Choosing the right acoustical underlayment

April 5, 2016

Photo ©

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—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:

The composition of these products can be:

This brown underlayment installed below the green glue-down floorcovering is smooth and joint-less, allowing for a uniform top surface.

Common underlayments for wood include:

The key attributes all these options share is a firm base and absence of moisture. When installing hardwood, the surface must always be clean, flat, and dry. In sound-control situations, wood underlayments are typically floated over or glued to the top floor. It is rarely acceptable to use nails. However, there are underlayment options offering a ‘sleeper’ system, which allows the user to nail 19-mm (¾-in.) solids without compromising installation. It is important to note this method will not penetrate the actual acoustical product, and therefore, will not compromise the acoustic value.

Tile and stone
The standard installation method for tile or vinyl is a thin-set mortar, also referred to as the wet-set installation method. Due to this install method, and the moisture associated with it, wood and wood composite underlayments are not recommended. With a thin-set installation, it is important to select an acoustical underlayment unaffected by moisture, and that has the ability to bridge cracks and subfloor irregularities. Acceptable pads include variations of recycled rubber, cork, foam, and sheeted goods. It is important to note subfloors should have an L360 for deflection.

Due to the softness of sheet vinyl flooring, it has limited resistance to footfall noise and easily indents. There are two types of energy primarily affecting luxury vinyl tile (LVT) and luxury vinyl planks (LVP). The first is dynamic energy, which is the transfer of energy to the floor from people walking or rolling loads across the surface. These actions can indent the floor and transfer sound to the space below.

Static energy, on the other hand, is brought about by objects in place for extended periods of time, such as a desk or chair. This too, can lead to indentions. Therefore, when selecting an underlayment for vinyl, it is critical to select a firm product that can withstand the effects of both dynamic and static energy.

Foam underlay helps control noise and gives rigidity to the covering vinyl flooring applications. Photo ©
Foam underlay helps control noise and gives rigidity to the covering vinyl flooring applications.
Photo ©

Carpet is the most commonly installed floorcovering application. Carpet inherently offers some level of sound-control. When incorporating an acoustical underlayment, it is important to select one with the right amount of firmness and cushion. The appearance of the face fiber will be retained far longer by the addition of a cushion as opposed to a bare hard surface below.

In some cases, a compromise must be met, as the best combination of firmness and cushion may not necessarily provide the highest level of acoustical value. Additionally, the underlayment must have some density; otherwise, it can lead to premature stretching of the carpet, damage to the backing system, and possible delamination.

A prime example of how advanced planning can deliver superior acoustical performance is the Pacifica Honolulu project. This 46-story building has a residential interior area of about 39,950 m2 (430,000 sf) and 489 residential apartments. To minimize noise between units, an acoustic underlayment was necessary. Designers needed an underlayment system specially designed for use under tile and stone floors that would increase the IIC over the existing floor/ceiling assembly.

The project utilized a 5-mm (1/5-in.) thick underlayment system, made of a hemi-cyrstalline thermoplastic material, offering high energy impact with low weight, which will remain unchanged after repeated impact loads. Its design ensures that it will not rot, swell, or absorb water, and is anti-microbial.

Jamie Vallee is the Impacta product manager. He has been with Sound Seal for eight years and has extensive knowledge of acoustic underlayments. He can be reached via e-mail at[1].


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