Insulating Combustible Pipes in Plenum Spaces

Plenum spaces in buildings tend to be “out-of-sight, out-of-mind” kinds of spaces because they are typically hidden behind ceiling panels or below floors. That being said, they are incredibly important building components. Plenum spaces are designed to house HVAC systems, computer and telephone network cables, and pipe runs. They are also utilized to provide pathways for return air flow for HVAC systems.

While they may simply seem like a convenient way to hide unsightly duct work, piping or cables, they actually create a unique environment in the building that system designers have to carefully consider when designing the building. Since plenum spaces house HVAC systems, they require airflow to operate successfully. This airflow creates an oxygen-rich environment that could become a potential fire hazard if exposed to flames. The fire hazard presented by the oxygen rich environment is further compounded by the HVAC systems that run through a plenum. 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 in 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. Relatively recent code changes have adopted more stringent interpretations of the types of materials permitted in plenums. This has led to some questions about the types of materials that are allowed in plenum applications. We’ve outlined the code changes below, and how JM products meet the requirements of plenum spaces.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 (commonly referred to as 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 (e.g., a combustible plastic pipe), then it has to be enclosed in a noncombustible raceway or in 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 be listed and labeled as meeting the 25/50 fire rating. This means that while all the materials in the assembly don’t have to have a 25/50 fire rating, the outer material does need to meet this rating, 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 had to adapt to meet the new testing and subsequent listing and labeling requirements.

2. The impact of the code:
For many designers, the new requirements 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 specific fire conditions 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 fiberglass pipe insulation, Micro-Lok HP®, 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-Lok HP still meets the 25/50 requirements even when installed over combustible pipes, and it is now listed with Intertek, the testing agency, and the product packaging is labeled to show compliance with the code requirements.

This has given insulation system designers more options in the materials they can specify for insulating combustible pipes in plenum spaces, including plenum wrap and Micro-Lok HP Pipe insulation.

3.  When and where you should use Micro-Lok HP vs plenum wrap:

While Micro-Lok HP and plenum wrap can both be used in plenums, they are not necessarily interchangeable products in every application because they have different fire ratings and different price points.

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, in a typical building, most codes only require a listed and labeled 25/50 fire rating. This means plenum wrap’s fire rating performance goes beyond what the code requires. Since plenum wrap can be a more expensive material than Micro-Lok HP, and the fabrication and installation process can also be more involved than installing Micro-Lok HP, 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.

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-Lok HP being used in a number of different plenum applications for a variety of different structures, including:

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 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 rated 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-Lok HP product page.


1. This summary is a high–level overview of the recent changes in the code.  Please consult the 2018 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. 2018 International Mechanical Code, P 70: Section 602.2.1.5 Materials Within Plenums