Ventilation

This chapter establishes requirements for the ventilation of occupied building spaces, covering natural, mechanical, and specialized systems like those for parking garages and uninhabited spaces.

401 — General

401.1 Scope. This chapter shall govern the ventilation of spaces within a building intended to be occupied. Mechanical exhaust systems, including exhaust systems serving clothes dryers and cooking appliances; hazardous exhaust systems; dust, stock and refuse conveyor systems; subslab soil exhaust systems; smoke control systems; energy recovery ventilation systems and other systems specified in Section 502 shall comply with Chapter 5.

401.2 Ventilation required. Every occupied space shall be ventilated by natural means in accordance with Section 402 or by mechanical means in accordance with Section 403.

401.3 When required. Ventilation shall be provided during the periods that the room or space is occupied.

401.4 Intake opening location. Air intake openings shall comply with all of the following:

Intake openings shall be located a minimum of 10 feet (3048 mm) from lot lines or buildings on the same lot. Where openings front on a street or public way, the distance shall be measured to the centerline of the street or public way.
Mechanical and gravity outdoor air intake openings shall be located not less than 10 feet (3048 mm) horizontally from any hazardous or noxious contaminant source, such as vents, streets, alleys, parking lots and loading docks, except as specified in Item 3 or Section 501.2.1.
Intake openings shall be located not less than 3 feet (914 mm) below contaminant sources where such sources are located within 10 feet (3048 mm) of the opening.
Intake openings on structures in flood hazard areas shall be at or above the design flood level.

401.5 Intake opening protection. Air intake openings that terminate outdoors shall be protected with corrosion-resistant screens, louvers or grilles. Openings in louvers, grilles and screens shall be sized in accordance with Table 401.5, and shall be protected against local weather conditions. Outdoor air intake openings located in exterior walls shall meet the provisions for exterior wall opening protectives in accordance with the International Building Code.

401.6 Contaminant sources. Stationary local sources producing airborne particulates, heat, odors, fumes, spray, vapors, smoke or gases in such quantities as to be irritating or injurious to health shall be provided with an exhaust system in accordance with Chapter 5 or a means of collection and removal of the contaminants. Such exhaust shall discharge directly to an approved location at the exterior of the building.

A table appears here in the adopted code. Refer to the official source for the full table.

402 — Natural Ventilation

[B] 402.1 Natural ventilation. Natural ventilation of an occupied space shall be through windows, doors, louvers or other openings to the outdoors. The operating mechanism for such openings shall be provided with ready access so that the openings are readily controllable by the building occupants.

[B] 402.2 Ventilation area required. The minimum openable area to the outdoors shall be 4 percent of the floor area being ventilated.

[B] 402.3 Adjoining spaces. Where rooms and spaces without openings to the outdoors are ventilated through an adjoining room, the opening to the adjoining rooms shall be unobstructed and shall have an area not less than 8 percent of the floor area of the interior room or space, but not less than 25 square feet (2.3 m2). The minimum openable area to the outdoors shall be based on the total floor area being ventilated.

Exception: Exterior openings required for ventilation shall be permitted to open into a thermally isolated sunroom addition or patio cover, provided that the openable area between the sunroom addition or patio cover and the interior room has an area of not less than 8 percent of the floor area of the interior room or space, but not less than 20 square feet (1.86 m2). The minimum openable area to the outdoors shall be based on the total floor area being ventilated.

[B] 402.4 Openings below grade. Where openings below grade provide required natural ventilation, the outside horizontal clear space measured perpendicular to the opening shall be one and one-half times the depth of the opening. The depth of the opening shall be measured from the average adjoining ground level to the bottom of the opening.

Worked example

Scenario: An architect designs a small, single-story office building where the client prefers to maximize natural ventilation in the workspaces.

Explanation: The design incorporates large operable windows and doors in each occupied office. To comply with §402.1, these openings are easily reachable by occupants, allowing them to control the ventilation manually. The total area of these openings must meet the minimum free area required by code relative to the floor area of the space.

403 — Mechanical Ventilation

403.1 Ventilation system. Mechanical ventilation shall be provided by a method of supply air and return or exhaust air. The amount of supply air shall be approximately equal to the amount of return and exhaust air. The system shall not be prohibited from producing negative or positive pressure. The system to convey ventilation air shall be designed and installed in accordance with Chapter 6.

403.2 Outdoor air required. The minimum outdoor airflow rate shall be determined in accordance with Section 403.3. Ventilation supply systems shall be designed to deliver the required rate of outdoor airflow to the breathing zone within each occupiable space.

