The plenum areas within a data center present unique challenges when it comes to protecting against fire.  These oxygen-rich hidden areas within dropped ceilings or under an elevated floor manage return airflow to HVAC equipment and house vast networks of plastic-coated mechanical pipes and cabling. Material combustibility and the toxicity of plastics and various wrappings when exposed to flame have led to changes in codes and the development of materials suited for the plenum space.

Sadly, history bears out the potential threat these spaces provide. A February 1975 fire at the World Trade Center in New York was linked to cables installed in plenum spaces and led to changes in codes governing plenums. A New York Board of Fire Underwriters report stated, “The worst and most hazardous condition is when wires and cables with combustible insulation are run through plenums to service the floor above. This provides a double hazard by introducing combustibles into the air conditioning. It should be noted that the mass of cables to supply communication equipment in many office occupancies is sufficient to sustain a substantial fire. While an individual cable is extremely difficult to ignite, a group of cables lying parallel will burn intensely, similar to the situation that exists with a group of logs in a fireplace.”

While much attention has focused on the toxicity risk associated with combustible cabling in the plenum, a less considered threat may be the networks of PVC and plastic-coated pipes connecting mechanical systems in the plenum area. Changes to the building code have generated questions about the types of materials permitted in the plenum.  

The World Trade Center fire and other similar cases prompted listing agencies, such as Underwriters Laboratories (UL), to develop reproducible tests based on standards to evaluate fire safety in the plenum space. These tests helped inform multiple changes to the International Building Code and International Mechanical Code.

Codes and Noncombustibles

Section 602.2.1 of the 2012, 2015, and 2018 International Mechanical Code applies to materials used within plenums, including the mechanical pipes. The section specifies that materials shall be noncombustible and shall be listed and labeled with a flame spread index not more than 25 and a smoke developed index not more than 50 when tested in accordance with ASTM E84 or UL 723. There is an exception — Exception #5 — which applies for combustibles fully enclosed within materials listed and labeled for installation within a plenum.

ASTM E84 Standard Test Method for Surface Burning of Building Materials or UL 723 Standard for Test for Surface Burning Characteristics of Building Materials are “reaction to fire” test standards. These methods measure the flame spread and amount of smoke produced by the materials used, not the hourly fire-resistance. 

Hourly fire-resistance ratings are determined when building elements or assemblies are tested according to  ASTM E 119 Standard Test Methods for Fire Tests of Building Construction and Materials or UL 263 Standard Fire Tests of Building Construction and Materials. These test standards provide fire-resistance ratings. 

Any materials installed in the plenum, including the insulation or plenum wrap used to wrap mechanical pipes, should be in full compliance with a listing organization and its requirements for covering materials on the insulated plastic pipe assembly as mandated by the building code, at a minimum.

Other Factors

Life safety will always be the most urgent priority when specifying materials in the plenum area. But other factors can also influence material choice, especially with regard to the materials protecting pipes. From an installation perspective, a material’s ease of use on the job site as well as its maintenance requirements should be considered. Because plenum areas can be challenging to access and difficult to work within, it is important that the insulating material used on pipes be lightweight and sized at an appropriate width for ease of handling. The material must also be dependable when it comes to sealing. A material that stays sealed once it is installed, can help limit callbacks — an even bigger consideration at a time of social distancing and contact-free service.

As data centers are designed to project a clean and pristine aesthetic, an insulating material’s surface matters. Even in spaces that are typically unseen, sanitation and a clean look are important. When evaluating an insulating material’s jacketing for thermal and moisture resistance, it’s important to consider how easily the material can be maintained. A polymer jacketing surface, such as the material used on Owens Corning SSL II® with ASJ Max Fiberglas™ Pipe Insulation,  can be easily wiped down to maintain a clean appearance and a hygienic atmosphere.  

Finally, acoustic comfort is another consideration, albeit unrelated to fire concerns. Plenums in data centers are often located above drop ceilings and in areas with plenty of hard surfaces that allow sound waves to reverberate back into a space or flank into other areas.  Installing a material, such as mineral wool, in the wall assemblies can not only help mitigate flanking noise but deliver a fire-resistant material to support the most precious commodity in any fire: time for occupants to egress the center.

Building and fire codes will naturally be the most important factor when supporting safety in the plenum. But other factors — including a material’s handling profile, its maintenance requirements, and the opportunity to mitigate other challenges such as noise — can help achieve both performance and safety requirements in plenum spaces.  Plenums may be out of sight, but they should never be out of mind when it comes to supporting life safety.