Poly-Iso Foam vs. Fiberglass for Rooftop Ducts

Rooftop ducts pose a unique set of variables that can influence insulation material selection. The simple fact that they are outdoors sets a different bar for performance requirements than typical indoor HVAC applications. As a result, installers and specifiers frequently look for specific material features and performance benefits when they select a material for rooftop ducts.

  • Weight
  • Weatherability
  • Durability
  • Insulating Performance
  • Code Adherence

Given these details, fiberglass is often a common choice for insulating rooftop ducts. It is lightweight and easy to fabricate. It meets building code requirements for both indoor and outdoor applications. When used outdoors, fiberglass needs to be jacketed. Most installers choose a flexible, weather-proof jacketing that protects the insulation from water intrusion and helps prevent weather-related damage. Additionally, fiberglass meets all building code requirements for 25/50 flame spread/smoke developed. This is a substantial differentiator between fiberglass and many other insulating options such as foam insulations or certain thicknesses of rubber insulations.

For these reasons, historically, fiberglass has been a very popular choice for rooftop ducts. However, recently, polyisocyanurate (poly-iso) foam board, like XSPECT® ISO-foam APF board, has been introduced to the industry as an alternative to fiberglass for rooftop ducts. While it offers a number of performance benefits that make it ideal for roof top duct applications, many poly-iso foams do not meet the 25/50 flame-spread/smoke developed building code requirements. That said, authority having jurisdiction (AHJ) have the authority to grant exceptions/variances to the code, where reasonable, and in the case of poly-iso foam, many AHJs have deemed it appropriate for roof top duct applications to include poly-iso foam because they are outside the building envelope.

The reason AHJs have made this concession for poly-iso foam is because it offers a number of benefits that make it ideal for rooftop duct applications. Poly-iso foams are closed cell, meaning water can’t penetrate easily into the cells, making the insulation water-resistant. As water is a known conductor, preventing water intrusion is crucial to ensuring that the system operates efficiently and effectively. This means that XSPECT is more likely to insulate to its published values if it gets wet because it is less likely to absorb water1. That said, even though poly-iso foam is closed cell, it still needs to be covered with a weather-proof jacketing, just like fiberglass. This is a code requirement for all insulation systems used on rooftop ducts2.

Beyond the closed-cell nature of poly-iso foam, it is also light weight and offers high compressive strength when compared to fiberglass (poly-iso >16 psi vs. fiberglass <1 psi). This can make it much easier to utilize and fabricate on a rooftop, and its compressive strength can help prevent damage from external impact.

Poly-iso foam board also offers one of the highest R-values per inch of any rigid insulation (R 6.0 at 1-inch thick). This can be a highly desirable feature in areas of the country where building codes are shifting toward insulations that offer higher R-values. Bear in mind, that the while poly-iso foam maintains excellent insulating values throughout its life, the values tend to experience marginal degradation over time. The reason for this is the gas that fills the open cells begins as butane, and over time is replaced by oxygen. Butane is more resistant to temperature fluctuation than oxygen. Over time, this causes the R-value of the insulation to slightly deteriorate.

XSPECT poly-iso foam can be a great alternative insulation for rooftop ducts, as long as your AHJ has approved the insulation for that application. This is a crucial component to ensuring that the insulations you specify and select meet the code requirements of the application. For more information about using poly-iso foam or fiberglass for your rooftop duct application, please visit the XSPECT page on our website.

We will be touching on XSPECT insulation and more in our upcoming webinar, “Designing for LEED & Maximizing HVAC System Energy Efficiency,” on May 16th at 2:00 PM EDT. Click here to register for the webinar.


  1. Thermal Insulation Handbook, McGraw Hill 1981
  2. 2018 IMC, Section 614.12, p. 75: Weatherproof barriers