Exception: Where the registered design professional demonstrates that an engineered ventilation system design will prevent the maximum concentration of contaminants from exceeding that obtainable by the rate of outdoor air ventilation determined in accordance with Section 403.3, the minimum required rate of outdoor air shall be reduced in accordance with such engineered system design. 403.2.1 Recirculation of air. The outdoor air required by Section 403.3 shall not be recirculated. Air in excess of that required by Section 403.3 shall not be prohibited from being recirculated as a component of supply air to building spaces, except that: Ventilation air shall not be recirculated from one dwelling to another or to dissimilar occupancies. Supply air to a swimming pool and associated deck areas shall not be recirculated unless such air is dehumidified to maintain the relative humidity of the area at 60 percent or less. Air from this area shall not be recirculated to other spaces where more than 10 percent of the resulting supply airstream consists of air recirculated from these spaces. Where mechanical exhaust is required by Note b in Table 403.3, recirculation of air from such spaces shall be prohibited. All air supplied to such spaces shall be exhausted, including any air in excess of that required by Table 403.3. Where mechanical exhaust is required by Note g in Table 403.3, mechanical exhaust is required and recirculation is prohibited where more than 10 percent of the resulting supply airstream consists of air recirculated from these spaces. 403.2.2 Transfer air. Except where recirculation from such spaces is prohibited by Table 403.3, air transferred from occupiable spaces is not prohibited from serving as makeup air for required exhaust systems in such spaces as kitchens, baths, toilet rooms, elevators and smoking lounges. The amount of transfer air and exhaust air shall be sufficient to provide the flow rates as specified in Section 403.3. The required outdoor airflow rates specified in Table 403.3 shall be introduced directly into such spaces or into the occupied spaces from which air is transferred or a combination of both.

Exception: Where the registered design professional demonstrates that an engineered ventilation system design will prevent the maximum concentration of contaminants from exceeding that obtainable by the rate of outdoor air ventilation determined in accordance with Section 403.3, the minimum required rate of outdoor air shall be reduced in accordance with such engineered system design.

403.2.1 Recirculation of air. The outdoor air required by Section 403.3 shall not be recirculated. Air in excess of that required by Section 403.3 shall not be prohibited from being recirculated as a component of supply air to building spaces, except that:

Ventilation air shall not be recirculated from one dwelling to another or to dissimilar occupancies.
Supply air to a swimming pool and associated deck areas shall not be recirculated unless such air is dehumidified to maintain the relative humidity of the area at 60 percent or less. Air from this area shall not be recirculated to other spaces where more than 10 percent of the resulting supply airstream consists of air recirculated from these spaces.
Where mechanical exhaust is required by Note b in Table 403.3, recirculation of air from such spaces shall be prohibited. All air supplied to such spaces shall be exhausted, including any air in excess of that required by Table 403.3.
Where mechanical exhaust is required by Note g in Table 403.3, mechanical exhaust is required and recirculation is prohibited where more than 10 percent of the resulting supply airstream consists of air recirculated from these spaces.

403.2.2 Transfer air. Except where recirculation from such spaces is prohibited by Table 403.3, air transferred from occupiable spaces is not prohibited from serving as makeup air for required exhaust systems in such spaces as kitchens, baths, toilet rooms, elevators and smoking lounges. The amount of transfer air and exhaust air shall be sufficient to provide the flow rates as specified in Section 403.3. The required outdoor airflow rates specified in Table 403.3 shall be introduced directly into such spaces or into the occupied spaces from which air is transferred or a combination of both.

403.3 Outdoor airflow rate. Ventilation systems shall be designed to have the capacity to supply the minimum outdoor airflow rate determined in accordance with this section. The occupant load utilized for design ofthe ventilation system shall not be less than the number determined from the estimated maximum occupant load rate indicated in Table 403.3. Ventilation rates for occupancies not represented in Table 403.3 shall be those for a listed occupancy classification that is most similar in terms of occupant density, activities and building construction; or shall be determined by an approved engineering analysis. The ventilation system shall be designed to supply the required rate of ventilation air continuously during the period the building is occupied, except as otherwise stated in other provisions of the code.

With the exception of smoking lounges, the ventilation rates in Table 403.3 are based on the absence of smoking in occupiable spaces. Where smoking is anticipated in a space other than a smoking lounge, the ventilation system serving the space shall be designed to provide ventilation over and above that required by Table 403.3 in accordance with accepted engineering practice.

