Building Pressurization

Deliberately controlling which way air flows between spaces.

Quick Answer

Building pressurization is the intentional control of air pressure differences between spaces, or between a building and the outdoors. A positively pressurized space pushes air outward, while a negatively pressurized space draws air in. Designers use it to control odors, contaminants, infiltration, moisture, and smoke movement.

The Full Picture

Air always moves from higher pressure to lower pressure. Mechanical designers use this to their advantage by supplying and exhausting different amounts of air in different spaces. If a space gets more supply than exhaust, it runs at positive pressure; if it gets more exhaust than supply, it runs at negative pressure.

Whole-building pressure matters for envelope performance. A slightly positive building resists unconditioned outdoor air leaking in, which can reduce drafts and humidity problems, while a building at strong negative pressure can pull in moist or polluted air through the envelope and complicate door operation. Many designs aim for modest positive pressure.

Space-to-space relationships matter even more in specialized buildings. Hospitals use positive pressure for operating rooms and negative pressure for airborne infection isolation rooms. Laboratories typically run negative relative to corridors to keep hazardous fumes contained, and kitchens are kept negative so cooking odors do not migrate into dining areas. Cleanrooms run positive to keep particles out.

Pressurization also appears in life safety. Stairwell and elevator hoistway pressurization systems keep smoke out of exit paths during a fire, and the adopted building code defines when they are required. These systems are tested on completion, and testing is a key part of closeout and commissioning.

Real Examples

→Hospital isolation room: An airborne infection isolation room runs at negative pressure relative to the corridor, so air flows into the room and contaminated air is exhausted rather than escaping.

→Commercial kitchen: The kitchen hood exhaust is balanced with makeup air so the kitchen stays slightly negative, which keeps cooking odors out of the adjacent dining room.

→Smoke-control stairwell: A high-rise includes a pressurized exit stair. A fan injects air to keep the stair at a higher pressure than the floor, and acceptance testing confirms the design pressure difference.

Common Misconceptions

People assume: Positive pressure is always better.

Actually: The right pressure depends on the space. Isolation rooms, labs, and kitchens need negative pressure on purpose, and excessive positive pressure can push humid air into wall assemblies in some climates.

People assume: Pressure relationships are set once and stay that way.

Actually: Open doors, dirty filters, damper positions, and fan changes all shift pressures, which is why controls, monitors, and periodic testing are part of the design.

Frequently Asked Questions

What is the difference between positive and negative pressure?

A positive-pressure space has higher pressure than its surroundings, so air flows out of it. A negative-pressure space has lower pressure, so air flows into it.

How is pressurization achieved?

By balancing supply and exhaust or return airflow so a space receives more or less air than it removes, often with controls and airflow-measuring devices.

Where is negative pressure required?

Typical examples include airborne infection isolation rooms, many laboratories, and kitchens and bathrooms where odors and contaminants should stay contained.

How is pressurization verified?

Through testing and balancing, differential pressure monitors, and, for smoke-control systems, acceptance testing required by the building official.