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The Insulation Myth: Rockwool vs. Fiberglass in Double-Stud Soundproof Wall Assemblies
In the realm of building acoustics, the quest for superior soundproofing has led many to believe that Rockwool Safe’n’Sound, a mineral wool insulation, offers significantly better performance than standard fiberglass insulation. This perception is especially common in the construction of high-STC-rated double-stud walls. However, controlled lab data and manufacturer tests do not support this claim. In fact, insulation type plays a relatively minor role in the overall STC rating of a properly constructed double-stud wall. When examining test results and authoritative sources, it becomes clear that the commonly held belief—that Rockwool substantially improves STC over fiberglass—is a myth.
A typical high-performance double-stud wall comprises two separate 2×4 stud walls, with a 1-inch air gap between them, filled with insulation, and finished with two layers of 5/8-inch drywall on both the inner and outer faces. This assembly is designed to maximize mass, decoupling, and damping—all core principles in soundproof construction. Industry sources and acoustical consultants consistently report that this configuration achieves an STC of approximately 63 when filled with standard fiberglass batt insulation (WoodWorks 18).
A key misconception arises when comparing Rockwool to fiberglass in this context. Rockwool is often marketed as a superior acoustic product, but lab comparisons reveal that the performance difference is negligible. For example, Johns Manville, a manufacturer of both fiberglass and mineral wool products, states directly: “For both sound transmission and sound absorption, mineral wool and fiberglass are nearly identical. The tiny differences are undetectable to the human ear” (Johns Manville 3).
Acoustic consultant Russ Berger, whose firm has overseen testing of various wall assemblies, reinforces this point. In one study, a double-stud wall filled with fiberglass insulation and finished with one layer of 5/8-inch drywall per side achieved an STC of 56. Adding a second layer of drywall increased the STC to 63 (Berger 5). The improvement resulted from increased mass—not from a change in insulation type. Berger emphasizes that “how the materials are put together is often more important than what materials are selected” (Berger 6).
The notion that Rockwool offers a measurable STC advantage is further dispelled by Soundproofing Company, a respected acoustic consulting firm. In its technical literature, the firm notes that “fiber insulations work very similarly” and recommends standard fiberglass as a cost-effective and acoustically valid option (Soundproofing Company). Moreover, they assert that fiberglass can outperform higher-density materials at certain frequencies, particularly in low-frequency damping.
Even Rockwool’s own published testing does not demonstrate a marked STC improvement in double-stud walls. One example, based on data from Riverbank Acoustical Laboratories (Report No. TL95-195), shows a single-stud wall with 5/8-inch gypsum and Rockwool AFB insulation achieving STC 52—respectable, but not substantially different from fiberglass in comparable conditions (“AFB Acoustical Fire Batt”).
Critically, experts agree that an STC difference of fewer than 3 points is generally imperceptible to most occupants (Owens Corning 7). Variations of 1–2 points in STC between assemblies are often within the margin of testing error and environmental conditions. Thus, any claim that Rockwool Safe’n’Sound boosts STC by more than a couple points compared to fiberglass is not substantiated by data and would not lead to a noticeable difference in real-world performance.
In conclusion, while both fiberglass and Rockwool provide effective cavity damping in double-stud wall systems, the type of insulation used does not meaningfully affect the STC rating when other construction variables are held constant. The superior performance of a double-stud wall—typically STC 63—results from its decoupled framing, multiple layers of drywall, and air-tight construction. The evidence clearly shows that swapping fiberglass for Rockwool does not yield a “magic” increase in soundproofing performance.
Works Cited
Berger, Russ. “High STC Wall Assemblies: Testing and Field Results.” Russ Berger Design Group, 2015. Presentation.
Johns Manville. “Comparing Fiberglass and Mineral Wool for Sound Control.” Johns Manville Acoustics Bulletin, 2019, https://www.jm.com/en/blog/2019/october/sound-control-mineral-wool-vs-fiberglass/.
Owens Corning. Sound Control Solutions: Design Considerations and STC Performance. Owens Corning, 2018.
Riverbank Acoustical Laboratories. “Test Report TL95-195: Rockwool AFB Wall Assembly.” Riverbank Acoustical Labs, 1995. Referenced by Rockwool. https://www.rockwool.com/siteassets/o2-rockwool/documentation/brochures/commercial/afb-acoustical-fire-batts-steel-studs-brochure.pdf
Soundproofing Company. “Insulation in Soundproof Walls.” SoundproofingCompany.com, 2020, https://www.soundproofingcompany.com.
WoodWorks. “Acoustic Assemblies and STC Ratings.” WoodWorks Acoustics Guide, 2020.
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