ASHRAE 62.1 (Ventilation and Acceptable Indoor Air Quality)
ASHRAE 62.1 is the American Society of Heating, Refrigerating and Air-Conditioning Engineers standard governing ventilation rates and indoor air quality in commercial and institutional buildings. It defines the minimum outdoor air ventilation rate per zone calculated from the zone area, the zone occupancy, and the zone air distribution effectiveness. The standard is referenced by the International Mechanical Code (IMC) and most state building codes; compliance is mandatory for new construction and major renovation in most U.S. jurisdictions.
What is the Ventilation Rate Procedure?.
The VRP calculates the required outdoor air rate (Voz) per zone as Voz = (Rp × Pz + Ra × Az) / Ez, where Rp is the people component (CFM per person), Pz is the zone population, Ra is the area component (CFM per square foot), Az is the zone area, and Ez is the zone air distribution effectiveness. The people and area rates depend on the occupancy classification (office space, classroom, retail, hospital, laboratory). The zone air distribution effectiveness depends on the supply and return air configuration (ceiling supply with ceiling return, ceiling supply with floor return, floor supply with ceiling return). System-level outdoor air calculations roll up zone calculations through a ventilation efficiency factor.
What is the IAQ Procedure?.
The IAQ Procedure is an alternative to the VRP that calculates outdoor air based on maintaining specific contaminant concentrations below defined limits (CO2, formaldehyde, VOCs, particulate matter). The IAQ Procedure is more design-intensive and requires monitoring and documentation; it is used where the VRP rate is impractical or where unusual contaminants are present. Combination procedures (VRP for baseline plus IAQ adjustments for specific contaminants) are also recognized.
What is demand-controlled ventilation (DCV)?.
DCV uses CO2 sensors to modulate outdoor air based on actual occupancy rather than design occupancy. When a zone is under-occupied, the outdoor air rate reduces to maintain the area component; when occupancy increases (and CO2 rises), the outdoor air rate increases to maintain the people component. ASHRAE 62.1-2022 expanded DCV applicability and added explicit calibration requirements for CO2 sensors. DCV is a primary energy-saving strategy in commercial buildings with variable occupancy.
Frequently asked.
What is the difference between ASHRAE 62.1 and ASHRAE 62.2?
ASHRAE 62.1 governs commercial and institutional buildings. ASHRAE 62.2 governs single-family and low-rise multifamily residences. The two standards parallel each other in approach but apply to different building types; the residential rates are typically lower and the procedures simpler.
Does ASHRAE 62.1 apply internationally?
ASHRAE 62.1 is a U.S. standard but is referenced internationally as a benchmark. European projects typically reference EN 16798-3 (formerly EN 13779) which has similar structure but different numerical values. Middle Eastern, Asian, and Latin American projects sometimes cite ASHRAE 62.1 directly or use it as the basis for project specifications.
What are typical Rp and Ra values?
For office space, Rp is 5 CFM per person and Ra is 0.06 CFM per square foot. For a classroom (ages 9 plus), Rp is 10 CFM per person and Ra is 0.12. For a hospital patient room, Rp is 25 CFM per person and Ra is 0.18. The 2022 edition includes a comprehensive table of occupancy categories with their Rp and Ra values; the values reflect both bioeffluent dilution (Rp) and material-emission dilution (Ra).
Is CO2 the contaminant ASHRAE 62.1 controls?
No. CO2 is used as a proxy for human occupancy (people generate CO2 in proportion to their metabolic rate) and as a DCV control signal, but ASHRAE 62.1 itself does not specify a maximum CO2 concentration. The VRP delivers a ventilation rate that, in a steady-state model, limits CO2 to approximately 700 ppm above outdoor levels. The IAQ Procedure includes specific contaminant limits for compounds where data exists.