FSGLASSTM insulating glass units configured with aluminum and stainless steel spacers are constructed with a dual seal configuration where polyisobutylene (PIB) is the primary seal and structural silicone is the secondary seal.

  • PIB is the primary seal of the insulating glass unit preventing moisture vapor transmission into the unit’s sealed space. The silicone secondary seal acts as the adhesive that holds the unit together in both conventional and structurally glazed systems. Both the PIB and structural silicone must maintain adhesion to the glass and spacer throughout the life of the unit.
  • Silicone is uniquely resistant to ultraviolet light and is the only sealant capable of being exposed long term to UV, temperature extremes and harsh atmospheric conditions. The high strength of silicone makes it ideal for structurally glazed applications.

Insulating glass units are constructed with a single component spacer consisting of a black thermoplastic with integrated desiccant and polyisobutylene (PIB). VTS is chemically bonded to the glass and to the secondary silicone sealant, eliminating the need for added vapor barriers, acrylic adhesives, additional desiccant or PIB. 

Black is the standard sealant color for insulating glass. When black is specified, both the PIB and silicone will be black as is the case when gray is specified. Please note, the color of the PIB and silicone materials are not identical due to material differences and will be distinguishable. 



Both colors of sealant provide the same structural performance and long-term durability expected of the primary and secondary seal of our insulating glass.

The Gray silicone/PIB dual seal construction has certain inherent visual characteristics that are not readily apparent with a Black silicone/PIB dual seal construction. These include the following:

  • Gray color variation. Given the inherent variation in compounding both silicone and PIB by suppliers, visual differences may occur.
  • Color match between silicone and PIB. While every effort is made to match these visually, the compounding variation mentioned above precludes a perfect match. Additionally, a slight contrast in color is required in order to identify any sealant inconsistencies.
  • Black specks within the silicone and PIB. These are due to the existence of carbon black in the sealant manufacturing process.
  • Dark lines or streaks in the silicone. These may occur as a result of the edge deletion process used to remove certain coatings around the perimeter glass edge. With black silicone/PIB these are not visible; however, with the light gray color they may be visible when viewed from close distances.

None of these inherent product characteristics would be considered cause for rejection.