Acoustical flooring underlayments serve multiple purposes. First and foremost, they provide noise dampening qualities. Secondly, many support the floor by retaining its thickness throughout the life of the installation. This is where synthetic rubber underlayments provide additional value.
Resilience Matters
A common misconception is that all underlayments perform the same. In reality, quality
underlayments are made from highly resilient materials that absorb impact and quickly return to their original thickness. This resilience is critical because wood flooring is heavy, and once furniture is added, the true test begins. The underlayment must support not only the floor, but also the furnishings. If it loses thickness over time, it also will lose its overall support of the floor.
When selecting an acoustical underlayment, focus on two key attributes: density and resiliency. Higher mass (density) and greater resistance to compression – often measured in Newton pounds of force at 25 percent and 65 percent indentation force deflection (IFD) – are measures of resilience. Shore A hardness is another metric sometimes used to show how well the product resists impressions in small areas, such as corners or joints.
Once an acoustical underlayment compresses and loses height, its acoustical performance drops significantly, and the floor may feel harder and noisier. Think of it this way: a good acoustical underlayment acts like a shock absorber. It cushions impact, reduces sound
transmission, and maintains its structure under pressure. Without these qualities, the flooring system can fail to deliver the comfort and quiet consumers expect.
Acoustic Basics
Flooring underlayments are tested as part of a system to simulate a typical
floor/ceiling assembly. The floor, underlayment, adhesive, substrate, and the
ceiling assembly are combined to evaluate performance against IIC, Delta IIC, HIIC, and STC
ratings. These values show how the assembly performs in various noise tests, from footsteps to airborne sound.
Manufacturers often test multiple assemblies to provide a broader understanding of their product’s capabilities. Generally, denser acoustical underlayments, like synthetic rubber, tend to resist deflection better than some other types. It is not uncommon to find these types of underlayments used in applications requiring vibration control.
Installation Considerations
Beyond material properties, consider installation methods and substrate compatibility. The underlayment should work over concrete slabs, wood subfloors, and floors treated with leveling compounds. Ideally, it should be suitable for glue-down or floating installations.
As with any flooring project, success starts with proper substrate testing. Check for moisture and flatness, as prep work may be needed. Underlayments cannot correct substrate problems; they will conform to irregularities, which can telegraph through the flooring or create voids under wood floors. A little extra time spent on prep can prevent costly callbacks later.
Acoustical underlayment materials may include cork, synthetic rubber, recycled rubber or cork/rubber blends, foam pads, recycled cellulose fiber materials, and dimpled or peel-and-stick membranes. For additional information, refer to the Underlayments: Sound Control/Acoustical section of the NWFA’s Installation Guidelines on page 110 at NWFA.ORG/TECHNICAL-GUIDELINES/.
Moisture and Vapor Retarders
NWFA Guidelines along with most flooring manufacturers provide guidance on vapor retarder requirements, including whether one is required and the recommended class rating. Underlayments vary in their permeance (per ASTM E96), and vapor retarders generally serve one purpose: protecting against moisture vapor emission. When your acoustical underlayment does not publish a moisture permeability, consider using both a vapor retarder and an acoustical underlayment. This combination satisfies manufacturer requirements while delivering a quieter, more enjoyable installation. It’s a simple step that pays off in long-term durability.
Ease of Use
Finally, a good acoustical underlayment should be easy to handle and install. It should not be prone to damage simply from working on top of it. The old saying applies: “You get
what you pay for.” Investing in a high-quality acoustical underlayment ensures long-term performance, comfort, and peace of mind. Acoustical underlayments should not be seen as an afterthought, but as a critical component of a successful flooring project.
Randy Dye is the product and marketing manager for the flooring division of Leggett and Platt, a manufacturer of hard surface underlayment. He can be reached at randy.dye@leggett.com.