What is ASHRAE Standard 62.1?
ASHRAE 62.1, 'Ventilation for Acceptable Indoor Air Quality,' dictates the minimum outdoor air requirements for commercial and institutional buildings. It balances the need to exhaust bioeffluents (people odors/CO2) and off-gassing from building materials against the energy costs of conditioning outside air.
The most common method for calculating these rates is the Ventilation Rate Procedure (VRP). The VRP is prescriptive, meaning engineers apply specific mathematical formulas based on the space type and occupancy.
How to Calculate Breathing Zone Ventilation (Vbz)
The foundation of the VRP is calculating the outdoor airflow required in the breathing zone (Vbz) of a single space. This requires two components: the people outdoor air rate and the area outdoor air rate.
The formula is: Vbz = (Rp x Pz) + (Ra x Az).
Where 'Rp' is the required airflow per person, 'Pz' is the peak zone population, 'Ra' is the required airflow per square foot of area, and 'Az' is the zone floor area. The Rp and Ra values are pulled directly from tables in the ASHRAE standard based on the occupancy category (e.g., office space, classroom).
Calculation Example: Office Space
Let's calculate Vbz for a standard office space of 1,000 square feet designed for 10 occupants.
According to ASHRAE tables for an office, the people rate (Rp) is 5 cfm/person, and the area rate (Ra) is 0.06 cfm/sq ft.
People component: 5 cfm/person x 10 people = 50 cfm.
Area component: 0.06 cfm/sq ft x 1,000 sq ft = 60 cfm.
Total Breathing Zone Ventilation (Vbz) = 50 + 60 = 110 cfm of outdoor air.
Factoring in Zone Air Distribution Effectiveness (Ez)
Supplying 110 cfm to the room does not guarantee 110 cfm reaches the occupants' breathing zone. The air distribution method matters. Overhead heating mixes differently than floor displacement ventilation.
Engineers must calculate the Zone Outdoor Airflow (Voz) by dividing the Breathing Zone Ventilation (Vbz) by the Zone Air Distribution Effectiveness (Ez). Voz = Vbz / Ez.
For example, typical overhead cooling has an Ez of 1.0, so Voz = 110 / 1.0 = 110 cfm. However, overhead heating of warm air has an Ez of 0.8, meaning Voz = 110 / 0.8 = 137.5 cfm. You must supply more air to compensate for poor mixing.
- Ceiling supply of cool air (Ez = 1.0)
- Ceiling supply of warm air >= 15°F above space temp (Ez = 0.8)
- Floor supply of cool air/displacement ventilation (Ez = 1.2)
Frequently asked questions
Why does ASHRAE 62.1 use both people and area calculations?
People require ventilation to dilute CO2 and odors, while the area component dilutes VOCs off-gassing from building materials like carpets, paints, and furniture, which occurs even when the room is empty.
What happens if a room is empty? Do I still ventilate?
Yes. If a space is expected to be occupied, the area-based ventilation (Ra x Az) must be maintained even during unoccupied periods to purge building off-gassing, unless specific purge procedures are used.
How does multi-zone system calculation differ?
When a single central air handler serves multiple zones, the VRP becomes complex. You must calculate the uncorrected outdoor air intake and apply a system ventilation efficiency factor (Ev) that accounts for the critical zone requiring the highest percentage of outside air.
Are there alternatives to the Ventilation Rate Procedure?
Yes, ASHRAE 62.1 also offers the Indoor Air Quality (IAQ) Procedure, which allows for lower outdoor air rates if an engineer can objectively prove (through air cleaning technology or mass balance calculations) that contaminant levels are kept below accepted thresholds.