ASHRAE Standard 62.2: Airflow, Exhaust, and Intake Rules

ASHRAE Standard 62.2 sets the residential ventilation requirements for low-rise dwellings in the United States, covering how much outdoor air a home needs, how contaminated air must be exhausted, and how the system has to be tested and documented. It applies to dwellings of three stories or fewer above grade, including manufactured housing, and because the International Residential Code and the International Energy Conservation Code reference it, most jurisdictions treat its provisions as mandatory for new construction and major renovations.¹

How Much Outdoor Air the Whole House Needs

The required continuous airflow, in cubic feet per minute, equals 0.03 multiplied by the total floor area in square feet, plus 7.5 multiplied by the number of bedrooms plus one. The “plus one” accounts for occupants in common areas outside bedrooms.¹

A three-bedroom, 2,000-square-foot house works out to (0.03 × 2,000) + (7.5 × 4) = 90 CFM. Larger homes and higher bedroom counts scale the number up. This figure is the baseline every other decision has to satisfy.

Continuous or Intermittent Operation

A fan does not have to run every minute of every day. Intermittent operation is allowed if the fan cycles at least once every three hours and its airflow rate during the on-cycle is increased to compensate for the off-time.² A fan running 20 minutes out of every 60 is on one-third of the time, so it needs to move roughly three times the continuous rate while it runs.

Infiltration Credit

Leakier homes already pull outdoor air through gaps in the envelope. When a blower door test has been performed, the standard allows a credit that subtracts the estimated effective annual infiltration rate from the required ventilation rate, reducing the mechanical airflow the fan actually has to supply.³

The credit has a hard cap. Infiltration can offset no more than two-thirds of the total required ventilation, so at least one-third stays under mechanical control. The credit applies only to non-attached dwelling units such as single-family homes, not townhouses or apartments with shared walls.³

Exhaust-Only, Supply-Only, or Balanced Systems

The standard does not dictate which system you install, but each option handles pressure differently, and the wrong choice for your climate can cause moisture problems inside walls.

Exhaust-only systems pull air out of the house, with makeup air entering through envelope leaks or passive inlets. The house is slightly depressurized. This suits cold climates, where the goal is to push warm, moist indoor air outward rather than draw humid outdoor air into wall cavities.
Supply-only systems push filtered outdoor air into the house, with stale air escaping through leaks and exhaust paths. The house is slightly pressurized. This suits hot or mixed climates, where incoming air can be filtered and dehumidified before reaching living space.
Balanced systems use separate intake and exhaust fans moving roughly equal volumes, keeping pressure near neutral. This approach works in any climate and is the only one that supports a heat- or energy-recovery ventilator.

For balanced systems, total supply and exhaust airflows must be within 20 percent of each other. If one side moves significantly more air than the other, the pressure effects mimic those of an unbalanced system.

Kitchen and Bathroom Exhaust

Whole-building ventilation dilutes pollutants across the home, but kitchens and bathrooms produce concentrated moisture and odors that need removal at the source. These local exhaust rates run independently of the whole-building system.

Kitchens

A vented range hood must deliver at least 100 CFM when it runs. Other kitchen exhaust devices, such as downdraft systems, must provide 300 CFM or a capacity of five air changes per hour based on kitchen volume, whichever the design achieves. If an intermittent exhaust fan cannot move five air changes per hour, a vented range hood is required. Continuous exhaust in enclosed kitchens must provide five air changes per hour.⁴

Kitchen exhaust must vent directly outdoors. Recirculating range hoods that filter grease and return the air to the kitchen do not satisfy the standard.

Bathrooms

Bathrooms need 50 CFM for intermittent fans or 20 CFM for continuous operation.⁵ These fans can be controlled by a manual wall switch, an occupancy sensor, or a humidistat. A dual-speed fan can serve both roles: low-speed continuous operation for whole-building ventilation, boosted to a higher speed for local exhaust when the shower runs.

Combustion Safety and Makeup Air

Powerful exhaust fans can depressurize a home enough to reverse the draft in a furnace flue or water heater chimney, pulling combustion gases back indoors. If a home has atmospherically vented combustion appliances or solid-fuel-burning appliances inside the envelope, the total exhaust flow of the two largest exhaust fans cannot exceed 15 CFM per 100 square feet of livable space at full capacity.⁶

Direct-vent appliances, which draw combustion air from outside and exhaust through a sealed pipe, are exempt. So are summer cooling fans designed to operate only with windows open. If a range hood and dryer together exceed the threshold, the options are reducing net exhaust flow or providing powered makeup air. Gravity-operated and barometric dampers are specifically prohibited for makeup air, because they do not reliably open at the low negative pressures that cause backdrafting.⁶

Where the Outdoor Air Intake Goes

Intakes that are part of the ventilation design must sit at least 10 feet from known contamination sources, including exhaust vents, plumbing stacks, and areas where vehicles idle. They must also be located where snow, landscaping, or stored materials will not block them.⁷

Several exceptions allow tighter spacing on constrained sites:

A wall intake can be as close as 3 feet, measured as a stretched-string distance, from contamination leaving through the roof or from a dryer exhaust.
An intake can be as close as 5 feet from an exhaust termination serving the same dwelling unit, provided it is at least 2 feet horizontally from any exhaust directed downward and at least 2 feet below any exhaust aimed horizontally toward the inlet.
No minimum separation is required between operable windows and local exhaust outlets in kitchens and bathrooms.
A combined exhaust/intake fitting requires no minimum separation if the manufacturer can demonstrate exhaust concentration in the intake stream stays below 10 percent.

