What You Need to Know About Using Micro-Lok HP in Plenum Spaces

Plenum spaces in buildings are designed to house HVAC systems, computer and telephone network cables, and pipe runs. They are engineered to provide pathways for airflows in HVAC systems. Typically, building designers will implement plenum space as drop-ceilings or raised floors.

The constant airflow that is necessary for a plenum space to operate successfully also creates an oxygen-rich environment that could become a potential fire hazard if exposed to flames. Additionally, in the event of a fire, the HVAC systems present in a plenum could “inhale” smoke from the fire and distribute it throughout the building. Given the unique risks at play regarding plenum spaces, they require careful attention in terms of safety and design, and building codes have detailed requirements regarding what is and isn’t allowed in plenums.

Recent code changes have adopted more stringent interpretations of the types of materials permitted in plenums, and they have led to some questions as to what is and isn’t allowed for plenum applications. We’ve outlined the code changes below, and how Johns Manville has responded to ensure that our products still meet the needs of the market. 1 In addition to reading this blog, however, you should always check your local building code requirements for confirmation regarding what is required in terms of material specifications.

  1. How the codes have changed and why it’s relevant:
    Since plenum spaces are areas where flames can easily spread, building codes specifically state that any material used in a plenum must have a flame spread index of 25 and a smoke developed index of 50 (25/50) as tested per ASTM E-84, “Standard Test Method for Surface Burning Characteristics of Building Materials.”  If the material does not meet 25/50, like a plastic pipe, then it has to be enclosed in a noncombustible raceway or a material that does meet the 25/50 fire rating.2

    The code’s most recent evolution has strongly emphasized that the insulation used to cover or enclose the combustible material must belisted and labeledas meeting the 25/50 fire rating. This means that whileallthe materials in the assembly don’t have to have a 25/50 fire rating, the outer material does, and it has to be listed as such with an agency, and the packaging has to be labeled to indicate the 25/50 fire rating.  After this change in emphasis in the code, many materials that were previously code-compliant for use over combustible materials in plenums now had to meet the new testing and subsequent listing and labeling requirements.
  2. The impact of the code:
    The evolution in the code caused many materials that were previously permissible in plenums to fall out of code-compliance because they were not listed and labeled as meeting 25/50 in a configuration over combustible materials.
    For many designers, this limited the variety of materials that were available for use in plenums, largely leaving them to rely on plenum wrap to fulfill the needs of plastic pipe insulation applications. Plenum wrap is a blanket insulation that has a robust fire rating and is commonly used in plenum spaces. It is made from inorganic fibers that are encased in a scrim-reinforced foil, and it can withstand exposure to flames for 1-2 hours.
    At Johns Manville, we realized that designers need access to more insulating materials in order to be able to specify the most optimal products for the application. To help meet this need, we had our preformed fiber glass pipe insulation, Micro-LokHP®,tested in an assembly over combustible materials to confirm that it still meets the 25/50 fire rating in this configuration. The testing confirmed that Micro-LokHPstill meets the 25/50 requirements, and it is nowlistedwith Intertek, the testing agency, and the product packaging islabeledto show compliance with the code requirements. It is currently the only preformed fiber glass pipe insulation that is listed and labeled as meeting 25/50 in an assembly over combustible materials.

  3. When and where you should use Micro-LokHPvs plenum wrap:
    While Micro-LokHPand plenum wrap can both be used in plenums,they are not necessarily interchangeable products in every application becausethey have different fire ratings. Plenum wrap’s 1-2 hour fire rating is necessary for specific applications, like Type 1 Noncombustible Building Designs, which strictly require noncombustible materials, or for applications where the insulation is likely to be exposed directly to flames, such as grease ducts.

    However, since the code for a plenum in a typical building generally only requires a listed and labeled 25/50 fire rating, plenum wrap’s fire rating performance goes beyond what the code requires. Since plenum wrap can be a more expensive material than Micro-LokHP, and the fabrication and installation process can also be more involved than installing Micro-LokHP, it may not be the best pipe insulation alternative for a budget-conscious application that only requires a 25/50 fire rating. Given the differences in cost and installation processes between the two products, plenum wrap could potentially be a more expensive and labor-intensive solution for this kind of application.

    That said, if the material to be insulated is oddly shaped, or if the design doesn’t leave enough room for a preformed, rigid insulation, using a listed and labeled blanket material, like plenum wrap, is a reasonable alternative.
  4. Micro-Lok HP  has already been used successfully over PVC in a number of different plenum space applications:
    Some engineers and installers may still be reticent to specify Micro-Lok HP to insulate plastic pipes in plenums out of concern that the insulation may not meet the code requirements; however, we have several examples of Micro-LokHPput to good use in a number of different plenum applications for a variety of different structures:
    a.     Stadiums
    b.     Schools
    c.     Assisted living/care homes
    d.     Automobile dealerships
    e.     Data Centers
    f.      Recreational facilities
    While Micro-Lok HP is not a complete replacement for plenum wrap in every situation, it is a code compliant, third-party tested alternative to plenum wrap when the code calls for a 25/50 fire rating product. If you have more questions about using Micro-Lok HP in plenums, please contact our technical support team at 1-800-654-3103, or visit the Micro-LokHP product page.



  1. This summary is a high –level overview of the changes in the code.  Please consult the 2015 International Mechanical Code2 for specific details.  This summary is provided for your convenience only and is not intended to be used or relied upon by anyone as a substitute for professional engineering design and documentation required by building code, contract or applicable law. 
  2. 2015 International Mechanical Code, P 70:Section 602.2.1.1 Materials Within Plenums