Choosing the right acoustical underlayment

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by Jamie Vallee
Professionals involved with the installation of flooring must choose from a large selection of materials for a variety of projects. Whether selecting options for stone, tile, vinyl, hardwood, laminate, or carpet, project teams are expected to be experts, know the ideal solution for every scenario, and deliver it on time and on budget. However, beyond this baseline is the need to deliver a space that performs behind-the-scenes to reduce and isolate foot fall or impact noise, and allow the space to function as intended—whether it is a commercial, multi-residential, or public space.

Acoustical floor underlayments are becoming increasingly important on new and retrofit construction projects. As with any construction project, it is most cost-effective to create an acoustically correct floor during the planning stage, rather than try to correct the problem later. By being more informed upfront, construction teams and product specifiers can eliminate the guesswork and choose a solution that performs to deliver acoustically correct flooring in any environment.

Understanding IIC, DELTA, and STC
Sound is classified in two basic types:

structure-borne noise; and
airborne noise.

Structure-borne noise—commonly referred to as footfall noise—includes activity directly impacting the floor beneath the user or object, such as the dragging of furniture or stomping of feet. Airborne noise is the noise that surrounds the user, such as a television in the background, or music being played in another room.

Many project teams neglect to specify the critical ratings—Impact Insulation Class (IIC), and Sound Transmission Class (STC)—denoting noise control performance. These play an important role in guiding how well flooring will perform acoustically.

IIC is used to evaluate the performance of the floor/ceiling assemblies when structure-borne noise from certain activities is inflicted. Generally, floor underlayments are lightweight and designed specifically for impact isolation. The higher the number on the IIC scale, the better the performance. Weight and mass do not relate to the performance in reduction of structure-borne noise.

This system is applied in blocked sections to provide a calibrated surface board as the underlayment. It will then be covered by a glue-down floorcovering. Photos courtesy SoundSeal
This system is applied in blocked sections to provide a calibrated surface board as the underlayment. It will then be covered by a glue-down floorcovering.
Photos courtesy SoundSeal

The delta is the highest impact insulation class provided by an acoustical underlayment. It is the truest measurement of the product’s sound-controlling properties. The delta is determined by a series of tests performed on the floor and underlayment. First, an IIC measurement is taken on the floor assembly prior to the inclusion of the acoustical underlayment and the IIC ratings are reported. The same test is then executed with the underlayment in place, resulting in a higher IIC rating. The difference between these two results is referred to as the delta rating.

STC is a value for evaluating the performance of interior walls and the floor/ceiling assembly to stop or insulate airborne noise. The higher the STC number, the better the ability of the assembly to block noise. Weight and mass play a major factor in the overall STC rating—the heavier the structure, the higher the STC ratings will be.

It is necessary to understand the effect of airborne noise and address the issue of noise transference early in the design process. By selecting building materials with appropriate STC and IIC ratings and by ensuring proper installation, developers can help eliminate noise complaints and avoid potential noise problems in the future.

There are many local building codes for noise that can vary from state to state. However, 
the International Building Code (IBC) applies nationally. This code, in addition to local and state amendments, calls for minimum STC 
and IIC ratings between 45 and 50 for design.

Planning for performance
There is no one-size-fits-all solution when looking for an effective floor underlayment. Higher-quality underlayments are typically designed and marketed by application, as different floorcoverings require different underlayment needs. Depending on the flooring that is chosen, there are various floor options which include:

glue down—providing stability of the floors, and no movement;
floating—offering a relatively quick and easy installation; and
nailed, in which real hardwood is used instead of engineered.

The composition of these products can be:

recycled rubber;
synthetic cork; or
wood composites.

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