Exception: The occupant load is not required to be determined based on the estimated maximum occupant load rate indicated in Table 403.3 where approved statistical data document the accuracy of an alternate anticipated occupant density. 403.3.1 Zone outdoor airflow. The minimum outdoor airflow required to be supplied to each zone shall be determined as a function of occupancy classification and space air distribution effectiveness in accordance with Sections 403.3.1.1 through 403.3.1.3. 403.3.1.1 Breathing zone outdoor airflow. The outdoor airflow rate required in the breathing zone (Vbz) of the occupiable space or spaces in a zone shall be determined in accordance with Equation 4-1. Vbz = RpPz+ RaAz (Equation 4-1) where: Az Zone floor area: the net occupiable floor area of the space or spaces in the zone. Pz Zone population: the number of people in the space or spaces in the zone. Rp People outdoor air rate: the outdoor airflow rate required per person from Table 403.3. Ra Area outdoor air rate: the outdoor airflow rate required per unit area from Table 403.3.

Exception: The occupant load is not required to be determined based on the estimated maximum occupant load rate indicated in Table 403.3 where approved statistical data document the accuracy of an alternate anticipated occupant density.

403.3.1 Zone outdoor airflow. The minimum outdoor airflow required to be supplied to each zone shall be determined as a function of occupancy classification and space air distribution effectiveness in accordance with Sections 403.3.1.1 through 403.3.1.3.

403.3.1.1 Breathing zone outdoor airflow. The outdoor airflow rate required in the breathing zone (Vbz) of the occupiable space or spaces in a zone shall be determined in accordance with Equation 4-1. Vbz = RpPz+ RaAz (Equation 4-1) where: Az Zone floor area: the net occupiable floor area of the space or spaces in the zone. Pz Zone population: the number of people in the space or spaces in the zone. Rp People outdoor air rate: the outdoor airflow rate required per person from Table 403.3. Ra Area outdoor air rate: the outdoor airflow rate required per unit area from Table 403.3.

403.3.1.1 Breathing zone outdoor airflow. The outdoor airflow rate required in the breathing zone (Vbz) of the occupiable space or spaces in a zone shall be determined in accordance with Equation 4-1.

Vbz = RpPz+ RaAz (Equation 4-1)

where:

Az Zone floor area: the net occupiable floor area of the space or spaces in the zone.

Pz Zone population: the number of people in the space or spaces in the zone.

Rp People outdoor air rate: the outdoor airflow rate required per person from Table 403.3.

Ra Area outdoor air rate: the outdoor airflow rate required per unit area from Table 403.3.

A table appears here in the adopted code. Refer to the official source for the full table.

403.3.1.2 Zone air distribution effectiveness. The zone air distribution effectiveness (Ez) shall be determined using Table 403.3.1.2.

403.3.1.2 Zone air distribution effectiveness. The zone air distribution effectiveness (Ez) shall be determined using Table 403.3.1.2.

A table appears here in the adopted code. Refer to the official source for the full table.

403.3.1.3 Zone outdoor airflow. The zone outdoor airflow rate (Voz)' shall be determined in accordance with Equation 4-2.

403.3.1.3 Zone outdoor airflow. The zone outdoor airflow rate (Voz)' shall be determined in accordance with Equation 4-2.

403.3.2 System outdoor airflow. The outdoor air required to be supplied by each ventilation system shall be determined in accordance with Sections 403.3.2.1 through 403.2.3 as a function of system type and zone outdoor airflow rates.

403.3.2.1 Single zone systems. Where one air handler supplies a mixture of outdoor air and recirculated return air to only one zone, the system outdoor air intake flow rate (Vot) shall be determined in accordance with Equation 4-3. Vot=Voz (Equation 4-3) 403.3.2.2 100-percent outdoor air systems. Where one air handler supplies only outdoor air to one or more zones, the system outdoor air intake flow rate (Vot) shall be determined using Equation 4-4. Vot=∑all zonesVoz (Equation 4-4) 403.3.2.3 Multiple zone recirculating systems. Where one air handler supplies a mixture of outdoor air and recirculated return air to more than one zone, the system outdoor air intake flow rate (Vot) shall be determined in accordance with Sections 403.3.2.3.1 through 403.3.2.3.4. 403.3.2.3.1 Primary outdoor air fraction. The primary outdoor air fraction (Zp) shall be determined for each zone in accordance with Equation 4-5. where: Vpz = Primary airflow: The airflow rate supplied to the zone from the air-handling unit at which the outdoor air intake is located. It includes outdoor intake air and recirculated air from that air-handling unit but does not include air transferred or air recirculated to the zone by other means. For design purposes, Vpz shall be the zone design primary airflow rate, except for zones with variable air volume supply and Vpz shall be the lowest expected primary airflow rate to the zone when it is fully occupied. 403.3.2.3.2 System ventilation efficiency. The system ventilation efficiency (Ev) shall be determined using Table 403.3.2.3.2 or Appendix A of ASHRAE 62.1.

