Debunking Bogus Underlayment Ratings

Acoustic underlayment is primarily used to improve IIC ratings in flooring systems (read Comparing Sound Ratings). With so many different types of acoustic floor underlayment available, there’s ongoing debate over which materials perform best and whether many of the published ratings are even legitimate. This article takes a closer look at those claims and helps expose bogus underlayment ratings.

Bogus Sound Ratings

Some distributors and manufacturers promote unrealistically high STC and IIC ratings for their floor underlayments. These exaggerated claims persist largely because of confusion over what the ratings actually represent. While sound isolation products for walls can sometimes have misleading data, floor underlayment ratings tend to take this to an extreme. Marketing materials often claim that a thin underlayment, sometimes only 1/16” to 1/4” thick, can achieve STC or IIC ratings above 70, which is physically impossible. A quick online search will reveal numerous examples of such inflated claims.

To put this into perspective: laboratory tests show that 2” of solid rubber over 6” of concrete, with an additional 4” of concrete poured over the rubber, performs at IIC 64 and STC 72. When the rubber layer is reduced to 1”, the rating drops to STC 69. Reduce it again to 3/8”, and the rating falls to STC 54. These results make it clear; if 2” of rubber achieves STC 72, and 3/8” achieves STC 54, there is no way that 1/16” of any material could produce ratings above 70. The same principle applies to IIC performance, which also declines as rubber thickness and concrete mass decrease.

Characteristics Of A Quality Underlayment

Starting first with the characteristics of a soundproof flooring underlay suited for isolating impact footfall noise. Resilience is key to isolating impact footfall noise as it allows a cushion for the energy of sound (pressure of a footstep). Proper resilience can be created within a remarkably thin profile assuming that layer is truly resilient. Meaning it can properly deflect (compress) when put under stress and return to the original form when not under stress. A proper soundproof underlayment does not need to be soft like carpet pad but must be resilient like rubber. Flexible vinyls, cork, rolls of composite material, or similar, are not significantly resilient products. Performance of these non-rubber underlayment products is generally 1/3 to 1/2 that of GenieMat® RST when comparing the same thickness profile.

To effectively isolate impact footfall noise, a soundproof flooring underlayment must have one critical quality: resilience. This property allows the material to compress under pressure (like a footstep) and rebound to its original form, dissipating sound energy in the process. A proper soundproof underlayment doesn’t need to be soft like carpet padding; it needs to be resilient like rubber. Materials such as flexible vinyl, cork, or composite rolls generally lack true resilience and therefore deliver far less acoustic benefit. When comparing products of equal thickness, non-rubber underlayments typically achieve only one-third to one-half the performance of GenieMat® RST rubber underlayment.

View our GenieMat® RST rubber underlayment.

Performance When Used Over Concrete Or Over Wood

The performance of any underlayment depends heavily on the sub-floor type (concrete or wood) and the ceiling construction below (resilient or fixed). For that reason, any product that claims a “universal” IIC or STC rating should be treated with skepticism. Products boasting high Delta IIC ratings should also be considered relevant only to concrete installations, since Delta IIC testing is not valid for wood structures.

Concrete Sub-Floor

Concrete is rigid and massive, which means it doesn’t flex or absorb vibration well. Adding a thin, resilient material such as rubber introduces the resilience that concrete lacks, resulting in substantial gains in impact noise isolation. These gains increase even more when the ceiling below is resilient. For example, when installed using resilient clips or resilient channel systems, since both layers contribute to overall flexibility and decoupling.

Wood Sub-Floor

Wood sub-floors are lighter and naturally more resilient, which means the structure already moves under impact. As a result, adding a thin rubber layer does not yield the same level of improvement as it does over concrete, since both the structure and underlayment flex together. If the ceiling below is not resilient, we recommend a thicker underlayment such as GenieMat® RST10 or thicker to achieve noticeable improvements in isolation. If the ceiling below is resilient, a thinner underlayment like GenieMat® RST05 or GenieMat® RST02 can deliver excellent results. There is a limit to how much resilience a structure can benefit from. Once that threshold is reached, adding thicker layers, such as using GenieMat® RST10 beneath a resilient ceiling, provides minimal or no added value compared to thinner products like GenieMat® RST05 or RST02.