Interpreting ASHRAE 62.1

ASHRAE Standard 62.1 is best known for its regulation of the amount of ventilation air delivered to each space in a commercial building by HVAC systems through various ventilation approaches to system design

By Carol Qing Li, PE, LEED AP, Interface Engineering, Los Angeles August 23, 2019

Learning Objectives

  • Understand the evolution of the ASHRAE 62.1 ventilation standards to eliminate confusion about changes. 
  • Review available tools for calculating air quality and ventilation needs. 
  • Understand best practices with pros and cons to three ventilation compliance procedures. 

All building components, including envelope, lighting, HVAC system, building automation and controls, outside air treatment and domestic water heating play a role in creating a sustainable environmentConditioning outside air for ventilation is a big contributor to energy use in a building and is one of the primary factors in achieving sustainable and integrated design. 

Numerous studies spanning several decades have documented the adverse health effects associated with indoor air pollution, ranging from minor discomfort, decreased worker productivity, to respiratory illness and even cancer. There are many ways to improve indoor air quality, including source contaminant control, keeping HVAC systems clean, filtration systems and increased ventilation rates. 

ASHRAE Standard 62: Ventilation for Acceptable Indoor Air Quality and its subsequent incarnations as Standard 62.1 (commercial and institutional) and Standard 62.2 (residential), serves as one of the most prominent ventilation standards and sets up the minimum ventilation requirements to provide IAQ that is acceptable to human occupants to minimize adverse health effects. 

ASHRAE Standard 62.1 includes four separate partsThe standard itself has been written in mandatory and enforceable language since 1999. The standard by itself is not a code nor it is enforceable, unless it has been adopted or referenced by an authority having jurisdiction. Portions of this standard, such as ventilation rate tables and formulas, have already been incorporated into some of the model building codes such as international mechanical code and thus become mandatory requirements.  

Appendices include two types: normative appendices and informative appendices. Normative appendices are part of the standard and provide alternative procedures for compliance to the standard. Informative appendices are merely intended to provide knowledge with no conformance requirements. ASHRAE 62.1-2016 includes appendix A through K, with only Appendix A and B being normative appendices and part of the standard. 

The user’s manual is intended to be used as a companion to Standard 62.1, which is written to be code enforceable and therefore contains only mandatory language. The user manual contains explanatory material, detailed information, figures and examples to aid user in designing, installing and operating buildings. It provides information on how to use and apply Standard 62.1 with practical examples of compliance and explains the basis of ventilation rate requirement and calculation methods. 

The guideline covers design, construction and commissioning of IAQ. Going far beyond the minimum requirements, it provides information and tools building designers can use to achieve IAQ sensitive building design integrated with other design goals, budget constraints and functional requirements. 

ASHRAE Standard 62 has served as one of the most prominent ventilation standards since it was first published in 1973. The standard is updated regularly by the ASHRAE Standing Standard Project Committee and has been revised every three years since 2001. The standard has undergone key revisions over the years, reflecting the expanding knowledge, experiences and research related to ventilation and air quality.  

The purpose of the standard remains unchanged, which is to specify minimum ventilation rates and other measures intended to provide IAQ acceptable to human occupants by minimizing adverse health effects.  

Means of ventilation 

Building ventilation has long been recognized for its role in occupant health, comfort and productivity. Ventilation is the intentional introduction of outside air into or removing air from a space and is mainly used to control IAQ by diluting and displacing or removing indoor pollutants. Ventilation of a building can be achieved by means of natural ventilation, mechanical ventilation or combination of both. 

Natural ventilation refers to intentionally designed passive methods of introducing outside air to a space without the use of mechanical system. Natural ventilation can only be achieved within certain distance of a building perimeter and there is no treatment to the outside air. 

Mechanical ventilation refers to any system that uses mechanical means, such as fans, to introduce outside air into a space or removing air from a space. This includes positive pressure ventilation, exhaust ventilation and balanced systems that use both supply and exhaust ventilation. In addition, outside air will normally be treated prior introducing to a building. 

Outdoor air quality 

Part 1 to 3 of the standard is about purpose, scope and definitions. Part 4 of ASHRAE Standard 62.1 —defined acceptable air quality for ventilating buildings based on U.S. federal criteria  promulgated in 1975 for several outdoor contaminants. Ventilation air also is considered not acceptable if the concentration of any contaminant exceeded one-tenth of the threshold limit value issued by the American Conference of Governmental Industrial Hygienists. If the outdoor air did not meet these requirements, filtration or other air treatment was required to meet these criteria. In addition to the U.S. Environmental Protection Agency National Ambient Air Quality Standard requirements for outdoor air contaminant levels, ASHRAE Standard 62-1981 contained a table of limits for 28 additional compounds derived from current practices in various state, provinces and other countries. 

