DPWES Letters to Industry

Land Development Services
12055 Government Center Parkway
Fairfax, Virginia 22035
703-324-1980, TTY 711

January 12, 2005

Ventilation Air Policy, Intermittent and Variable Occupancies, Occupant Load Calculations


The Virginia Uniform Statewide Building Code (USBC) incorporates by reference the International Mechanical Code (IMC), 2000 edition, which in turn prescribes the minimum amount of outdoor air required for the ventilation of various indoor spaces - based on the occupancy of each such space and its estimated maximum occupant load or other parameters as prescribed by the IMC. The IMC’s tabular values for Required Outdoor Ventilation Air are derived from ASHRAE 62-1989, although ASHRAE 62-1989 itself is not incorporated within the VUSBC. In occupancies where the primary sources of air contamination are the occupants themselves, the prescribed ventilation rates (mathematically) limit the rise of carbon dioxide within such spaces to about 700 parts per million (ppm) above ambient outdoor air levels. Thus, if the outdoor air has a carbon dioxide level of 300 ppm (as presumed in ASHRAE 62-1989), a steady state condition of about 1,000 ppm carbon dioxide will be reached within a space with an occupant loading and outdoor ventilation rate prescribed for that space by the IMC. ASHRAE 62-1989 states: “Comfort (odor) criteria are likely to be satisfied if the ventilation rate is set so that 1,000 ppm carbon dioxide is not exceeded.” ASHRAE 62-1989 also implies that short-term exposure to contaminant levels as high as 1.25 times the steady-state value (e.g., 1.25 times 1,000 ppm carbon dioxide) should be safe, provided the required air quality (e.g. 1,000 ppm carbon dioxide, maximum) is provided over the entire occupancy period. In cases of “intermittent or variable occupancy,” it is sometimes possible to provide ventilation rates lower than those prescribed by the IMC and still maintain indoor air quality comparable to that described above.


In cases of “intermittent or variable occupancy” where the primary sources of air contamination are the occupants themselves, and outdoor ventilation air is the principal means of maintaining indoor air quality, equivalence to the IMC prescribed ventilation rates may be demonstrated through mathematical modeling of carbon dioxide concentrations within the space. Carbon dioxide concentrations so modeled must be shown to be no higher than 750 ppm above outdoor ambient conditions (which may be assumed at 400 ppm for this locality), with exceedance of this threshold being limited to 15 minutes and 1,000 ppm above outdoor ambient conditions during a single occupied period. “Average occupancy” calculations, such as described in ASHRAE 62-1989 Section, may not be used as an alternative for demonstrating equivalence.

A computer program has been developed for modeling indoor air quality in "intermittent and variable occupancy" situations that will be used to verify design and supporting calculations. Should you request a “code modification” for reduced ventilation capacities in qualifying situations, we will use this program to check your design and supporting calculations during the permit application process. Copies of the computer program are available from Land Development Services, by sending a request to:

Guy Tomberlin, Chief Plumbing Inspector
Commercial Inspections Division
12055 Government Center Parkway, Suite 630
Fairfax, Virginia 22035-5504
Phone: 703-324-1910
TTY: 711
Fax: 703-324-1846

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