The 5-foot exception does not apply between an intake and any vent serving a combustion water heater or furnace.⁷ Forced air inlets must also include rodent and insect screens with mesh no larger than half an inch.⁸

Air Filtration

Any mechanical system supplying air through more than 10 feet of ductwork and including a heating or cooling component must have a filter rated MERV 6 or higher. All recirculated and mechanically supplied outdoor air must pass through the filter before reaching thermal conditioning equipment. Evaporative coolers are the exception.⁸

Filters have to be installed in a way that minimizes bypass around the frame, and they must be accessible from within the living space. The required filter size and type must be prominently displayed on or near the filter access door, so a future homeowner knows what to buy.⁹ MERV 6 is a floor. Higher-rated filters capture more particulates but create more airflow resistance, so ductwork and fans have to be sized for the pressure drop.

Attached Garage Isolation

Attached garages release carbon monoxide, fuel vapors, and other pollutants that migrate through shared walls, ceilings, and penetrations. All joints, seams, penetrations, and openings between a garage and occupied space must be sealed with caulk, gaskets, weatherstripping, or foam. Doors between the garage and living space must be gasketed or substantially airtight.⁸

HVAC systems serving the living space cannot supply air to or return air from the garage. If the air handler or ducts run through the garage, total duct leakage must not exceed 6 percent of total fan flow when tested at 0.1 inches of water pressure, with the garage door open to outside air during the test.⁸

Fan Sound Limits

Whole-building ventilation fans must operate at 1.0 sone or less, roughly the sound of a quiet refrigerator. Local exhaust fans in kitchens and bathrooms are allowed up to 3.0 sones.¹

Several categories are exempt from these limits:

HVAC air handlers and remote-mounted fans installed outside habitable spaces, bathrooms, and hallways, with at least 4 feet of ductwork between the fan and the nearest intake grille.
Fans serving only a toilet room rather than a full bathroom.
Fans with a minimum airflow setting above 400 CFM, which are exempt from the demand-controlled sound limit.¹⁰

Field Airflow Testing

After installation, a technician measures actual airflow at each vent using calibrated flow hoods or similar instruments to confirm the system delivers the required rates in place.¹¹ Nominal fan ratings almost never match installed performance. A fan rated 110 CFM in a laboratory may deliver 80 CFM at the end of a duct run with two elbows, so designers build in a safety margin at the sizing stage.

For balanced systems, measured supply and exhaust airflows must be within 20 percent of each other. The installed system must match the performance specified on the compliance certificate, and many jurisdictions require a signed testing report before issuing the final mechanical permit.⁵

Documentation for the Homeowner

The standard requires that homeowners receive documentation of what was installed, how it performed at testing, and how to maintain it.⁸ That package must include:

The measured airflow rates from field testing, not just rated fan capacity.
A description of the system type installed and how it operates.
A maintenance schedule for filter replacement, intake screen cleaning, and fan inspection.
Identification of which switches or controls operate which fans.

Manual switches controlling whole-building ventilation must be labeled to alert occupants that the switch controls the home’s air quality system and should generally be left on. Controls for obvious functions such as a toilet exhaust fan do not need additional labeling. Filter housings must display the required filter designation so replacements can be bought without consulting the original mechanical plans.⁹

Applying the Standard to Existing Homes

Retrofitting an older house to the full new-construction rates is not always practical. Appendix A of the standard sets modified requirements for existing homes when local exhaust in kitchens and bathrooms falls short of the standard rates.

For each bathroom lacking a 50 CFM fan and each kitchen lacking a 100 CFM range hood, the shortfall is calculated as an airflow deficit. If the local building authority allows it, an operable window in the room reduces the deficit by 20 CFM. The remaining deficit is totaled across all kitchens and bathrooms, and one-quarter of that total is added to the whole-building mechanical ventilation rate.¹⁰

When exhaust equipment is replaced during a renovation, the new equipment must meet the full local exhaust requirements. The relaxed rules apply only to rooms not being renovated, where existing equipment stays in place. Replacing a single bathroom fan triggers the full 50 CFM requirement for that room even if the rest of the house is grandfathered under Appendix A, so the sequence of which rooms to touch during a renovation has direct consequences for the mechanical scope.

1
ASHRAE. Standards 62.1 and 62.2
2
ASHRAE. Interpretation IC 62.2-2003-1
3
ASHRAE. ANSI/ASHRAE Addendum m to ANSI/ASHRAE Standard 62.2-2013
4
ASHRAE. ANSI/ASHRAE Addendum c to ANSI/ASHRAE Standard 62.2-2013
5
U.S. Department of Energy. ASHRAE Standard 62.2 Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings
6
ASHRAE. 2015 Addenda Supplement to ANSI/ASHRAE Standard 62.2-2013
7
ASHRAE. ANSI/ASHRAE Addendum f to ANSI/ASHRAE Standard 62.2-2022
8
ASHRAE. ANSI/ASHRAE Addendum h to ANSI/ASHRAE Standard 62.2-2019
9
ASHRAE. ANSI/ASHRAE Addendum a to ANSI/ASHRAE Standard 62.2-2022
10
ASHRAE. ANSI/ASHRAE Addendum e to ANSI/ASHRAE Standard 62.2-2022
11
Lawrence Berkeley National Laboratory. Measuring Residential Ventilation System Airflows: Part 2 – Field Evaluation of Airflow Meter Devices and System Flow Verification