In the first two parts of this series, we explored the fundamentals of sound transmission and its interaction with barriers, particularly in the context of firestop systems. Part 1 introduced ASTM E90, the standard test method for measuring airborne sound transmission loss (STL) across a barrier in a controlled laboratory environment, and how this STL data is then evaluated and transformed into Sound Transmission Class (STC), a single-number metric that summarizes a barrier’s ability to attenuate airborne sound, using the standard contour curve outlined in ASTM E413. Part 2 delved into the physical nature of sound, explaining how sound waves interact with solid barriers that break up the wave energy. Some energy is reflected, some is dissipated within the material through mechanisms like damping and vibration transforming sound energy into heat energy, and some is transmitted to the other side of the barrier at a reduced decibel level. And we learned that the difference in sound energy measured before and after the barrier equals the sound transmission loss that is quantified by ASTM E90 across a spectrum of frequencies spanning 5 octaves. In this third installment, we address the complex challenge of integrating firestop systems into barriers that must meet both fire-resistance and sound-attenuation requirements, ensuring safety and acoustical performance are achieved without compromise.
Very often, it is mistakenly asked, what is the rating of your firestop product? And if you have ever attended a level 1 firestop training course, within the first 10 minutes you know the answer to that question. Of course, the firestop product is not rated. To illustrate the point as to why, consider that a product may be tested in a 1-hour wall penetration, but the very same product may be tested in a 4-hour floor penetration, sometimes at the same application thickness. Does the product carry a 1-hour rating? a 4-hour rating? The answer is neither, because performance is based on several interdependent elements working together, and ratings vary based on individual system performance. The product is only one part of that system. The answer is no different when someone asks what the STC of a firestop product is. Similar to the previous example, what if the product is tested in an STC 50 wall? What if it is tested in an STC 70 wall? You don’t build entire sound barriers out of firestop materials. You build the barrier, and test how the treated deviation in the barrier affects performance. Just as it is with firestop systems, the outcome is dependent on the interaction of multiple components that make up the system. As with firestop systems, the sound performance system carries the rating, not the product. This distinction is critical for designers and specifiers to understand, as it underscores the need to evaluate firestop systems holistically when addressing both fire and sound performance.
Designing barriers to meet both fire-resistance and STC requirements involves distinct but somewhat complementary approaches. In firestop design, the process begins with a barrier that meets the required fire-resistance rating (e.g., a 2-hour-rated wall per ASTM E119). When a penetration or joint breaches the rated barrier, a firestop system is installed to restore the original fire-resistance rating completely. The system must withstand the fire test duration equal to the barrier rating without failure (e.g., no flame passage before 2 hours for a 2-hour rated barrier). If the system fails prematurely, even if only by 1 minute, the firestop system is inadequate and must be improved upon and retested until it meets the barrier rating. That is the only choice the designer has because you cannot do anything to the barrier construction to bolster the fire-resistance performance of the penetration or joint.
As discussed in Part 2 of this series, the most effective way to enhance STC performance is to minimize air leakage across the barrier, as sound waves travel efficiently through air gaps. This principle aligns with firestop design, where airtight seals are essential for smoke control, as tested per UL 1479’s air leakage (L-rating) test. Firestop products that create robust, airtight seals are critical for maintaining both STC and smoke performance. For example, SpecSeal® caulks, sealants, and sprays are designed to fill gaps and provide a hermetic seal in order to resist transmission of smoke. SpecSeal® SSP Firestop Putty and putty pads, being dense mastic materials, not only seal air gaps but also dampen vibrations from penetrating items, reducing flanking losses caused by vibrational energy transfer. Similarly, SpecSeal® Firestop Blocks provide effective sealing and vibration damping, making them suitable for both fire and sound applications. Another excellent option is SpecSeal® CID (Cast-In-Place) Devices, which incorporate water- and airtight seals, ensuring minimal air leakage. As well, SpecSeal® Composite Sheet paired with SpecSeal® sealants, especially when installed on both sides of a barrier, provide both fire-resistance and sound attenuation by trapping air space and providing airtightness.
Besides looking simply at products, the designer can improve STC performance by incorporating firestop systems with low L-ratings, as verified by UL 1479. These are particularly effective, as they demonstrate proven resistance to air flow. The L-rating, expressed in cubic feet per minute per square foot (cfm/ft²) at a specified pressure, quantifies a system’s air tightness which directly correlates with improved smoke migration protection which indirectly improves STC performance. When selecting firestop systems, designers should prioritize systems with low L-ratings to optimize both sound and smoke mitigation.
Beyond sealing air gaps with mastic materials, certain firestop materials enhance STC performance by adding mass to the barrier, which reduces sound transmission by reflecting sound waves. SpecSeal® SSM Mortar, for example, contributes to sound attenuation by filling the opening around a penetration with the dense mortar and it also seals penetrations effectively. While the base barrier (e.g., gypsum board, concrete) primarily reflects sound energy due to its surface properties, the added mass of these firestop materials strengthens the overall system’s performance.
The choice of firestop material depends on the penetration type and barrier construction. For metallic pipes, which can transmit vibrations, mastic materials like putty or foam blocks are particularly effective at damping vibrational energy. For non-metallic penetrants, such as plastic pipes, sealants or cast-in devices will do well. If a collar or wrapstrip product is used with plastic pipe penetrations, they would need to be used in conjunction with a SpecSeal® sealant in accordance with the firestop design. Designers should consult ASTM E90 test reports that incorporate firestop materials in representative systems to verify their impact on STC performance in the intended assembly.
Annular space, the gap between the penetrating item and the barrier, can play a critical role in STC performance. Larger annular spaces increase the potential for air leakage and flanking paths, potentially degrading the barrier’s STC rating. Firestop systems that minimize annular space, or firestop devices such as EZ Firestop® Grommets or smaller EZ Path® devices, provide smaller potential for STC degradation by reducing air gaps and by occupying a smaller footprint on a barrier surface. These devices have been shown to perform well in ASTM E90 tests due to their ability to limit flanking losses. When designing penetrations, minimizing the size of the opening is a practical strategy. Ultimately, specifiers should review UL 1479 and ASTM E90 tested systems to balance fire-resistance and acoustical requirements while minimizing annular space.
To achieve optimal fire and sound performance, consider the following best practices:
Not all firestop applications require STC ratings, and not all sound barriers need fire-resistance ratings. However, when both are required—such as in multifamily housing, commercial buildings, or healthcare facilities—Specified Technologies offers a range of tested systems designed to meet these dual demands. For detailed performance data, consult STI’s technical resources, including UL 1479 and ASTM E90 tested systems, to select the appropriate system for your project.
Integrating firestop systems into sound-rated barriers requires a thorough understanding of both fire-resistance and acoustical principles. By prioritizing airtight seals, vibration damping, and mass addition, designers can ensure that firestop systems maintain the barrier’s STC rating while meeting fire-safety requirements. Specified Technologies provides a comprehensive portfolio of products and systems, backed by rigorous testing, to address these challenges. Whether you’re sealing a pipe penetration in a residential wall or a complex duct system in a commercial building, STI’s solutions offer the reliability and performance needed to contain fire, smoke, and sound effectively.
This article, by Tim Mattox, Senior Manager of Systems & Testing Development, originally appeared in the Spring Wrap-Up 2025 edition of The Burn.