TAB Journal Summer 2019 - 6

A properly constructed room cannot be over emphasized. Room
pressurization is not only dependent on the offset between the
supply and return/exhaust airflow, but also on the tightness
of the room construction. Far too often, the quick solution for
improving the room pressurization level (or room differential
pressure) is to install a door sweep and/or install gaskets
around the door jambs. In most cases, this a quick fix for the
construction team and only masks a much larger problem.
A loosely constructed room will allow the airflow to pass through
small openings around wall penetrations, electrical outlets, and
ceiling tiles rather than around the door where it was intended to
pass. Room construction is especially important when dealing
with cascading pressures where the neighboring spaces are
dependent on the transfer air through the door. If the door is
designed to transfer 200 CFM to the adjacent space, and only
50 CFM is passing through the door (due to the installation of a
door sweep or room leakage), then the adjacent room is deficient
by 150 CFM and most likely cannot maintain its differential
pressure requirements to adjoining rooms and corridors.
Can the room be too tight? In some cases, yes. While it may
be impressive that a room can obtain a +0.05 in. water column
differential pressure at only 50 CFM offset, this may not be
the best application for the room. For example: on a positive
pressure cleanroom that has been properly constructed and
sealed, you may consider not using door sweeps. While all
design and regulatory-body requirements have been met, the
best strategy for obtaining the room pressure differential has


not been utilized. Removing the door sweep and increasing the
room CFM offset to 200 CFM to achieve the 0.05 in. pressure
level will increase the directional airflow velocity.
Although we are still dealing with very low velocities when the
entry door is opened, the additional offset CFM will improve
the airflow barrier as laboratory personnel are passing through
the doorway when they enter the room. The same concept
for negative pressure rooms apply, and when low differential
pressure levels are found at the design CFM offset (and
increasing the offset is not an option), the room tightness should
be addressed, not solely the door tightness. In this instance,
door sweeps would be the last resort, and special attention
should be made to ensure the room is negative to all areas,
including ceiling spaces, which may be a path of return air to
other areas within the facility.
Current healthcare and laboratory standards only provide a
minimum level of room pressure differential with the door
closed, which does not address the directional airflow quantity
when the door is open while entering or exiting critical spaces.
For improved containment and infection control, these standard
committees may want to consider adding a minimum level of
directional airflow quantity (or room CFM offset) to obtain
these pressure requirements. While room differential pressure
levels are very important, and are the standard for gaging the
directional airflow, more emphasis should be placed on how
these pressure levels are achieved.



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TAB Journal Summer 2019

TAB Journal Summer 2019

Table of Contents for the Digital Edition of TAB Journal Summer 2019

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