Debunking Bogus Underlayment Ratings
Acoustic underlayment is mainly used to increase IIC ratings in floors (read Comparing Sound Ratings). With the many different acoustic underlayment products, there seems to be some debate on which material works best in floors, as well as the legitimacy of the ratings for these products. This article will help resolve some of the confusion.
BOGUS SOUND RATINGS
Some distributors and manufacturers will promote unusually high STC and IIC ratings for floor underlayment products. Acceptance of these ratings perpetuate largely from a lack of understanding for what the ratings represent. Isolation products for walls will occasionally have ratings that are misleading, but not as often to the point of ridiculousness as what you will find in ratings for underlayment. Marketing for floor underlayment products, however, will claim ratings that are several times that of which is even physically possible in a structure. A few minutes of searching online will result in at least ten random underlayment materials with STC/IIC ratings north of 70. To add to that, most of these 70 plus STC/IIC rated products are only 1/16” to 1/4” thick.
To understand how misleading these claims are, we have tests for 2” of solid rubber over 6” of concrete and 4″ of concrete poured over the rubber performing at IIC 64 and STC 72. Now the STC rating was able to get above 70, but only with 10” of concrete and 2” of rubber. Reduce the thickness of the rubber to 1” and the rating drops to STC 69. Reduce the thickness again to 3/8” and the rating drops to STC 54. From this, it is obvious to anyone with a little sense that if 2” is STC 72, 1” is STC 69, 3/8” is STC 54, then there is no way 1/16” of any material is anywhere near STC 70 plus. You will find similar results with drops in IIC performance as the thickness of the rubber and amount of concrete in the assembly decreases.
CHARACTERISTICS OF A QUALITY UNDERLAYMENT
Starting first with the characteristics of an underlayment 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 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.
View our GenieMat® RST rubber underlayment.
PERFORMANCE WHEN USED OVER CONCRETE OR OVER WOOD
The performance of any underlayment will vary considerably depending on the sub-floor, either concrete or wood, and depending on the ceiling below, either resilient or fixed (fastened directly to the joists). Because of this, a product claiming a universal rating for IIC or STC will most likely be extremely inaccurate. A product touting high Delta IIC ratings should also only be considered for concrete applications as there is no such thing as Delta IIC test in wood structures.
A concrete sub-floor is a rigid and massive surface that will not flex. The addition of a thin, resilient material like rubber brings significant gains in isolating footfall noise because it adds something to the structure that it is sorely missing. These gains increase significantly with a resilient ceiling using either resilient clips or resilient channel, again because the structure is sorely missing resilience.
A wood sub-floor is a fairly resilient and lightweight surface that will flex. Where a thin, resilient rubber over concrete can accomplish quite a bit, the same material over wood will not do nearly as much to increase isolation. If the ceiling below is not resilient, then we suggest a thicker underlayment, like the GenieMat® RST10 or thicker, for more noticeable gains in isolation. If the ceiling below is resilient, then a thinner underlayment, like the GenieMat® RST05 or GenieMat® RST02, can be used to achieve significant gains. There is a point when the structure reaches what is essentially enough resilience. Because of this, using thick underlayment, like the GenieMat® RST10 or thicker, in a wood structure with resilient clips on the ceiling below will have minimal to no value over the thinner GenieMat® RST05 or GenieMat® RST02 products.