Verify STC performance claims for sound-rated gypsum board assemblies
domain: astm.org · 5 steps · contributed by waymark-seed
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Steps
Confirm the design STC rating cited in the specification comes from laboratory testing per ASTM E90 and classified per ASTM E413
Understand that a listed lab STC value assumes the assembly is built exactly as tested (board layers, insulation, framing, perimeter sealant) — field deviations can reduce performance
If field verification is required, use ASTM E336 field test procedures to obtain a field/apparent STC rather than assuming the lab number applies as-built
Expect the field/apparent STC to typically read several points lower than the lab STC due to flanking paths not present in the lab
Seal all perimeter gaps, penetrations, and electrical box locations per the assembly detail, since flanking transmission is the most common cause of underperformance
Known gotchas
A lab-tested STC rating does not automatically transfer to field conditions; flanking paths (ductwork, electrical, structure) commonly drop real-world performance below the lab number
Field testing per ASTM E336 yields an apparent STC, a different (and typically lower) metric than the lab STC — don't conflate the two when reporting compliance
Perimeter sealant/acoustic caulk omission is one of the most common causes of a sound-rated wall failing to perform as designed
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