Determine ventilation requirements

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To determine ventilation requirements for your project, a bit of homework will be needed because ventilation requirements will vary depending on the project location. Ventilation codes are usually adopted based on what is included in the mechanical code. This may sometimes be a direct adoption of ASHRAE Standard 62.1, but often the requirements will be a modified version of ASHRAE 62.1. Specific requirements may vary from state to state or even city to city. If there is a mechanical (HVAC) engineer involved in the project who has experience working on other projects in the same city or state, they will likely be able to tell you which mechanical code is in effect and refer you to the ventilation requirements.

If the weather conditions are appropriate, natural ventilation may be used to reduce energy consumption associated with mechanical ventilation.

Relevance to BEM Practitioner During Early Stages

Ventilation rates can make up a large portion of the HVAC system loads in both hot and cold climates. The outside air required for providing suitable indoor air quality to the building occupants must usually be heated or cooled, and sometimes dehumidified or humidified), so that it doesn't lead to adverse thermal comfort conditions. During periods of mild weather, outdoor air rates can be increased to allow for free cooling (via an airside economizer in the HVAC system), or buildings with operable windows may take advantage of natural ventilation.

Because of the significance of ventilation loads, it is important, even in the early project stages, to use a reasonable estimate of the ventilation rates in the energy model. Some software tools may have ASHRAE 62.1 ventilation calculations built-in to the tools, although state/city-specific modifications may not be available. The basic ventilation rates in 62.1 haven’t changed too drastically since ASHRAE 62.1-2004, and most codes are based on some vintage of 62.1.  Therefore, if software has some vintage of 62.1 data built-in, it should provide a realistic representation during early stage analysis. However, small variations will still result in measurable energy impacts, so it will be important to refine the inputs after the early stages because the ventilation rates used in the energy model are likely to be reviewed carefully for projects seeking code compliance or participating in beyond-code programs.

Calculations and Inputs for Mechanical Ventilation Rates

Ventilation calculations require a two-step process where, first, ventilation requirements are determined for each zone in the building, and second, the HVAC systems are analyzed to define how they will effectively provide the ventilation to each zone. Refer to the tutorial below for more information on how to input ventilation rates.

Considerations for Early-Stage Design

Assumptions for Space Types

Ventilation rate requirements are variable depending on the type of space in the building. For example, a conference room will have people packed into the space and need more outside air than a single-person private office. The conference room is also often empty, so it may utilize occupancy or CO2 sensors to reduce outside air flow during unoccupied periods. However, during early stages, floor plans may not yet be designed and detailed space or zoning diagrams may not yet be determined, so assumptions must often be made.

If it is still so early that the building programming information is not yet available, then it is may be reasonable to create a model where all zones in the model are assigned a ventilation category most similar to the building type itself. For example, an office building could use the "office space" ventilation rates throughout the whole model. However, even making some basic assumptions about area percentages used for other purposes (corridors, conference rooms) would result in a more appropriate model, so if you (or another project team member) have experience with similar building types, then consider using similar area percentages as in past projects.

If the building programming information is available, then use the space percentages defined in the program. The modeling approach should be to assign ventilation rates to an appropriate area percentage of the building according to the program (or your best assumption). While the exact location (on the floor plan) of each zone type may still be unknown, at least the outdoor air rates will be appropriate for calculating the effects on HVAC system energy.

Software Tool Selection

Because ventilation calculations are a bit complex in terms of defining rates at the zone level, and at the HVAC system level, software tools that have built-in capabilities for performing ASHRAE 62.1 calculations based on simple space-type categorization can reduce modeling time (and spreadsheet calculation time) significantly.

Impacts of Increased (Mechanical) Ventilation Rates

Cost Impacts

Many studies suggest that providing additional ventilation (beyond the code requirements) can lead to improved work performance, reduced sick building syndrome symptoms, and reduced absence of employees.[1][2]

However, additional ventilation will result in additional energy consumption for the heating and cooling systems to temper the outdoor air before supplying it to the building spaces. The above studies highlight that the financial benefits associated with improved productivity greatly outweigh the additional energy costs, however energy impacts are important to understand.

