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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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
KG, it is my understanding that the 6mm cork underlayment generally solves this issue. I'm not sure if the sealed concrete is structural or a topping but a wood floor over cork over concrete should give you an IIC over 55. Here's a link to some manufacturer's data.
http://www.greenbuildingsupply.com/DocumentFiles/24.pdf
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?
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.
http://www.woodsthebest.com/Flooring/underlay/noiseReductionUnderlayRatings.htm