Product Compatibility: The Dangers of Using Untested Systems

In order to protect building walls from water intrusion, the components of the building envelope must be compatible with one another, and must be properly integrated to form a comprehensive moisture control system. The weather-resistive barrier (WRB), window flashing, and sealant must work together to keep water out of the walls. While the primary purpose of the sealant is to form a waterproof seal between the WRB, window and flashing, adhesion is not the only concern. The sealant must be compatible with the WRB and flashing in order to assure long term moisture control system performance.

Most sealants on the market contain plasticizers. Certain plasticizers commonly found in caulk and sealant have the effect of liquefying the asphalt portion of asphalt self-adhering flashings and film/asphalt/film laminated flashing. These plasticizers can also be present in other flexible membranes such as EPDM and flexible plasticized PVC (sheet vinyl), as well as a variety of other building materials.

Figure 1: Liquefied asphalt bleeding from window.
Figure 1: Liquefied asphalt bleeding from window.

Over time, rubberized asphalt (also known as bitumen) can be softened and liquefied by contact with incompatible sealant. The effect is accelerated by the high temperatures that can occur in walls. In lab testing, beads of various sealants were applied directly to the rubberized asphalt side of self-adhering flashings. With certain sealants, the asphalt was liquefied by the sealant and then traveled along the sealant bead to areas where the bead was no longer in contact with the asphalt. As shown in Figure 1, in extreme cases of incompatibility, the asphalt can be liquefied to the point that it bleeds from behind the window and flows onto the face of the exterior cladding. In Figure 2, a lab test shows how the wrong sealant can flow from flashing along the sealant bead onto adjacent materials.

A similar effect can occur with film/asphalt/film laminations. The plasticizers in the sealant can migrate through the film and attack the asphalt layer. The asphalt can be severely softened and liquefied by plasticizers.

As illustrated above, incompatibility between components of the building envelope can cause severe problems. Therefore, it is important to make sure that all components are compatible. All components of Fortifiber Building Systems Group have been tested for compatibility. Moistop Sealant® adheres to and is compatible with Moistop PF® Flashing, Moistop neXT® Flashing, FortiFlash®, FortiFlash® Butyl, FortiFlash® Commercial, Moistop E-Z Seal® Flashing, FortiFlex® Flashing, all Jumbo Tex® products, PlyDry®, WeatherSmart®, WeatherSmart® Commercial and Weather Tex® weather-resistive barriers. If a sealant other than Moistop Sealant is used with Fortifiber building envelope products, the contractor is taking a risk on the compatibility of that sealant with the other components.

Figure 2: Effect of sealant/flashing incompatibility.
Figure 2: Effect of sealant/flashing incompatibility.

It is equally important to follow the flashing manufacturer’s installation guidelines. Even with compatible products, excessive use of sealant with self-adhesive flashings can overwhelm the rubberized asphalt. For this reason, Fortifiber strongly recommends against the practice of using a “knockdown bead” or “buttering the flange” with sealant. Excess sealant should always be tooled off the window flanges and flashing.