Don’t Duck the Ductwork

June 20, 2008
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Manbars need to be half-inch diameter steel and welded in place five inches on center.

With today’s global uncertainty, many enterprises and government agencies are building sensitive compartmented information facilities (SCIF).

In particular, those who perform sensitive work for the government are building these types of spaces. A SCIF space is a space where sensitive information may be stored, used, discussed or processed. The goal of these types of rooms is whatever type of information is available in the space cannot be heard or seen outside that space. There are some unique challenges involved when designing the mechanical systems for such a space.

Beyond the manbars, a 1.5 hour fire damper is needed.


In a typical mechanical design, many ducts enter and leave the SCIF space in the plenum ceiling. If the building exists, there may even be some ducts that do not service the area but pass through it. For an SCIF facility, each duct that is greater than 96 square inches that passes to or from the SCIF area to an unrated space shall be protected with manbars. However, any duct that has one dimension less than six inches does not require manbars. Figures 1.1, 1.2 and 1.3 show typical manbar construction in ductwork passing from an SCIF area to a nonrated area.

Figure 1.1 illustrates that the manbars shall be half-inch diameter steel. The bars must be welded in place five inches on center. The walls between the SCIF areas and non-SCIF areas must have a 2-hour firewall rating. In addition to the manbars at the wall penetration, a 1.5-hour fire damper is needed for all duct penetrations.

Ducts are not the only equipment that need manbar protection through the SCIF perimeter wall. Pipes also need to be protected if they are greater than 96 square inches or greater than 6 inches in direction. An inspection port is also required at the wall penetrations from SCIF to non-SCIF. The inspection port is typically located on the SCIF side of the penetration. The port is typically located on the secure side so no one from the non-secure side can tamper with the port. It is acceptable, however, to locate the port on the non-secure side as long as it is locked. Most inspection ports installed are hinged access doors as shown in Fig. 1.3, but an inspection window is also acceptable.

Sound can travel through a ductwork system at any size. To meet accreditation standards, proper sound attenuation is needed for each duct penetration to or from the space. There are several ways security can meet this requirement. The first method, less popular than some, is to place sound baffles directly inside the ductwork. The sound baffles must be metal and positioned no more than 6 inches apart.

A hinged access door on the security side for inspection can work, or you can use an inspection window.


The baffles work well, but a preferred method for sound attenuation is to strategically locate sound generators so no coherent sound passes through. The generators need to be connected to the building electrical system and should run continuously. The key with sound generators is to balance them so they are not so loud as to be distracting to people working below the ceiling locations.

There are several challenges in making an area’s mechanical systems SCIF rated and accredited, but with careful design and construction, the space can safely be used for many years.

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