IIC (Impact Insulation Class)

Impact Insulation Class (IIC) is a single-number rating of a floor assembly’s ability to reduce impact sound transmission (footsteps, dropped objects, furniture movement) to the room below. Defined in ASTM E989, the rating uses a standardized tapping machine (ISO 140-6) to generate impact noise on the floor, and the resulting sound levels are measured in the room below. A higher IIC indicates better impact insulation. The rating procedure is similar to STC: a reference contour is fitted to the measured third-octave band data from 100 Hz to 3150 Hz. IBC 2021 requires IIC 50 between dwelling units. Typical values include IIC 25–35 for bare concrete slabs, IIC 45–55 for concrete with carpet, and IIC 55–70 for floating floor systems. The international equivalents are Ln,w and L’nT,w per ISO 717-2. IIC is critical in multi-story residential buildings where impact noise is the most common complaint between neighbours.