In earlier versions of ASHRAE Standard 62, the requirement for outdoor air filtration or air cleaning when outdoor air did not meet the criteria was unspecific. Standard 62.1-2004 addressed outdoor air quality in much improved code language, requiring an assessment of outdoor air quality and nearby contaminant sources in all buildingsOutdoor air not meeting EPA standards called for prescribed levels for PM10, PM2.5 and ozone. (PM10 is particulate matter 10 micrometers or less in diameter; PM2.5 is particulate matter 2.5 micrometers or less in diameter.) 

ASHRAE 62.1-2016 requires that when PM10 exceeds EPA standard, a minimum efficiency reporting value 6 filter shall be provided to clean the outdoor air at any location before its introduction to occupied space. If PM2.5 cannot satisfy EPA requirements, MERV 11 filter shall be provided before outdoor air being introduced to a building. Air cleaning devices having 40% volumetric ozone removal efficiency shall be provided if ozone cannot meet EPA outdoor air quality requirements.  

EPA air data gives access to air quality data collected at outdoor monitors across the United States.﷟HYPERLINK “” 

Outdoor air distribution system and equipment 

The 1981 version of Standard 62 added a number of requirements related to ventilation systems and equipment. The requirements addressed a range of important topics including location of air intakes to avoid cross-contamination, acceptable duct materials and construction and capturing indoor contaminants as close to the source as practicable. 

The requirement to avoid entrainment of outdoor contaminates was replaced by a table of minimum separation distances from outdoor air intakes to various outdoor sources. The separation distance requirements are presented in Table 5.5.1 and further explanation and information has been provided in Appendix B. 

The requirements to deliver ventilation air to occupants were replaced by air change and system efficiency factors used to calculate outdoor air intake rates. Section 5 of Standard 62.1 addresses the following for systems and equipment: 

  • Ventilation air distribution: Provide means to adjust airflows to ensure outdoor airflow reaches each zone as required at any load condition. 
  • Exhaust duct location: Design negative pressure exhaust ducts to prevent leakage of potentially harmful exhaust contaminants into the building. 
  • Ventilation system controls: Design to ensure minimum ventilation at any load condition. 
  • Airstream surfaces: Use duct materials that resist microbial growth and erosion per UL 181ASTM C1338 or ASTM D3273. 
  • Outdoor air intakes: Require minimum separation distance from outside air intakes to various outdoor sources and protect outside air intake openings to prevent rain intrusion, snow entrainment and bird nesting.  
  • Local capture of contaminants: Reduce risk of recirculation of indoor contaminants by providing local exhaust directly to outdoors.  
  • Combustion air: Reduce pollutant from combustion appliances by providing sufficient outside air for combustion and sufficient air for removal of combustion products. 
  • Particulate matter removal: Must provide MERV 8 filter upstream of wet surface devices such as cooling coils. 
  • Dehumidification systems: Reduce dampness in buildings during mechanical cooling. For system with dehumidification, the relative humidity in occupied space should be limited to 65% maximum relative humidity. 
  • Drain pans: Ensure proper condensate drainage without moist air carry-over and/or flooding 
  • Finned tube coils and heat exchangers: Reduce dirt accumulation and resulting water droplet carry-over and potential microbial growth. 
  • Humidifiers and water spray systems: Reduce waterborne contaminants. Ensure water purity for water spray systems including steam and direct-evaporative humidifiers and other water spray systems. 
  • Access for inspection, cleaning and maintenance: Reduce dirt accumulation in air distribution system. 
  • Building envelope and interior surfaces: Reduce intrusion of water and water vapor and uncontrolled condensation on interior surfaces. 
  • Buildings with attached parking garages: Reduce infiltration of vehicle exhaust. 
  • Air classification and recirculation: Reduce recirculation of dirty air to cleaner spaces. 

Ventilation requirements and compliance procedures 

Increased ventilation can improve IAQ, however it also may increase building energy use if not design carefully. Recommendations for minimum outdoor air date back to early 19th century when Thomas Tredgold published an estimate of 4 cubic feet per minute per person.  

