What ventilation rates does ASHRAE 62.1 prescribe?
The Ventilation Rate Procedure
ASHRAE Standard 62.1-2022 'Ventilation and Acceptable Indoor Air Quality' defines the minimum outdoor air requirements for commercial buildings using the Ventilation Rate Procedure (VRP). At the zone level, the breathing zone outdoor airflow is calculated as:
**V_bz = (Rp × Pz) + (Ra × Az)**
Where Rp is the per-person outdoor air rate (CFM/person or L/s·person), Pz is the design zone population, Ra is the per-area outdoor air rate (CFM/ft² or L/s·m²), and Az is the zone floor area. This dual-component approach recognises that building-related contaminants (materials, furnishings) require ventilation proportional to floor area, while occupant-related contaminants (bioeffluents, CO₂) require ventilation proportional to population.
### Typical Prescribed Rates
For a standard office space, ASHRAE 62.1-2022 prescribes Rp = 5 CFM/person (2.5 L/s·person) and Ra = 0.06 CFM/ft² (0.3 L/s·m²). So a 100 m² open-plan office zone designed for 8 people requires:
V_bz = (2.5 × 8) + (0.3 × 100) = 20 + 30 = 50 L/s
The per-area component often dominates in modern low-density offices — a trend that has strengthened over successive editions of the standard.
### Zone Air Distribution Effectiveness (Ez)
The zone outdoor airflow V_oz = V_bz / Ez, where Ez accounts for the effectiveness of the air distribution system in delivering outdoor air to the breathing zone. Ceiling-supplied cool air achieves Ez = 1.0 (ideal — no penalty). Floor-supplied displacement ventilation can achieve Ez = 1.2 (better than ideal — the supply air 'pools' in the occupied zone). However, ceiling-supplied warm air for heating achieves only Ez = 0.8 because the warm supply air tends to stratify near the ceiling, bypassing the occupied zone.
### System-Level Calculation for Multi-Zone Systems
For VAV systems serving multiple zones, Appendix A of ASHRAE 62.1 provides the system ventilation efficiency (Ev) calculation. The system-level outdoor air intake V_ot = V_ou / Ev, where V_ou is the sum of all zone outdoor airflows and Ev is determined by the 'critical zone' — the zone requiring the highest fraction of outdoor air relative to its primary airflow. The larger the disparity between the critical zone and the average, the lower the system ventilation efficiency and the more total outdoor air the system must condition.
ASHRAE 62.1-2022 Prescribed Ventilation Rates for Common Spaces
People outdoor air rate (Rp) and area outdoor air rate (Ra) for typical commercial building space types per ASHRAE 62.1-2022 Table 6.2.2.1.
| Occupancy Category | Rp (L/s·person) | Ra (L/s·m²) | Default Occupant Density | Example V_bz (100 m² zone) |
|---|---|---|---|---|
| Office space | 2.5 | 0.30 | 5 persons/100 m² | V_bz = 2.5×5 + 0.30×100 = 42.5 L/s |
| Open-plan office | 2.5 | 0.30 | 8 persons/100 m² | V_bz = 2.5×8 + 0.30×100 = 50 L/s |
| Conference/meeting room | 2.5 | 0.30 | 50 persons/100 m² | V_bz = 2.5×50 + 0.30×100 = 155 L/s |
| Classroom (age 9+) | 5.0 | 0.60 | 35 persons/100 m² | V_bz = 5.0×35 + 0.60×100 = 235 L/s |
| Retail sales floor | 3.8 | 0.60 | 15 persons/100 m² | V_bz = 3.8×15 + 0.60×100 = 117 L/s |
| Corridor | — | 0.30 | — | V_bz = 0.30×100 = 30 L/s |
| Hotel guest room | 2.5 | 0.30 | 2 persons/room (30 m²) | V_bz = 2.5×2 + 0.30×30 = 14 L/s |
🔑 Key Takeaways
- ✓Ventilation rate V_bz = (Rp × Pz) + (Ra × Az) — combines per-person and per-area components
- ✓For typical offices: Rp = 2.5 L/s·person and Ra = 0.3 L/s·m² (5 CFM/person and 0.06 CFM/ft²)
- ✓The per-area component often dominates in modern low-density offices — do not overlook it
- ✓Zone air distribution effectiveness (Ez) penalises heating mode (Ez = 0.8) compared to cooling (Ez = 1.0)
- ✓VAV multi-zone systems must use Appendix A calculations — the critical zone determines system ventilation efficiency