6.2 Outdoor Air Rates & IMC Table 403.3.1.1

403.3.1.1 requires every occupiable space to receive the outdoor airflow determined from Table 403.3.1.1. The system shall have the capacity to supply that rate and shall supply it continuously during occupancy, except where another provision (demand control, the dwelling-unit 1-in-4-hour exception) says otherwise. The occupant load shall be not less than the table’s estimated maximum occupant load rate. Exception: approved statistical data may document a different anticipated density. “We never have that many people” is not approved statistical data.

The equation you must be able to write

Equation 4-1 (403.3.1.1.1.1):

Vbz = Rp × Pz + Ra × Az

• Az = zone floor area = net occupiable floor area (ft²).
• Pz = zone population = number of people in the zone. Default Pz = (occupant density / 1000) × Az.
• Rp = people outdoor-air rate from the table (cfm/person).
• Ra = area outdoor-air rate from the table (cfm/ft²).

Both products are required whenever the table publishes both numbers. The people term accounts for bioeffluents and occupancy-driven contaminants. The area term accounts for building materials, furnishings, and background sources that do not leave when the last person goes home. Using only people or only area is the most common M2 trap on this table.

Then Equation 4-2: Voz = Vbz / Ez. Ceiling supply of cool air, Ez = 1.0, so Voz = Vbz. Ceiling supply of warm air with ceiling return, Ez = 0.8, so Voz is 25 percent larger than Vbz. A 100 percent outdoor-air DOAS serving several zones uses Vot = Σ Voz. A single-zone mixed-air unit uses Vot = Voz. 403.3.1.2: outdoor air introduced by an exhaust system does count toward the table’s outdoor-air requirement — a toilet exhaust that pulls replacement air from a ventilated corridor is not a second outdoor-air tax on that toilet, but the corridor still needs its own 0.06 cfm/ft².

2024 Table 403.3.1.1 values used in this section

Confirmed against the 2024 IMC table (GSA / ICC Chapter 4 posting). Do not substitute a memory of ASHRAE 62.1-2010 or an older IMC printing.

Note e: rates are per water closet, urinal, or adult changing station. Higher rate = intermittent; lower rate = continuous while occupied. Note f: rates are per room unless otherwise indicated; same intermittent / continuous split. Note b: mechanical exhaust required; recirculation from the space is prohibited (air contained entirely within the space may recirculate). The 2024 table does not publish a standalone “janitor closet” row — classify a soiled-utility room by the most similar listed occupancy or send contaminant-generating work to Chapter 5, but do not invent a cfm/ft² from an older handbook.

Smoking. 403.3.1.1: except for smoking lounges, the table rates assume no smoking in occupiable spaces. Where smoking is anticipated in a space other than a smoking lounge, provide ventilation over and above the table in accordance with accepted engineering practice. A “we allow vaping in the break room” note does not let you keep the ordinary break-room row.

Worked examples (calculate these, then check the workshop table)

Example 1 — Office, 2,000 ft². Density 5/1000 → Pz = 10 people. Rp = 5, Ra = 0.06. People outdoor air = 5 × 10 = 50 cfm. Area outdoor air = 0.06 × 2,000 = 120 cfm. Vbz = 50 + 120 = 170 cfm. People-only (50) or area-only (120) both fail. With ceiling supply of cool air, Ez = 1.0, Voz = 170 cfm. With ceiling supply of warm air and ceiling return, Ez = 0.8, Voz = 170 / 0.8 = 212.5 cfm. That 42.5 cfm difference is why you cannot stop at Vbz on a heating-dominated VAV.

Example 2 — Classroom, age 9 plus, 900 ft². Density 35/1000 → Pz = 0.035 × 900 = 31.5 people. Rp = 10, Ra = 0.12. People = 10 × 31.5 = 315 cfm. Area = 0.12 × 900 = 108 cfm. Vbz = 423 cfm. An ages 5–8 classroom of the same area uses density 25/1000 (Pz = 22.5), same Rp and Ra: people = 225, area = 108, Vbz = 333 cfm. Using the wrong classroom row costs 90 cfm.

Example 3 — Retail sales, 4,000 ft². Density 15/1000 → Pz = 60. Rp = 7.5, Ra = 0.12. People = 7.5 × 60 = 450 cfm. Area = 0.12 × 4,000 = 480 cfm. Vbz = 930 cfm. Area actually exceeds people on this row. A candidate who “always uses the people column” undersizes outdoor air by more than half.

Example 4 — Restaurant dining room, 1,500 ft². Density 70/1000 → Pz = 105. Rp = 7.5, Ra = 0.18. People = 7.5 × 105 = 787.5 cfm. Area = 0.18 × 1,500 = 270 cfm. Vbz = 1,057.5 cfm. The adjacent kitchen (cooking) row is 20/1000, Rp 7.5, Ra 0.12, plus 0.7 cfm/ft² exhaust and Note b (no recirculation). A 400 ft² cooking kitchen needs Vbz = 7.5 × 8 + 0.12 × 400 = 60 + 48 = 108 cfm outdoor air and 0.7 × 400 = 280 cfm exhaust. Transfer air from the dining room can make up that exhaust only if 403.2.2 is met — Section 6.3.

Example 5 — Corridor, 800 ft². Public-space corridors: no density, no Rp, Ra = 0.06. Vbz = 0.06 × 800 = 48 cfm. There is no people term to drop under demand control; 403.3.1.3’s floor is the area column, which is the entire rate.

Example 6 — Demand control in the 2,000 ft² office. After hours, four people remain. 403.3.1.3 allows the people component to follow the actual count: 5 × 4 = 20 cfm. The area component cannot fall below 120 cfm while the building is expected to be occupied. Minimum outdoor air = 140 cfm, not zero, and not the 50 cfm people-only design figure.

Example 7 — Exhaust-only rooms. Private toilet (Note f): 25 cfm continuous or 50 cfm intermittent, per room. Public toilet with three water closets (Note e): 50 × 3 = 150 cfm continuous or 70 × 3 = 210 cfm intermittent. Education locker/dressing room, 400 ft²: 0.25 × 400 = 100 cfm exhaust. Sports locker, 400 ft²: 0.5 × 400 = 200 cfm. Enclosed parking garage, 10,000 ft²: Table 403.3.1.1 and 404.1 full-on rate 0.75 × 10,000 = 7,500 cfm. 404.1 automatic operation (UL 2075 CO detectors and nitrogen dioxide detectors) may cycle between 0.75 cfm/ft² and a standby 0.05 cfm/ft² (500 cfm here). Accessory offices at a public garage shall be held at positive pressure (404.2).

Calculation workshop

Work every row with the 2024 numbers. If your Vbz does not match the last column, you dropped a term.

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