Basic Documentation

Chapter 3—Room Pressurization Design
Criteria
Chapter 3 discusses the following topics:
• Design Considerations
• Room Pressurization Reference Data
• Room Pressurization Factors
• Leakage Area
Design Considerations
As stated previously in this guide, the positive or negative room static pressurization
relationship between two spaces determines the potential for transfer airflow between them.
Unintentional openings that allow transfer airflow in or out of a negatively or positively
pressurized room are cumulatively referred to as the room’s leakage area. However, if there
is no room leakage area (the room is sealed off), then there can be no airflow in or out of the
room, even though a static pressure difference exists between the room and other areas.
In buildings, rooms cannot generally be totally sealed off. Except for extreme situations, such
as Biological Level 4 laboratories, there is little reason to try to maintain a perfect seal or
barrier between pressurized spaces. Personnel, equipment, and contents must be allowed to
continually enter and leave such spaces. Thus, a perfect seal or barrier is not a practical
solution for the prevention of unwanted air transfer.
This consideration leads to the fundamental reason for maintaining a differential static
pressure relationship between two spaces. Aside from a perfect seal or perfect isolation, the
next best way to prevent unwanted air transfer is to maintain a differential pressure that only
allows air transfer in an acceptable or desired direction. The differential static pressure
relationship between spaces is then created and maintained by a properly designed and
controlled ventilation system.
The most commonly used means to maintain a room at a negative or positive pressure is by
airflow tracking. Airflow tracking, also referred to as volumetric airflow tracking, maintains a
fixed differential airflow or offset between the total air supplied to the room and the total air
exhausted from the room. The total air supplied to and the total air exhausted from a room to
which airflow tracking is applied is considered to be the air that is provided by a fan powered,
ducted, ventilation system.
Figure 1 illustrates the airflow relationship of a negatively pressurized room where the total
room exhaust airflow exceeds the total room supply airflow by the fixed airflow tracking offset.
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