Basic Documentation

Room Pressurization Control Application Guide
Figure 3 shows that when a room has as little as a 0.2 sq. ft. leakage area, a small change in
differential airflow such as only 25 cfm causes a rather large variation in the resulting
differential pressure value. Whereas, the same 25 cfm differential airflow variation for a room
having a 1.0 sq. ft. leakage area would exhibit a much smaller differential pressure variation.
Except for biological laboratory rooms where highly contagious pathogens are present, and
very critical clean rooms, little is gained by attempting to make the room exceptionally tight.
Rather, a room’s static pressure can be maintained at a more stable value if the total room
leakage area is perhaps between 0.5 and 1.0 sq. ft.
Note also that if room leakage area were significantly greater than about 1.5 sq. ft. the
resulting leakage area curve would lie close to the bottom of the chart in Figure 3. Trying to
maintain a 0.01 Inch static pressure differential for that much leakage area requires very high
differential airflows. Experience indicates that a negative 0.01 inches w.c. room differential
pressure (typical for chemical laboratory rooms) cannot be maintained in rooms that have a
leakage area much greater than about 1.5 square feet due to the excessively high differential
airflow required. Thus, except for very large rooms, a reasonable room tightness of between
0.5 and 1.5 sq. ft. of leakage area is recommended when a 0.01 inches w.c. room differential
pressure is desired.
It is also very important to consider the effect that opening a room door will have on room
pressurization. Opening a single width door of average size will increase a room's leakage
area by approximately 20 square feet or more. The resulting room leakage area curve would
essentially lie horizontally along the bottom of the graph and result in a near zero differential
pressure value for the room. With an open door, no appreciable differential static pressure
value can be maintained without an excessively high amount of differential airflow.
Consequently, it should be realized that a room’s differential pressure drops to near zero
anytime a door is opened. However, the occurrence of near zero differential pressure should
not be interpreted as a failure to contain contaminants or prevent an undesirable air transfer,
since the proper directional airflow (inward for a negative room and outward for a positive
room) will still be maintained even with a (temporary) large increase in room leakage area.
16 Siemens Building Technologies, Inc.