating at a rate of 1 cfm for every 100 square feet of occupiable space plus 7.5 cfm per occupant. requirements (1 cfm of mechanical ventilation for every 100 square feet of occupiable space, plus an additional 7.5 cfm per occupant).
  • Bathroom doors should be undercut to allow makeup air to enter the room.

    To control condensation in cold and mixed climates, it’s important to properly insulate and support exhaust ducts that run through unconditioned spaces. Condensation buildup, which happens primarily during cold winter weather, can reduce exhaust efficiency, damage ceilings and walls, and lead to soggy insulation. Use galvanized ducts, not flex duct, for exhaust ductwork. Routing ducts so there’s a minimum of bends helps keep condensation to a minimum. In an attic, the exhaust duct should rise above the insulation layer, and then slope gently toward the termination in a gable-end wall. Sloping the duct allows any condensation to drain away from the ceiling grille.

    Roof terminations invite roof leaks and are sometimes obstructed by heavy snowfalls. In a home with a hipped roof, however, a roof termination may be unavoidable.

    The Code

    Discharge air to the outside

    Bath fanClick for slide show
    The International Residential Code covers exhaust ventilation system requirements in Chapter 15. Among the provisions of Chapter 15 is section M1501.1: "The air removed by every mechanical exhaust system shall be discharged to the outdoors. Air shall not be exhausted into an attic, soffit, ridge vent or crawl space."

    Illustration: Code Check HVAC 2nd Edition.
    click to buy


    Passive air inlets are unnecessary

    Although some books recommend that homes with exhaust-only ventilationMechanical ventilation system in which one or more fans are used to exhaust air from a house and make-up air is supplied passively. Exhaust-only ventilation creates slight depressurization of the home; its impact on vented gas appliances should be considered. systems include passive air inlets — that is, deliberate holes in the wall to allow the entry of fresh outdoor air when exhaust fans are operating — several studies have shown the passive air inlets are almost always unnecessary. Among the reasons:

    • Most homes have an area of envelope leakage greater than even a dozen passive air vents, which typically measure 4 square inches each.
    • Although passive vent manufacturers refer to their products as inlets, the devices often operate as exhaust outlets. The stack effectAlso referred to as the chimney effect, this is one of three primary forces that drives air leakage in buildings. When warm air is in a column (such as a building), its buoyancy pulls colder air in low in buildings as the buoyant air exerts pressure to escape out the top. The pressure of stack effect is proportional to the height of the column of air and the temperature difference between the air in the column and ambient air. Stack effect is much stronger in cold climates during the heating season than in hot climates during the cooling season. and wind tend to overwhelm the weak force induced by exhaust fans, allowing the vents to operate as exhaust portals as often as they act as inlets.

    A study of 22 Vermont homes equipped with passive air vents found that the exhaust fans pulled most of their makeup air from random envelope leaks. Homes with exhaust-only ventilation systems usually do not require passive air inlets.

    Don't forget the garage

    Bathrooms and kitchens are obvious places for spot ventilation, but an exhaust fan in the garage also is a good idea. With a timer linked to the door opening mechanism, the fan will pull dangerous exhaust fumes out of the garage after a vehicle has left or for a set amount of time after the vehicle pulls in and the door closes. The fan will keep fumes and odors out of the house.


    • With a remote fan, use soft, flexible duct for the first 5 ft. to reduce fan noise.
    • Insulate ducts in unconditioned spaces to reduce condensation.
    • For ceiling-mounted fans, choose galvanized steel or PVC duct, which have less airflow resistance than flexible vinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate). or aluminum.
    • Install a UL-listed fan in a shower stall or over a tub, and put it on a circuit protected by a ground-fault circuit interrupter.
    • Ceiling inlets are more effective than wall inlets at
      gathering heat and moisture.
    • Always duct exhaust air outside, never into an attic or a basement.


    LEED for HomesLeadership in Energy and Environmental Design. LEED for Homes is the residential green building program from the United States Green Building Council (USGBC). While this program is primarily designed for and applicable to new home projects, major gut rehabs can qualify. EQ4 (Environmental Quality) offers up to 3 points for above-code ventilation practices.

    NGBSNational Green Building Standard Based on the NAHB Model Green Home Building Guidelines and passed through ANSI. This standard can be applied to both new homes, remodeling projects, and additions. Under Ch. 9 — Indoor Environmental Quality: up to 17 pts. for whole building ventilation per ASHRAE 62.2A standard for residential mechanical ventilation systems established by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers. Among other requirements, the standard requires a home to have a mechanical ventilation system capable of ventilating at a rate of 1 cfm for every 100 square feet of occupiable space plus 7.5 cfm per occupant. (902.5).