403.3.2.1 Single zone systems. Where one air handler supplies a mixture of outdoor air and recirculated return air to only one zone, the system outdoor air intake flow rate (Vot) shall be determined in accordance with Equation 4-3.

Vot=Voz (Equation 4-3)

403.3.2.2 100-percent outdoor air systems. Where one air handler supplies only outdoor air to one or more zones, the system outdoor air intake flow rate (Vot) shall be determined using Equation 4-4.

Vot=∑all zonesVoz (Equation 4-4)

403.3.2.3 Multiple zone recirculating systems. Where one air handler supplies a mixture of outdoor air and recirculated return air to more than one zone, the system outdoor air intake flow rate (Vot) shall be determined in accordance with Sections 403.3.2.3.1 through 403.3.2.3.4.

403.3.2.3.1 Primary outdoor air fraction. The primary outdoor air fraction (Zp) shall be determined for each zone in accordance with Equation 4-5. where: Vpz = Primary airflow: The airflow rate supplied to the zone from the air-handling unit at which the outdoor air intake is located. It includes outdoor intake air and recirculated air from that air-handling unit but does not include air transferred or air recirculated to the zone by other means. For design purposes, Vpz shall be the zone design primary airflow rate, except for zones with variable air volume supply and Vpz shall be the lowest expected primary airflow rate to the zone when it is fully occupied. 403.3.2.3.2 System ventilation efficiency. The system ventilation efficiency (Ev) shall be determined using Table 403.3.2.3.2 or Appendix A of ASHRAE 62.1.

403.3.2.3.1 Primary outdoor air fraction. The primary outdoor air fraction (Zp) shall be determined for each zone in accordance with Equation 4-5.

where:

Vpz = Primary airflow: The airflow rate supplied to the zone from the air-handling unit at which the outdoor air intake is located. It includes outdoor intake air and recirculated air from that air-handling unit but does not include air transferred or air recirculated to the zone by other means. For design purposes, Vpz shall be the zone design primary airflow rate, except for zones with variable air volume supply and Vpz shall be the lowest expected primary airflow rate to the zone when it is fully occupied.

403.3.2.3.2 System ventilation efficiency. The system ventilation efficiency (Ev) shall be determined using Table 403.3.2.3.2 or Appendix A of ASHRAE 62.1.

A table appears here in the adopted code. Refer to the official source for the full table.

403.3.2.3.3 Uncorrected outdoor air intake. The uncorrected outdoor air intake flow rate (Vou) shall be determined in accordance with Equation 4-6. You = D Lallzones RpPz + Lallzones RcAz (Equation 4-6) where: D Occupant diversity: the ratio of the system population to the sum ofthe zone populations, determined in accordance with Equation 4-7. where: Ps = System population: The total number of occupants in the area served by the system. For design purposes, Ps shall be the maximum number of occupants expected to be concurrently in all zones served by the system. 403.3.2.3.4 Outdoor air intake flow rate. The outdoor air intake flow rate (Vot) shall be determined in accordance with Equation 4-8.

403.3.2.3.3 Uncorrected outdoor air intake. The uncorrected outdoor air intake flow rate (Vou) shall be determined in accordance with Equation 4-6.

You = D Lallzones RpPz + Lallzones RcAz (Equation 4-6)

where:

D Occupant diversity: the ratio of the system population to the sum ofthe zone populations, determined in accordance with Equation 4-7.

where:

Ps = System population: The total number of occupants in the area served by the system. For design purposes, Ps shall be the maximum number of occupants expected to be concurrently in all zones served by the system.

403.3.2.3.4 Outdoor air intake flow rate. The outdoor air intake flow rate (Vot) shall be determined in accordance with Equation 4-8.

403.4 Exhaust ventilation. Exhaust airflow rate shall be provided in accordance with the requirements in Table 403.3. Exhaust makeup air shall be permitted to be any combination of outdoor air, recirculated air and transfer air, except as limited in accordance with Section 403.2.

