Containment Negative Air Pressure Calculator

Calculate the required exhaust airflow (CFM) to maintain negative air pressure within a containment area, accounting for room volume, air changes per hour, leakage, and pressure differential requirements.

Room Dimensions

Ventilation Parameters

Typical: 6–12 ACH for general containment; 12–20 ACH for infection control Extra airflow added to compensate for envelope leakage (typically 10–20%) Typical: 2.5–8 Pa (0.01–0.03 in. w.g.) below adjacent spaces

Openings / Leakage Area

Estimated total area of gaps around doors, penetrations, etc. Typical value: 0.60–0.70 for cracks and gaps
Results will appear here.

Formulas Used

1. Room Volume:
V = L × W × H  (ft³)

2. Base Exhaust CFM (ACH Method):
Qbase = (V × ACH) / 60  (CFM)

3. Exhaust with Leakage Compensation:
Qexhaust = Qbase × (1 + Leakage% / 100)  (CFM)

4. Crack Leakage Flow (Orifice / Bernoulli Equation):
v = Cd × √(2 × ΔPlb/ft² / ρ)  (ft/s)
Qleak = v × Acrack × 60  (CFM)
where ΔPlb/ft² = ΔPPa × 0.020885,  ρ = 0.0765 lb/ft³

5. Maximum Allowable Supply CFM:
Qsupply = max(0, Qexhaust − Qleak)  (CFM)

6. Negative Pressure Verification:
Net Outflow = Qexhaust − Qsupply − Qleak ≥ 0  → Negative pressure maintained

Assumptions & References

  • Standard air density assumed: ρ = 0.0765 lb/ft³ (at 70°F, sea level).
  • Discharge coefficient Cd = 0.60–0.70 is typical for building envelope cracks and gaps (ASHRAE Fundamentals).
  • Recommended ACH for containment: 6–12 ACH general; 12–20 ACH for infection control isolation rooms (CDC/ASHRAE 170).
  • Target negative pressure: minimum 2.5 Pa (0.01 in. w.g.) per CDC Guidelines for Environmental Infection Control; 8 Pa (0.03 in. w.g.) recommended for robust containment.
  • Leakage factor of 10–20% accounts for imperfect sealing of walls, floors, ceilings, and penetrations.
  • Supply air must always be less than exhaust air to maintain net negative pressure relative to adjacent spaces.
  • References: ASHRAE Standard 170-2021, CDC Guidelines for Environmental Infection Control (2003, updated 2019), OSHA 29 CFR 1926.1153 (silica containment), EPA RRP Rule containment guidance.
  • This calculator provides design estimates. Final system design should be verified by a licensed mechanical engineer and tested with a pressure gauge or manometer.

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