Sound Transmission Class (STC) and Impact Insulation Class (IIC) Between Dwelling Units

In multifamily building construction you need to insulate the dwelling units for sound. When using the International Building Code all common interior walls, partitions and floor/ceiling assemblies between adjacent units and between units and hallways, stairs and service areas need to be insulated. There are two types of sound that need to be addressed, the first is sound transmitted through the air and the second is sound transmitted through the building structure.

Sound Transmission Class (STC) is a rating of how well a building partition attenuates airborne sound. The STC number is roughly the reduction in decibels that a partition creates in the 125 Hz to 4,000 Hz range. Walls and floor/ceiling assemblies need to be insulated for airborne sound to achieve a STC of 50 or more. Typically you can find the sound rating for assemblies in the same location that you found the fire rating for the assembly. You can also contact manufacturers for more information on assemblies that use their products. If you choose to create your own assembly and have it tested, it must have a minimum STC of 45 after it is tested. Where you have openings and penetrations in the assembly they must be adequately insulated to maintain the STC with the only exception being entry doors.

Impact Insulation Class (IIC) is a rating of how well a floor/ceiling assembly attenuates structure borne sound. The IIC number is roughly the reduction in decibels that a partition creates in the 100 Hz to 3,150 Hz range when tapped by the testing machine. The easiest way to achieve the required IIC rating of 50 is to use carpet and underlayment as your flooring which can give you 20+ points. There are also a number of underlayment options for harder surfaces. You can also have the assembly field tested if you don’t have an official rating and again you need to achieve a 45.

Now on the subject of field testing vs using an established rating, it is my opinion that field testing should only be chosen if the particular assembly is necessary to the building design or will create serious savings. Otherwise it isn’t worth the brain damage and testing costs.


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8 thoughts on “Sound Transmission Class (STC) and Impact Insulation Class (IIC) Between Dwelling Units”

  1. The IIC rating of a condo unit floor above another condo unit should be 75 or more to match what is provided by carpet and reasonably thick carpet padding. Some condos prohibit hardwood flooring entirely to make sure noise transmission into lower units is not increased. So no, 1/4-inch cork is not adequate on concrete.

  2. It is important for any specifier to do their due diligence regarding sound control in multi-family, public use buildings and to collect proper documentation. This process will protect the best interest of all parties involved.

    There are a lot of smoke and mirrors regarding sound control. Mostly regarding manufactures claims on data sheets. I would advise requesting a laboratory tests supporting any manufacture published data sheets. These test should be done by a NVLAP accredited testing facility. NGC Testing Services is a nationally recognized and accredited facility.

    All test should conform with ASTM standard test method for laboratory measurement of airborne sound transmission loss of building partitions and elements – Designation E90-04 / E413-04. The test will include the specimen of the floor ceiling assembly, the wear layer, the underlayment and substrate information along with any ceiling assemblies. This portion of the test will arrive at a STC (Sound Transmission Class) rating. Any number at or above a STC of 50 is acceptable.

    Next the test is regarding the IIC (Impact Insulation Class). This test is by far the most important as hardest to achieve acceptable results for any manufacture as it pertains to foot fall noise from above occupants.

    This test shall be done in accordance with the ASTM sound transmission through floor ceiling assemblies using the tapping machine – designation E 492 – 04 / E 989 – 89. the uncertainty limits of each tapping machine location met the precision requirements of section 11.3 of ASTM E 492 – 04. This portion of the test will result in the IIC rating. Once again any number at or above a 50 is desirable.

    Their are many other considerations when specifying sound control systems that can not be covered here. Good Hunting.

  3. Edward, I think you can specify that they use a laminate flooring and underlayment that a manufacturer has tested to provide an IIC of 50 or greater.

    The underlayment choice is really key for sound insulation and like the cork manufacturer that I linked they’ll typically have some sample tests. The cork provides an IIC of 60 with a wood finish over wood joists.

    I’m sure that laminate manufacturers have preferred underlayments but the owner can generally choose. I’ve installed a couple laminate floors and there are many options. Here’s a chart showing several products yielding IIC over 50.

  4. We are rewriting our CCR’s. The question I have, is there a IIC Standard for Condo Floor/Ceilings Assembly using wood laminate floor & underlay which is acceptable or equal to Carpet on a 6″ concrete slab?

  5. Does 1/4 inch cork generally meet the 55 iic acoustic qualifications set by condominium associations for installation over sealed concrete floors and under prefinished flooring? Thanks, KG

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