Need to reach a certain IIC rating?

Here’s some sound advice to prepare yourself with before you contact an acoustic consultant or product supplier:

1.) IIC and STC ratings are dependent on entire assemblies, not any single element.

This means that you’ll need to have all of the details of your floor-ceiling assembly before you can expect real advice from an acoustician.  Ideally you’ll have an architectural drawing calling out each element and it’s thickness in the assembly:
  • Floor Finish (Floor Covering)
  • Underlayment/s
  • Subfloor system (concrete slab?  wood joist?  truss?  spacings, thicknesses, gauges, etc.)
  • Batt insulation in the ceiling cavity?
  • Resilient channels?
  • Ceiling details (material, number of layers, thicknesses?)
2.) The science of predicting IIC and STC ratings has not been perfected.

Much to the dismay of many people, acousticians can’t just add a couple of numbers together to determine your assembly’s predicted IIC rating.  Believe it or not, there is no set formula or equation out there that will accurately predict these ratings.  Currently, the only absolute way of determining a rating is by testing the actual assembly.  Even then, there are still many factors that can prevent you from obtaining an accurate rating.  Also, due to inevitable variations in construction conditions, one assembly’s rating should not be considered representative of another assembly, even an identical one.  This often-overlooked caveat is specified in the detailed description of the IIC test (ASTM standard E492).  It is, however, helpful to use test results as reference points of what you can expect from identical or similar assemblies.  With all this said, even the best prediction is likely to come with a +/- 3 dB window of uncertainty.

3.) More is not guaranteed to be better.

Of particular interest to IIC ratings one common misconception is that using an increased thickness of an underlayment will definitely result in a better IIC rating.  While this is generally true, this is a good time to familiarize yourself with the 'law of diminishing returns'.  Wikipedia defines this in the context of economics, but it’s also applicable to the physics of IIC ratings.  The initial doubling of a material will yield the highest return on value.  For example, suppose a 2mm underlayment improves an assembly’s IIC rating by 12 points.  Doubling it to 4mm might give you another 3-5 points of improvement.  Increasing the thickness beyond that point might add another 1-2 points for a few more millimeters, but you will see the amount of improvement begin to plateau with the increased thickness.

4.) Sometimes, you’re just gonna have to add a ceiling.

With the current trend of converting old factory buildings into residential units or trendy restaurants, exposed beams and decks are a desirable architectural feature. Unfortunately, in most cases, it’s going to be difficult to achieve a code-minimum IIC 50 rating by only treating the floor-side of the assembly.  Either way, it’s all about physics.  If you want to prevent sound from being transmitted from one space to another, your best bet is to add mass.  The most common ways of achieving this are by adding either an inch or more of gypsum-concrete or by adding a ceiling treatment below.

The above items should get you off to a good start with appropriate expectations when reaching out to an acoustical consultant for help with your IIC ratings.

International Building Code:

The International Code Council says the 'minimum' acceptable level of performance for both STC and IIC is 50-IIC). The “preferred” level of performance for STC and IIC is 60. 

An IIC and STC rating will generally be 5 points higher than a F-IIC and F-STC rating which indicate tests conducted in the field where flanking is not as controllable - hence the 5 point variance.

Calculating Soundproofing Requirements

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Determine The Sound Insulation Value of the 'Bare Sub Floor' (without floor coverings attached):
  • A typical 6" concrete slab will have a average baseline sound insulation rating of IIC 27. Keep in mind actual slab ratings will vary from a low 26dB to an exceptionally high value of 29dB IIC. 
  • A typical 8" concrete slab has a baseline rating of IIC 32dB (potential range is 28 to 33dB IIC).

In order to achieve your target (required sound rating of sub floor assembly with floor coverings attached) the formula works like this:

Sub Floor IIC ____ + Soundproofing Δ ____ IIC = Final IIC Rating

Δ IIC = The amount of sound reduction of the soundproofing product by itself. You should be able to add a couple decibels of sound value with wood, resilient flooring or carpet.

If You Have 6" concrete slab:

Assuming a low average value for 6" concrete (IIC 27) - you will need a soundproofing product with a minimum Δ 23 IIC to comply with the minimum building code IIC 50.

6" Concrete IIC 27 + Soundproofing Δ 23 IIC = IIC 50.

You will require:  Sound Insulation with a Δ IIC 23 rating when using Tile - other floor finishes may add to the value.

If You Have 8" concrete slab:

An 8" concrete slab with a baseline IIC 32 value will require a soundproofing product with a minimum Δ 18 IIC to comply with the building code IIC 50.

8" Concrete IIC 32 + Soundproofing Δ 18 IIC = IIC 50.

You should specify: Sound Insulation with a Δ IIC 18 rating or better with Tile to comply with minimum code - wood flooring and other floor finishes may add to the value.

Compare Competing Products Using Test Results

             Comparing Warranties Can Be Like Comparing Apples and Oranges

The ASTM E2179 'Delta' Test Is Your Best Product Evaluation Tool:

The ASTM E2179 Test protocol -  tapping machine test on Tile installed on a 6" Concrete Slab with no insulated ceiling


These results show rather conclusively that QT delivers more sound insulation in thinner applications VS a variety of competing sound insulation materials when tested on the same sub floor.

QT rubber is so versatile most any kind of flooring can be installed directly on top of the product while most competing sound insulation products have limited use. Tile floor coverings can be bonded direct to QTscu for residential and light commercial areas as tested by the Tile Council of North America. QT products are also impervious to the elements so it can be used as underlay or non-slip floor covering indoors or out.

QT Sound Insulation is composed of 94% recycled rubber granules reclaimed from used car tires mixed with multi-colored EDPM rubber 'flecs' recovered from rubber flooring production waste. These two recycled rubber components are cold bonded with a high quality urethane binder. Utilizing car tire rubber for flooring projects is good for two big reasons: (1) It benefits the environment by diverting millions of tons of waste tires from our landfills. (2) Sound tests show that QT Sound Insulation attenuates more sound than competing technologies.

QT Sound Insulation is impervious to the effects of moisture however, certain decorative floor coverings like wood and PVC sheet and tile have a specified moisture vapor transmission exposure limits. The upper end limit specified by most resilient and wood flooring manufacturers is 3 lbs. per 1,000 square feet per 24 hours (as measured by the Calcium Chloride Test). If moisture vapor transmissions exceed 3 lbs., the sub floor must be receive a vapor retarding 'barrier' capable of reducing vapor transmissions to specified levels . When Tile or Stone will be the floor covering finish, the moisture limit for QT is 5.5 lbs per 1000, square feet per 24 hours of exposure.

Radiant Heat Floor Systems? Our recycled rubber composite has a nominal BTU rating which means it has very low thermal resistance making it an efficient conductor of heat. When installing wood or resilient floor coverings on slabs with embedded radiant heating systems, always refer to the flooring manufacturer's installation guidelines and warranty limitations.

QTrbm adds sound attenuating mass to (noisier) wood sub floor assemblies. QTrbm is specified as a floating 'base mat' beneath  
poured Gypsum-Concrete applications as thin as 3/4".