Understanding Underlayment Ratings

The assemblies that separate suites in multi-family buildings, be it demising walls or floor-ceiling assemblies, have minimum acoustic requirements in the building code so that occupants can expect a reasonable degree of privacy and quiet in their space. These requirements are minimum ratings that are applied to both walls and floor-ceiling assemblies. The rating that most people are familiar with is STC. The other rating that is a little less well-known but arguably more important in vertical living is IIC. This is the rating that addresses impact noise from above like foot fall and moving furniture. Here are the definitions for each:

Sound Transmission Class (STC) – A numerical rating that describes how well a building element reduces airborne sound. It is used for interior walls, floor-ceiling assemblies, windows, doors, and exterior wall assemblies.

Impact Insulation Class (IIC) – Is the numerical rating that describes how well the floor-ceiling assembly reduces the structure-borne sound. The higher the IIC rating, the more effective the floor-ceiling assembly is at isolating or absorbing. IIC is tested in a laboratory setting.

Delta IIC (ΔIIC) – Shows how much a product improves the IIC performance when added to a specific assembly. For example, if a floor assembly has an IIC of 40 and when a sound underlayment is added, the construction achieves an IIC of 70, then the ΔIIC is 30. 

The STC and IIC ratings of a floor-ceiling assembly are the result of acoustic testing that evaluates the performance of the entire assembly.  All of the components, including the sub-floor, ceiling, rafting, underlayment and floor finish contribute to the overall acoustic performance of an assembly. The underlayment is only one component of the system and therefore, the STC and IIC ratings cannot be attributed to the underlayment alone. In order to properly evaluate the underlay, the test results should include information about the entire floor-ceiling assembly under which it was tested as results will vary for any underlayment depending on the composition of the assembly. Ideally, the underlayment manufacturer will provide third-party testing data for a number of different assemblies so that consumers can identify the assembly that most closely matches their project, giving them more predictable results. Another helpful rating to look at, when available, is the ΔIIC rating. This rating isolates the performance of the component alone, in other words, how much acoustic improvement is realized with the addition of that one component. What we are finding is that customers, contractors, property managers, and strata councils are starting to focus more on the ΔIIC rating rather than IIC rating when selecting or approving underlayments in high-rise/multi-family residential buildings. In older high-rises, where units are slowly being updated with new flooring, most often converting carpeting to a hard surface, strata councils are mitigating potential noise complaints by developing more stringent bylaws that address the underlayment requirements specifically. Many of these councils include details about minimum ΔIIC and require approval of the complete assembly before the tenant proceeds with the update. To illustrate the variation in results that are realized with different assemblies, below are some examples of the same underlayment, the Regupol Sonus 5mm, used with various assemblies.

Want to learn more about how to deal with noise complaints from neighbouring suites? Here is a great article: https://acoustical-consultants.com/built-environment/noise-investigations/what-property-managers-need-to-know-about-noise-complaints-from-neighboring-suites/

To access Regupol’s full library of testing data, please visit https://www.regupol.us/acoustics/acoustical-test-reports/

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