Do firestop sealants shrink? This is a question that has been bandied about at various industry meetings. The simple answer is yes. Sealants may exhibit various degrees of shrinkage dependent upon the environment in which they are installed. The majority of the firestop sealants on the market are latex based or silicone based, so we will constrain our discussion to those two types of sealant technology. For the most part, silicone sealants exhibit a negligible amount of shrinkage (generally <5%). Latex sealants, on the other hand, may shrink much more. Historically speaking, latex sealants have been on the market for more than 30 years, and while nothing has changed significantly, it is more of an issue today because of increased inspection, and specifically, destructive sampling where the dried sealant is cut and measured.
Latex based sealants dry rather than cure. The curing mechanism is quite simple and is somewhat analogous to a mud puddle drying. In a more humid environment, the sealant will draw moisture in and this can prolong the drying. Conversely, in an arid environment, the sealant will dry very quickly and the sealant may actually achieve its ultimate profile faster. Adding to the complexity of the appearance of the sealant, the geometry of the opening may induce more apparent shrinkage such as sealant applied across a larger span. In some cases, this is actually designed into the sealant. How can this be? The most optimal sealant profile to accommodate movement is an hourglass profile. In the past, installers might use a tooling knife to induce this concavity. However, some sealants will actually dry to be thicker towards the adhesion points (e.g. penetrant interface and barrier interface) and thinner towards the center.
So if a concave profile is ideal, why does it become an issue with some inspectors? The trouble with destructive testing is that often times a dry sealant application is being dissected and compared to a UL Certified System that reports a wet depth! When UL Systems are developed, the test lab measures the wet depth of the applied sealant and the resultant listings report the wet depth. It does not consider the shrinkage and the dried sealant profile. However, UL does reference the percentage of anticipated shrinkage according to ASTM C1241 and reports it (optionally) for the various latex based sealants. They also make a note in both the XHEZ and XHBN Guide Information that sealants may be prone to some degree of shrinkage and the systems report a wet depth unless otherwise indicated. According to ASTM C1241 testing, some sealants may shrink 20 to 25%. What does this mean? Well, if the installed depth was 1”, then it is possible to measure a dry sealant membrane that is almost a ¼” thinner. The listings cannot list a wet and dry depth because variations in annular space, opening shape, and penetrant size may produce a variety of different dry sealant profiles. One of the problems is variations in how inspectors perform cut tests on applied sealants. There is not always consistency. The IFC published a recommendation on how to perform cut tests. The IFC recommends measuring sealants at the adhesion points rather than towards the middle of the sealant to provide a better indicator of the original applied sealant thickness.
What can you do if you are flagged during an inspection and suspect that the sealant has shrunk? First, point the inspector to the UL Guide Information statements on sealant shrinkage. Next, provide the sealant listing card with the ASTM C1241 test results. Finally, verify if the inspector is following the IFC Recommendations when performing destructive testing. Please do not hesitate to contact STI Technical Services to enlist our help.