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Room pressurization: the physics behind isolation and clean room design
Pressurization control creates an intentional, controlled air leakage direction through a room's boundary, separate from the room's overall ventilation rate calculated by the ACH calculator. By making supply airflow either greater than exhaust (positive pressure) or less than exhaust (negative pressure), the room maintains a consistent pressure differential relative to adjacent spaces, driving air to flow in a controlled direction through any available leakage paths, primarily the door undercut.
The governing physics is the orifice flow equation: airflow through a leakage path is proportional to the leakage area times the square root of the pressure differential, scaled by a discharge coefficient. Critically, this means the relationship is not linear — doubling the pressure differential does not double the required airflow offset, since airflow scales with the square root of pressure. Achieving each successive increment of pressure differential becomes progressively easier as you have more excess offset airflow than the previous increment required.
Negative pressure isolation rooms
Airborne infection isolation rooms maintain negative pressure relative to the corridor, meaning exhaust airflow exceeds supply airflow. This ensures that if the door is opened, air flows into the room rather than contaminated room air escaping into the corridor. CSA Z317.2 and ASHRAE 170 specify minimum continuous pressure differentials, commonly 0.01 inches w.c. (2.5 Pa), verified by a visual indicator at the door, such as a smoke tube test showing inward airflow at the door undercut, or a continuous differential pressure monitor with alarm.
Positive pressure protective and clean rooms
Protective environment rooms for immunocompromised patients, and clean rooms for sensitive manufacturing or research, maintain positive pressure relative to adjacent spaces, with supply airflow exceeding exhaust. This prevents contaminated corridor air from entering the protected space when the door opens. Cleanroom applications typically target higher differentials (often 0.05 inches w.c. or more) and add fine filtration on the supply air to achieve both pressure and particulate control simultaneously.
Why door undercut and leakage area matter so much
A tighter room envelope (smaller leakage area) requires less airflow offset to achieve the same target pressure differential, which is why cleanroom and isolation room construction details, including door seals, sealed penetrations, and well-sealed wall assemblies, directly reduce HVAC system size and energy cost. Conversely, a leaky room (large undercut, unsealed penetrations) requires substantially more offset airflow to maintain the same pressure target, increasing both first cost and ongoing fan energy. Once you've sized your offset, verify total exhaust capacity using the exhaust fan sizing calculator and confirm the room's overall air change rate still meets its required ACH target for general ventilation.
Frequently Asked Questions
The offset is the supply-exhaust airflow difference needed to achieve target pressure, accounting for the room's leakage area. A tightly sealed room can achieve 0.01-0.03 in. w.c. with just 50-150 CFM offset. The relationship follows orifice flow: airflow is proportional to leakage area times the square root of pressure differential. For negative pressure rooms, exhaust exceeds supply by the offset amount; for positive pressure, supply exceeds exhaust. Use this calculator with your room's measured or estimated leakage area for an exact value.
Canadian healthcare facilities follow CSA Z317.2, aligned with ASHRAE 170, specifying a minimum continuous negative differential of 0.01 in. w.c. (2.5 Pa) relative to the corridor, verified by a door-mounted gauge or smoke tube test showing inward airflow. Protective environment rooms require the opposite: positive 0.01 in. w.c. minimum. Always verify against the current CSA Z317.2 edition for your province. Pair this with the ACH calculator to confirm the room also meets its general air change requirement.