In 1895, the American Society of Heating and Ventilating Engineers, now ASHRAE, adopted a minimum recommendation of 30 cfm per person. In 1914, ASHVE proposed a model code requiring 30 cfm per person as the minimum. By 1925, 22 states had adopted the requirement. The first ASHRAE Standard 62 appeared in 1973. In 1981 the standard was updated and reduced the outdoor air in response to the oil crises and energy conservation concerns. In the 1989 update to ASHRAE Standard 62, the minimum acceptable ventilation rate increased from 5 cfm per person to 15 cfm per person 

A new methodology for determining ventilation requirements was developed and first included in the 2004 standard. The per person ventilation requirements were selected to control contaminant sources associated with the number of occupants, including but not limited to body odor. The floor area requirements were based on contaminant sources associated with the size of space, such as materials and furnishings.  

There are three compliance approaches: 

  1. IAQprocedure is a performance-based design procedure in which the building outdoor air intake rates and other system design parameters are based on an analysis of contaminant sources, contaminant concentration limits and level of perceived indoor air acceptability. 

When using the IAQ procedure, all contaminant concentration and exposure values need to be documented and justified by reference to a cognizant authority. At present, no single organization develops acceptable concentrations or exposures for all indoor air contaminants, nor are values available for all contaminants of potential concern. In addition, meeting one, some or all of the listed values does not ensure that acceptable IAQ, as defined in the standard, will be achieved.  

However, with the development of new tools and researches on the contaminant source and its impact, this approach has been evolving now and being considered by project teams as a practical approach. 

  1. Ventilationrate procedure: This is a prescriptive-based procedure to design ventilation systems. The outdoor intake rates are determined based on space type/application, occupancy level and floor area. 
  1. Naturalventilation procedure: This is the prescriptive design procedure in which outdoor air is provided through openings to the outdoors. The openings shall satisfy the requirements of the standard in terms of dimension and distance to the outdoors. Combination of natural and mechanical ventilation methods can be used to provide adequate ventilation of a zone. 

Any of these procedures may be used to satisfy the ventilation required and are viewed as being a valid basis of design as long as they are properly documented to show compliance 

Ventilation rate procedure, however, is the most common method used for commercial building design. Calculation can be performed on zone basis or system basis. 

The 62MZCalc spreadsheet from the U.S. Green Building Council has been distributed together with ASHRAE 62.1 User Manual, which can be used to determine the ventilation rate for systems that serve multiple zones and that recirculate air from one or more zone to other zones. The spreadsheet calculates the ventilation rate for IAQ prerequisites (minimum requirement for compliance with ASHRAE Standard 62.1) and for LEED IAQ credit for enhanced ventilation.  


Documentation of all aspects of the project ventilation is required as it is defined within the standards.  

Appendix H summarizes the requirements of documentation contained in the body of the standard using a series of templates that summarize design criteria used and assumptions made to comply with Standard 62.1.  

Documentation for the IAQ procedures requires the design documents to include: 

  • Outdoor air qualityThe result of outdoor air quality will form the basis of ventilation strategy and ventilation system design. In particular, the outside air quality may require special air cleaning on the mechanical ventilation system. 
  • Ventilation criteria of the design: 
    • Ventilation rate as established in ASHRAE Standard 62.1 Table is a minimum requirement.  
    • Enhanced ventilation to obtain LEED credit. 
    • Client special requirement could set higher ventilation criteria.
  • Methodology used for compliance requires one to identify basic mechanical system features such as: 
    • Singlezone 100% outdoor air system. 
    • Underfloor, overhead, sideair distribution and return grille location. 
    • Supply air temperature, i.e., cooling or heating mode for ventilation analysis. 
    • Variable air volume or constant volume system. 
  • Calculate required outdoor air flow: 
    • Complete a separate ventilation rate procedure calculation for each ventilation system. 
    • Account for all occupied spaces in the calculation. Perform ventilation rate procedure calculations for worst case condition, which typically occur in the heating mode when supply airflow is lowest. 
    • Evaluate and document assumptions for all variables required for ventilation rate procedure calculation. For example, the diversity factor used for occupant, fraction of local recirculated air, etc. 
    • Special conditions corresponding to the system operation, such as demand ventilation control, etc. 
  • Document the compliance. 

Author Bio: Carol Qing Li an associate/senior mechanical engineer with Interface Engineering, and was previously with DLR Group. She has practiced engineering in Australia, China, New Zealand and the United States.