First, I am going to assume that you do not have radiant floor heat and that your subfloor is an 8" concrete slab (not lightweight concrete on capped wood floor).
With that said, cork is indeed a wonderful sound absorption product and has the added benefit of helping maintain good indoor air quality.
Measuring the transmission of sound
Your co-op is most likely concerned with sound isolation. Controlling the transmission of sound in a building is measured in one of three ways:
- NRC, or Noise Reduction Coefficient
- STC, or Sound Transmission Class
- IIC, or Impact Isolation Class
Noise Reduction Coefficient (NRC)is a measure of a material's ability to absorb sound reverberations over several frequencies within a given space such as a room. The quantity of noise absorbed will be affected by the reflectivity of the materials in the room such tile, hardwood, or sheetrock, all of which have high reflective properties. The NRC does not measure the sound transmission -- the passing of sound through a wall, floor, or ceiling -- but rather the level of sound within the room. NRC is expressed as a percentage; therefore an NRC of .50 means fifty percent of the sound generated will be absorbed and fifty percent will be "heard" in the room. Rooms with a lot of upholstered furniture will have less sound reverberation and therefore a higher NRC than rooms with sparse furnishings.
Sound Transmission Class (STC)rates a partition's (wall's) resistance to airborne sound transfer at the speech frequencies 125-4000 Hz. STC classifies a partition or wall assembly's resistance to airborne sound transmission in a single number: the higher the number, the better the isolation or the less sound that is transferred to the next room. The type of wall construction, wall finish and the existence of properly installed insulation in interior walls can impact the STC rating.
Impact Isolation Class (IIC)measures a floor/ceiling assembly's resistance to the transmission of structure-borne or impact noise -- such as footfall, furniture being moved across a floor, or items being dropped -- to the unit below. IIC is measured as a single number: the greater the number, the more sound isolation being achieved. You can find a good overview of ICC ratings on Findanyfloor.com.
Uniform Building Code requirements
The Uniform Building Code (UBC) contains requirements for sound isolation for dwelling units in Group-R occupancies, including hotels, motels, apartments, condominiums, monasteries and convents.?
The minimum IIC rating in many states, including New York, is 50.
Your choice of floor covering can have a significant effect on the amount of impact sound that is transmitted from your unit to the unit below, which is why the co-op board has asked you to review your choice carefully.
Cork, like carpet, can certainly help cushion the impact and therefore reduce the sound transmitted not only to the rooms below, but also to adjacent rooms on the same floor.
??As a means of satisfying the co-op, I would recommend the following material and installation prescription in your unit:
Select a plank-style, factory-finished cork floor product such as Qu-cork, a Greenguard-certified product. The planks are 1' x 3' x 7/16" thick with a decorative cork veneer wear surface over a high-density fiberboard substrate with a thin utilitarian cork backing. The planks have a tongue and a groove edge that "click" together.
The planks will minimize seaming and therefore the opportunity to transmit sound through "breaks" in the flooring material. Qu-cork products have an IIC rating of 71, well above the code minimum and above many carpet + pad combinations, which often range from 50 to 65, depending on the type of carpet and the quality of the carpet cushion (pad).
To maximize sound isolation, you will want to install the planks as a floating floor (not glued down) leaving a 1/4 " - 1/2 " gap at all walls, columns or any other fixed items such as built-in furniture/cabinetry and hold all base board and trim proud of the flooring material by approximately 1/4 ". By leaving these gaps, you can minimize sound reverberation in the walls and other assemblies.
If the prescription above does not satisfy the co-op, you canconsider adding an acoustical underlayment.
You need to consider your thresholds (entry doors, either front or balcony) and ensure that you have enough height to open doors with the new underlayment materials.
Here are some underlayment products to consider:
You could use a standard non-decorative cork underlayment in either sheet or tile form and glue down or float it on the concrete sub-floor. This material is available in different thicknesses, typically starting at 1/8" and going up to 1/2". One manufacturer is Acousticork, which can be purchased from Amazon.com. This can be applied with a no/low VOC adhesive -- see manufacturer's recommendations.
An alternative to a cork underlayment would be a CRI green-certified acoustical rubber underlayment such as QuietBLOCK from Fabricushion LTD, distributed by Future Foam. QuietBLOCK is 1/16" thick; is made of recycled automobile tires (and can be recycled at the end of its useful life, assuming it has not been glued down); and has an ICC rating of 70. Installation is made simple by a proprietary peel-and-stick poly film strip, called Moisture Block, which facilitates seaming of the material.
Alternatively, Dura Under Cushions Ltd. offers a CRI green-certified, recycled-content (85%) open cell acoustical rubber underlayment called Dura-Son which is distributed through Old County Flooring in New York (800-843-1379) and Patriot Flooring in New Jersey (866-444-4443). This product is 1/8" thick and has an FIIC (the F stands for field-tested) rating of 61. Both of the rubber underlayments are going to emit some odor for the first 24-48 hours, so plan on opening the windows and/or doors.
Also, theCarpet & Rug Institute (CRI)Indoor Air Quality Program lists several manufacturers' products that have been certified for low VOC emissions.
I sincerely believe you can state to your co-op, with confidence, that a cork floor (with or without the underlayment) will isolate sound transmission every bit as well as the average carpet.
Best of luck!