What you should know
- Pressure is created by airflow resistance and an air-volume imbalance, not by a pressure setpoint alone.
- Positive pressure protects a cleaner space; negative pressure helps contain a hazardous source.
- A fixed 10 or 15 Pa rule is not universal. The cascade must match the risk, doors, leakage and applicable requirements.
- Door position, exhaust variation and envelope leakage often explain unstable pressure readings.
Investigating low or unstable pressure?
Follow the detailed investigation guide and record the findings in the field worksheet.
Open investigation guideDownload worksheetHow a pressure cascade works
Air moves through openings from a higher-pressure space toward a lower-pressure space. The cleanroom system creates this direction by supplying and removing unequal air volumes while the room envelope, doors and transfer paths provide resistance.
For product protection, the cleaner room is commonly held positive to the adjacent less-clean space. For containment, a hazardous room may need to be negative. When both product protection and containment matter, nested zones, pressure sinks, airlocks or dedicated barrier systems may be required.
A positive room has net outward transfer; a negative room has net inward transfer. All paths must balance at the operating pressure.
Choose direction from the contamination risk
| Strategy | Primary purpose | Air direction | Critical coordination |
|---|---|---|---|
| Positive room | Protect product or clean process from adjacent contamination | Outward when openings exist | Door opening, relief path and sufficient makeup air to adjacent spaces |
| Negative room | Contain a hazardous, infectious or potent source | Inward when openings exist | Dedicated exhaust, safe discharge, door operation and failure response |
| Neutral or balanced zone | Avoid a deliberate directional relationship | Driven by local disturbances | Transfer paths and adjacent cascades still need review |
| Airlock or pressure sink/bubble | Separate conflicting requirements | Defined by the selected pressure arrangement | Door interlocks, purge or recovery time and simultaneous-door events |
Pressure setpoints are project-specific
Values such as 10 Pa or 15 Pa between rooms are common project examples, but they are not universal requirements. The selected differential must be large enough to create a robust directional relationship, yet practical for door opening, noise, envelope construction and control stability.
An illustrative cascade might be +20 Pa in a critical room, +10 Pa in its support room and 0 Pa in a corridor. That example is not a specification. Each boundary should be justified against the applicable standard, regulation, hazard and facility response.
| Boundary issue | Question |
|---|---|
| Door force | Can staff open, close and latch the door reliably at the proposed pressure? |
| Leakage | Can the walls, ceiling, penetrations and doors hold the cascade without excessive transfer air? |
| Process exhaust | How far and how quickly does exhaust volume vary? |
| Adjacent spaces | Where does offset air come from and where does it go? |
| Alarm philosophy | What delay prevents nuisance alarms but still detects a real loss of control? |
| Failure mode | What happens during fan failure, door hold-open, fire mode or power recovery? |
Estimate the airflow offset, then verify it
The supply-to-return or supply-to-exhaust offset must cover designed transfer air and uncontrolled leakage at the target pressure. A small, tight room may hold its pressure with modest offset. A leaky envelope or frequently opened large door may need far more air and still show unstable pressure.
Early estimates can use leakage assumptions and opening calculations, but actual envelope leakage should be measured or balanced during commissioning. Record door positions and system modes with every result.
Map every boundary
Identify doors, pass boxes, transfer grilles, service penetrations and construction joints.
Define intended flow
Mark the desired direction and pressure relationship at each boundary.
Build the air balance
List supply, return, exhaust and expected transfer air for every room and operating mode.
Select controls
Coordinate terminal airflow control, fan response, room sensors and exhaust diversity.
Test real events
Challenge door openings, process exhaust changes and normal operating sequences.
Troubleshooting unstable or low room pressure
- Confirm the sensor range, zero, tubing, reference location and calibration before changing airflow.
- Measure actual supply, return and exhaust volumes in the same operating mode.
- Check door seals, door closers, pass boxes and penetrations for new leakage paths.
- Trend pressure against exhaust status, FFU speed, terminal control and door events.
- Inspect filters and coils for rising resistance that reduces delivered airflow.
- Check whether another room or corridor pressure changed and moved the reference condition.
- Review fire, smoke-control and night-setback logic for unintended overrides.
Frequently asked questions
What pressure differential should a cleanroom have?
There is no single universal value. The differential must be selected from the contamination or containment risk, applicable requirements, opening size, envelope leakage, door operation and control stability. Values around 10 to 15 Pa are common project examples, not automatic rules.
Why does room pressure fall when a door opens?
An open door creates a much larger flow path and greatly reduces resistance between rooms. The pressure reading may collapse even when the net airflow direction remains correct, so door-open behaviour and recovery should be defined separately.
Can the cleanest room be negative?
It can when containment of a hazardous process is the controlling risk. Product protection may then require a barrier, local unidirectional zone, isolator or nested pressure arrangement.
Technical sources
Confirm the current controlled edition of every standard and the requirements that apply to your facility.
- ISO 14644-4:2022: Design, construction and start-upInternational Organization for Standardization
- EU Guidelines for Good Manufacturing Practice, Annex 1European Commission
Prepared by 360S Projects as a practical learning reference. Final design, validation and compliance decisions should be made by competent professionals using current project requirements, standards and manufacturer data.