Energy Code Compliance Impacts

Projects seeking code compliance using the performance approach may be negatively impacted by increased ventilation rates, depending on the version of the energy code in effect, so be sure to understand the requirements. ASHRAE 90.1's Energy Cost Budget (ECB) approach specifies that ventilation rates should be the same between the baseline and proposed design models, so no penalty is imposed. Similarly, California's Title 24 treats ventilation as a neutral input (baseline = proposed). ASHRAE 90.1's Performance Rating Method (PRM) treated ventilation as a neutral input in older versions, but starting with 90.1-2010, this is no longer the case.

The requirement of the PRM now states that: "If the minimum outdoor air intake flow in the proposed design is provided in excess of the amount required by the rating authority or building official then the baseline building design shall be modeled to reflect the greater of that required by the rating authority or building official and will be less than the proposed design."[3]

Natural Ventilation

Natural ventilation, unlike fan-forced ventilation, uses the natural forces of wind and buoyancy to deliver fresh air into buildings.[4] Energy savings may occur due to reduced fan energy, and reduced heating and cooling energy associated with tempering the outside air.

In the context of ASHRAE 62.1 ventilation requirements, there is a prescriptive approach for designing spaces that can be designated as naturally ventilated (and therefore not require mechanical ventilation). The prescriptive requirements dictate the size of openings along the facade (e.g. operable windows or louvers), and the distance from the openings within which a space is deemed to have acceptable ventilation.

The 2019 version of ASHRAE 62.1 introduces a performance-driven calculation approach for determining which spaces will receive adequate natural ventilation. It is called the Engineered System Compliance Path. It is less restrictive on design, but requires the design team to prepare detailed hourly calculations to demonstrate that adequate outside air is being supplied and not having a detrimental effect on air quality, or comfort. BEM software tools that include bulk-airflow capabilities could be used to calculate hourly data and contribute documentation required for this compliance path.

Refer to the Additional Resources section below for documents that provide further guidance on these calculation approaches.

Additional Resources

  • ASHRAE 62.1 - This document includes the calculation procedure for determining outside air ventilation requirements. A preview version of the 2019 Standard can be found here: 62.1-2019 Preview Version
  • ASHRAE 62.1 User's Manual - This document helps to apply the 62.1 requirements appropriately by providing more detailed explanations of the Standard's requirements, along with calculation examples.
  • 90.1 Performance Rating Method Reference Manual (PRM-RM) - The PRM-RM provides a detailed description of how to apply the 62.1 Ventilation Rate Procedure within the context of an energy code compliance model. This material can be found in section
  • Whole Building Design Guide - offers a nice primer on natural ventilation in buildings with a discussion of the building physics of wind-driven and buoyant effects, and design recommendations for naturally ventilated buildings.
  • ASHRAE 62.1: Using the Ventilation Rate Procedure - article from CSE Magazine with practical example of applying the 62.1 ventilation rate procedure.
  • The Feasibility of Natural Ventilation - a presentation by Arup at the San Joaqin ASHRAE Chapter discussing many challenges and approaches of natural ventilation designs.

Below are a few resources that may be helpful for determining which version of the ventilation codes apply at your project's location:

  • International Code Council – ICC compiles specific codes for certain states and allows users to view them for free. This does not include all state codes but does cover a broad swath of the country.
  • UpCodes – Similar to ICC, UpCodes makes many state-specific codes available in one place. However, in order to access all of the content, a subscription is required.

Links to external websites are provided as a convenience for further research, but do not imply any endorsement of the content or the operator of the external site, as detailed in BEMcyclopedia's general disclaimers.


  1. "Increased Ventilation Rates". LBNL Indoor Air Quality Scientific Findings Resource Bank.
  2. McArthur. "Rethinking ventilation: A benefit-cost analysis of carbon-offset increased outdoor air provision". Building and Environment. 169.
  3. "ASHRAE 90.1".
  4. "Natural Ventilation". Whole Building Design Guide.
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