403.5 System operation. The minimum flow rate of outdoor air that the ventilation system must be capable of supplying during its operation shall be permitted to be based on the rate per person indicated in Table 403.3 and the actual number of occupants present.

403.6 Variable air volume system control. Variable air volume air distribution systems, other than those designed to supply only 100-percent outdoor air, shall be provided with controls to regulate the flow ofoutdoor air. Such control system shall be designed to maintain the flow rate of outdoor air at a rate of not less than that required by Section 403.3 over the entire range of supply air operating rates.

403.7 Balancing. The ventilation air distribution system shall be provided with means to adjust the system to achieve at least the minimum ventilation airflow rate as required by Sections 403.3 and 403.4. Ventilation systems shall be balanced by an approved method. Such balancing shall verify that the ventilation system is capable of supplying and exhausting the airflow rates required by Sections 403.3 and 403.4.

Figure from IMC 2009 Section 403.

Worked example

Scenario: A design team is specifying a mechanical ventilation system for a new open-plan office on the second floor of a commercial building.

Explanation: The system will be designed to supply conditioned outdoor air to the space and return/exhaust an approximately equal amount of air. While a perfectly balanced system is ideal, the code allows for minor differences, enabling the space to maintain a slightly positive or negative pressure if beneficial for controlling airflow direction.

404 — Enclosed Parking Garages

404.1 Enclosed parking garages. Mechanical ventilation systems for enclosed parking garages shall be permitted to operate intermittently where the system is arranged to operate automatically upon detection of vehicle operation or the presence of occupants by approved automatic detection devices.

404.2 Minimum ventilation. Automatic operation of the system shall not reduce the ventilation airflow rate below 0.05 cfm per square foot (0.00025 m3/s· m2) ofthe floor area and the system shall be capable of producing a ventilation airflow rate of 0.75 cfm per square foot (0.0076 m3/s. m2) of floor area.

404.3 Occupied spaces accessory to public garages. Connecting offices, waiting rooms, ticket booths and similar uses that are accessory to a public garage shall be maintained at a positive pressure and shall be provided with ventilation in accordance with Section 403.3.

Worked example

Scenario: An architect is designing a two-level underground parking garage for a mixed-use development.

Explanation: Instead of running the ventilation fans continuously, the design incorporates carbon monoxide (CO) sensors and vehicle occupancy detectors throughout the garage. These devices automatically activate the ventilation system only when elevated CO levels or vehicle movement/presence is detected, thereby saving energy while still meeting the ventilation requirements, as permitted by §404.1.

405 — Systems Control

405.1 General. Mechanical ventilation systems shall be provided with manual or automatic controls that will operate such systems whenever the spaces are occupied. Air-conditioning systems that supply required ventilation air shall be provided with controls designed to automatically maintain the required outdoor air supply rate during occupancy.

Worked example

Scenario: A new retail store is being designed with an HVAC system that provides both heating/cooling and required ventilation air.

Explanation: The HVAC system will be equipped with controls that ensure the mechanical ventilation (supply of outdoor air) operates whenever the store is occupied, typically via a time schedule or occupancy sensors. Furthermore, the controls are designed to automatically maintain the required quantity of outdoor air, as mandated by §405.1.

406 — Ventilation of Uninhabited Spaces

406.1 General. Uninhabited spaces, such as crawl spaces and attics, shall be provided with natural ventilation openings as required by the International Building Code or shall be provided with a mechanical exhaust and supply air system. The mechanical exhaust rate shall be not less than 0.02 cfm per square foot (0.00001 m3/s · m2) of horizontal area and shall be automatically controlled to operate when the relative humidity in the space served exceeds 60 percent.

Worked example

Scenario: A residential builder is constructing a house with a crawl space and a non-vented attic.

Explanation: For the crawl space, the builder can either provide natural ventilation openings as per the IBC or install a mechanical exhaust and supply air system. If a mechanical system is chosen, the exhaust rate must be at least 0.02 cfm per square foot of crawl space area, as specified in §406.1, to prevent moisture buildup and ensure air quality.

Frequently asked questions

What are the requirements for naturally ventilating a space?

Natural ventilation must be provided through operable openings like windows, doors, or louvers, which must be readily accessible and controllable by occupants.

How does mechanical ventilation balance supply and exhaust air?

Mechanical ventilation systems should provide approximately equal amounts of supply air and return/exhaust air, though slight negative or positive pressure is permitted.

Can parking garage ventilation systems run intermittently?

Yes, mechanical ventilation systems for enclosed parking garages can operate intermittently if activated automatically by vehicle detection or occupant presence.