How to Select an Insulation Facing

When it comes to fiberglass insulation, there are a variety of facings available to meet the unique needs of varied HVAC and mechanical applications.  That said, when it comes to selecting a facing material, there’s are a variety of nuanced details that should influence your facing material selection. Facings serve a variety of purposes, ranging from aesthetics to limiting vapor drive. As you consider which facing is best for your application, there are a number of characteristics you’ll want to consider before you make your selection.

Today, we’ll compare the characteristics of four different kinds of facing offered by JM: foil scrim kraft (FSK), poly scrim kraft (PSK), all service jacket (ASJ), and polypropylene-coated (“poly-top”) ASJ.

In terms of functionality, facing is often used to limit vapor drive with the ultimate goal of preventing condensation. This is a two-part function of the facing (preventing vapor drive) and the insulation thickness (controlling temperature). System designers aim to prevent condensation for a myriad of reasons, including preventing mold or fungus growth, pipe corrosion, and inhibited thermal performance (if the insulation gets wet). Facing materials are used to minimize vapor drive between the ambient air outside the insulation and the surface of the pipe/duct, subsequently preventing condensation from forming on the surface of the pipe/duct or the surface of the insulation.

As you explore facing options to minimize vapor dive, the first thing you will want to consider is the facing’s “perm rating.” Perm ratings measure the water vapor permeability of a material, or the ability for water vapor to pass through the facing. Perm ratings for vapor barriers range between 0-10, and materials are classified as permeable or impermeable based on where their perm rating falls. Any material with a perm rating that is less than 0.1 is considered a Class I Impermeable Vapor Retarder (vapor barrier). As the perm rating moves higher, the facing becomes more permeable, falling into a Class II Semi-Permeable Vapor Retarder (ranging from 0.1 – 1) and a Class III Permeable Vapor Retarder (ranging from 1-10). Materials that have a perm rating that exceeds 10 should not be used as a vapor retarder because they are too permeable.1

By understanding the perm rating of the facing, you’ll have a better understanding of how it can perform in the real-world application. Perm ratings are crucial when insulating below ambient systems, chilled water systems, or systems in humid environments as condensation is much more likely to form in these types of applications. The facings outlined in this blog have very low perm ratings that fall in the Class 1 Impermeable Vapor Retarder category:

  • FSK: 0.02
  • PSK: 0.02
  • ASJ: 0.02
  • Poly-top ASJ: 0.01

Notably, the polypropylene coating on the poly-top ASJ adds a little extra punch to the perm rating. Additionally, it makes the facing slightly more water-resistant than the standard ASJ facing.

In addition to perm ratings, you’ll want to consider is whether the material is being used indoors or outdoors. In all outdoor applications, any faced fiberglass insulation, regardless of the type of facing, will need to be jacketed with a weather-proof jacketing. This jacketing could be a UV-resistant PVC, multi-layer laminate exterior cladding, or an aluminum metal jacketing. Since the material will be covered by the jacketing in an outdoor application, the aesthetics of the insulation and facing are rarely a concern for these types of applications. For this reason, most specifiers opt for FSK facing on materials that are going to be used in outdoor applications as it tends to be less expensive and more readily available. FSK facing is silver in color, just like aluminum foil, and it is one of the most common insulation facings on the market.

While ASJ and PSK facings could also be used outside, the material tends to be somewhat more expensive than FSK and thus they are less commonly used outdoors. ASJ and PSK facings are far more prevalent indoors because they are white and can offer a clean, uniform finish that comes close to matching white painted walls. These types of facing are very common exposed systems when aesthetics are a priority.

Finally, system designers should consider the puncture resistance of the facing. This is key to the handleability and durability of the material. The material needs to be cuttable and handleable during the installation, but it also needs to be able to resist damage as a punctured facing means an ineffective vapor barrier. In mechanical insulations, they measure puncture resistance in pounds per square inch.

  • ASJ: 55 PSI
  • Poly-top ASJ: 100 PSI

If you would like to learn more about insulation facing options and the innovative new, paper-free poly-top facing JM will be releasing in April, we’d like to invite you to attend our live webinar with Scott Witherow from Design Polymerics, Optimizing Insulation System Design with Mastics and Facings. We’ll be taking a deep dive into JM’s insulation facing options, and how to create sealed systems with mastics on April 17th at 2:00 PM ET.



1. What Is a Perm Rating?