    Choose a quiet fan that's not too powerful

    Exhaust fans are rated for both the volume of air they move per unit of time (measured in cubic feet per minute or cfm) and the amount of noise they make (measured in sones, where the lower the value the quieter the fan).

    An exhaust ventilation system does not need a powerful fan. According to the requirements of ASHRAEAmerican Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). International organization dedicated to the advancement of heating, ventilation, air conditioning, and refrigeration through research, standards writing, publishing, and continuing education. Membership is open to anyone in the HVAC&R field; the organization has about 50,000 members. Standard 62.2, a 2,000-square-foot house with three occupants requires only 42.5 cfm of mechanical ventilation.

    Oversized fans waste electrical energy and space heat. If they are very powerful, they can cause enough depressurizationSituation that occurs within a house when the indoor air pressure is lower than that outdoors. Exhaust fans, including bath and kitchen fans, or a clothes dryer can cause depressurization, and it may in turn cause back drafting as well as increased levels of radon within the home. to make a gas water heater backdraft. Choose a right-sized fan. The lower the sone rating, the quieter the fan.

    In most cases, a home equipped with an HRV(HRV). Balanced ventilation system in which most of the heat from outgoing exhaust air is transferred to incoming fresh air via an air-to-air heat exchanger; a similar device, an energy-recovery ventilator, also transfers water vapor. HRVs recover 50% to 80% of the heat in exhausted air. In hot climates, the function is reversed so that the cooler inside air reduces the temperature of the incoming hot air. or ERV(ERV). The part of a balanced ventilation system that captures water vapor and heat from one airstream to condition another. In cold climates, water vapor captured from the outgoing airstream by ERVs can humidify incoming air. In hot-humid climates, ERVs can help maintain (but not reduce) the interior relative humidity as outside air is conditioned by the ERV. does not require separate bath exhaust fans. The HRV's exhaust air stream is drawn from bathrooms and laundry rooms.


    Fans for kitchens and baths

    One example of a single-port fan is the common point-of-source exhaust fan found in bathrooms to remove humidity. They are relatively inexpensive, although quieter fans cost more than noisy ones.

    Unvented bathrooms that trap moisture are an invitation for mold and, in extreme cases, even structural decay. Opening a window while the shower is running is a good start, but will not reliably remove enough moisture to adequately address potential problems.

    Bath and kitchen exhaust fans must be sized right. Problems can occur when a fan is too weak or too powerful.

    A multi-port exhaust fan is a better option for venting large bathrooms or bathrooms with toilet or shower enclosures. This type of fan can be an inline fan mounted in the attic with a Y in the ductwork to draw from two locations or a multi-port fan box that draws from as many as six separate grilles. Multi-port exhaust fans can draw from several grilles — one over a shower, for example, and another at the tub or sink. Fan motors can be up to 50 ft. away from the grille to reduce noise. Pickup points should be located as close to the source of moisture as possible.

    One remote-mounted fan can handle exhaust from several bathrooms or laundry rooms. Fantech and Continental multi-port fans are contained in cylindrical housings that are mounted in the duct. Rectangular multi-port fans, like those made by American Aldes, have separate ducts for each pickup point. Tamarack Technologies makes a remote fan that is installed on an outside wall.

    Because they are mounted in remote locations, multi-port and inline fan units are practically silent at the grille despite their ability to move large volumes of air. Using multiple grilles, these fans can ventilate bathrooms with separate toilet or shower enclosures, back-to-back bathrooms, or function as part of a home’s ventilation system.

    When used for whole-house ventilation, a multi-port exhaust fan should run continuously or be controlled by a timer that operates the fan to meet ASHRAE 62.2A standard for residential mechanical ventilation systems established by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers. Among other requirements, the standard requires a home to have a mechanical ventilation system capable of ventilating at a rate of 1 cfm for every 100 square feet of occupiable space plus 7.5 cfm per occupant. requirements.


    Range hoods are ducted or recirculating

    Steam and grease generated during cooking can lower indoor air quality. Grease is especially insidious. Tiny particles less than a half-micron in size can float in the air for days at a time, eventually lodging in porous surfaces and contributing to the growth of mold. Although some cooking odors improve indoor air quality, others are sometimes considered objectionable. All ranges should be vented to the outside with an exhaust fan.

    Ducted hoods are better than recirculating hoods. Recirculating range hoods have grease and carbon filters that trap some contaminants, but recirculating fans are not nearly as effective as those vented to the outside and do nothing to expel moisture. There are three basic types of ducted range hoods:

    • Self-contained hoods. A fan, controls, lights and removable grease filters are housed in a simple hood mounted over the range or stovetop. Fans also can be incorporated into over-the-range microwave ovens.
    • Chimney style. These fans work essentially like a self-contained hood except that they have a decorative chimney-like extension that conceals the exhaust duct above the collector. Fans can be just above the hood or installed some distance away, minimizing noise. Chimney-style hoods can be mounted on a wall or over a cook surface in a kitchen island.
    • Downdrafts. Grease and moisture are pulled from the cooking surface into a collector at the back or in the center of the stove. They are less conspicuous than other options but do not remove pollutants as efficiently.

    Range-hood fans vary in capacity and price. Very large pro-style hoods can exhaust 1,200 cfm and top $5,000 in cost. Basic models are rated at 100 cfm and sell for $150 or less. Beware: high-capacity range hood fans can cause problems with combustion appliances like water heaters or wood stoves. For more information on the dangers of high-capacity range hood fans, see Makeup Air for Range Hoods.

    Key considerations:

    BackdraftingIndoor air quality problem in which potentially dangerous combustion gases escape into the house instead of going up the chimney.. High-powered exhaust fans running in tightly constructed homes can pull fumes from fireplaces or fuel-burning appliances that do not have sealed combustionCombustion system for space heating or water heating in which outside combustion air is fed directly into the combustion chamber and flue gasses are exhausted directly outside. chambers. Some states require that makeup air systems be installed when large range hoods are specified. In areas where radonColorless, odorless, short-lived radioactive gas that can seep into homes and result in lung cancer risk. Radon and its decay products emit cancer-causing alpha, beta, and gamma particles. is prevalent, any leaks between the basement and crawl space should be sealed. A radon mitigation system may be required. (For more information on the interaction between exhaust fans and radon entry into homes, see Exhaust-Only Ventilation Systems and Radon.)

    Capacity. The heat output of the range helps determine the capacity of the exhaust fan. Many manufacturers recommend that for every 100 BtuBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. of heat generated by the range, the fan should be able to move 1 cubic foot of air per minute (cfm). However, it is often advisable to minimize the size of a range hood exhaust fan, to reduce the need for makeup air and the potential for backdrafting problems. Large exhaust fans are incompatible with wood-burning fireplaces and some woodstoves. Island and downdraft are inherently less efficient and may require a larger fan than a wall-mounted model over the same cooking appliance.

    Height. In general, the distance between the cooking surface and the bottom of the collector increases with the capacity of the fan, from 18 in. to 24 in. for low-powered fans to 36 in. for high-capacity exhaust fans.

    Hood style. Manufacturers offer hoods in contemporary, sculptural shapes and nearly flat profiles but they are not as effective in trapping contaminants as larger, conventionally shaped hoods with high “capture efficiencies.”


    "Designing a Good Ventilation System"

    Exhaust-Only Ventilation Systems and Radon

    Makeup Air for Range Hoods

    Image Credits:

    1. Fine Homebuilding 178
    2. Krysta Doerfler / Fine Homebuilding
    3. Chuck Lockhart / Fine Homebuilding
    Tags: , , , , , ,
    Oct 5, 2012 5:28 AM ET

    Response to Denis Recchia
    by Martin Holladay

    It look like you posted the same comment and questions in three different places. To avoid multiple postings, all of the answers will appear here -- on the Q&A page.

    Oct 4, 2012 7:14 PM ET

    Humidity Problems
    by Denis Recchia

    Hopefully I can get myself out of this mess I got myself into :)

    I recently built a pretty tight house with my old fashioned dad, I think we ended up doing a pretty good job with making the house tight, but didnt include any mechanical ventilation in the home. Right now in early october we are seeing relative humidity levels around 75% with temperatures inside of around 70 deg F.

    How can I fix this? Should I install exhaust fans in the two bathrooms upstairs? My Dad says open a window, but its going to be winter very soon, and recently its been raining so much the humidity is worse outside than inside.

    Im moved into the house, so any mechanical ventilation i pick im sure is going to be a disaster. The home is double 2X4 walls with a 2 inch layer of closed cell foam with 10 inches of cellulose inside of it.

    Thanks in advance,


    Apr 23, 2010 4:52 PM ET

    Exhaust Fans Accessories
    by Anonymous

    Help me. Condo has no stovetop exhaust fan ventilation! Any way(s) to cope with this? Set off smoke detector in past.

    Oct 8, 2009 7:26 PM ET

    by Peggy Deras

    There is a safety issue about range hood exhaust fans that is not addressed in your article: Remote, and in-line blowers used in conjunction with a range hood fail miserably in the event of a fire on the cooking surface. The flame is pulled up through the hood and into the ductwork, quickly burning through and into wall, attic and roof.

    A range hood with a squirrel cage fan mounted within the hood canopy will stop flame at the fan, giving precious seconds to fight the fire on the rangetop.

    Grease fires are the most common residential fires.

    If you are confronted with such a fire, do not use an extinguisher or water.
    Instead, get down low and turn off the flame under the burning pot or pan.
    Then grab a towel and wet it at the sink and throw it over the pan.
    To see this exercise on video:

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