What is the best option for sound barrier flooring for multi-family housing?

Asked by Cynthia Grier
Ojai, CA

Looking for a hardwood product and adhesives that are non-toxic and as eco-friendly as possible.


Cynthia Phakos

Answered by Cynthia Phakos

Los Angeles, CA

Koffka Phakos Design

May 20, 2013

Acoustical separation of units is a most critical component in the design of multi-family housing. In renovating units we have been required to present our acoustical details to Home Owner Associations as well as the Building Departments for approval. 

Insulation between the ceiling joists and the wall studs and resilient channel attachments of the drywall to the structure help in mitigating the sound. Floors have their own complications. I think an understanding of the type of noise, rating systems, codes and methods to reduce the sound transmission is important in determining the best solution for your residence.

Types of Noise

There are four types of noise that affect the performance design of the floor/ceiling . The first two, airborne and impact noise, are something that you as a tenant can mitigate. 

  1. airborne,
  2. impact, 
  3. structural deflection, and 
  4. floor squeaks. 

Airborne noise. This noise is transmitted through the air and intervening partition and floor systems. Sources would be a voice, a barking dog or a television. Solutions include adding: absorptive materials (ie. wall and ceiling insulation); an air space by attaching the drywall with resilient channels, and; mass, as with multiple layers of drywall. 

Impact noise. This originates when one body strikes another, ie. footsteps, hammering, and falling objects. This noise can travel through the structure with little loss of energy if the structure is rigid and continuous. Solutions include soft flooring materials that absorb impact (ie carpet and pad) or resilient materials used as an underlayment for hard materials.

Structural deflection. This noise is different in that an impact load on the floor causes the structure to deflect such as the noise of a sumo wrestler living upstairs. This is beyond a tenant’s control as it involves the stiffness of the structure and so design of the building.

Floor squeaks. These occur from rubbing, as in the case of poorly installed wood planks. This can be corrected with the installation of a proper sub floor.

Multi-family Housing Codes

Minimum code requirements for STC (Sound Transmission Class) and IIC (Impact Insulation Class) ratings are typically 50. 

Some municipalities mandate a higher rating; the City of Redondo Beach sets a minimum requirement of 65 in condominiums. To start you should check with the City or Ojai for their requirements.

Another consideration is the building's quality level. Minimum code requirement are not necessarily good design. Housing can be grouped into 3 types:

  • Minimum (standard apartment): recommended STC/IIC 55
  • Medium (good apartments and normal condominiums): recommended STC/IIC 65
  • High (luxury condominiums): recommended STC/IIC 75

High ranges (75) are relatively easy to achieve by using carpet and pad, which explains the prevalence of carpet in multi-tenant housing. As an example, 4” thick concrete floor with no treatment or ceiling below has an IIC rating of 45. By adding carpet and a pad the IIC can increase to as much as 80. 

Using the STC and IIC Rating Systems

The sound rating classifications, established by HUD and the National Bureau of Standards, are recognized by building construction regulatory bodies and agencies and found in our building codes. 

As there are a number of components in the design of the floor/ceiling assembly, the resultant STC and IIC ratings are based on testing the total assembly. To properly design your floor’s acoustics, you will need to know the existing floor and ceiling structure of the building. 

Another method would be to find out the Delta IIC rating (ΔIIC) which is the rating for the material itself without considering the floor/ceiling assembly. This will give you a better comparison between the products you are evaluating without the contributions of concrete, sound rated ceilings, and other components.

An acoustic treatment for hard floors

A common cost-effective, acoustic treatment for ceramic tile, stone and wood floors in multi-family housing is the use of an Enkasonic mat

Made of nylon filaments forming a three-dimensional matrix, the mat obstructs the transmission of sound. The mat can be used for both wood and concrete floor systems. A 4” concrete slab with an Enkasonic mat on 7/16” wonder board (mass) and ceramic tile can achieve an IIC of 52.

Adding an underlayment

For higher levels of sound isolation, add an acoustic underlayment in both STC ratings and IIC ratings. The materials used in acoustic underlayment are usually recycled rubber, cork, a combination of rubber and cork (higher performing), mass load vinyl (MLV), foam, and a combination of MLV and foam. 

Of these, the recycled rubber and cork or combination would be the “Greener” solutions.

Cork Underlayments. Cork is known for having excellent acoustic and thermal properties, with 200 million cork cells per cubic inch, each cell effectively inhibits the transmission of sound and heat. Cork is non-toxic and rapidly renewable and does not support mold or mildew.

There are many cork products to consider, but the one that I have had good experience with is AcoustiCORK. The sound data is listed as a total system as discussed. Of interest is that higher IIC values are achievable with a floating wood floor installation and you might consider such an installation. This would eliminate gluing the floor making for a more VOC free installation.

Recycled Rubber as an Underlayment. Another option is a recycled rubber base underlayment used under floated or glue down hardwood or laminate flooring. Impacta by Regulpol is made of 92% recycled tires. The rubber is crush resistant and durable with good acoustic properties for wood and other hard surfaced floors. It is available in multiple thicknesses from 1/8”, up to a ½”. 

Underlayment installation

Many of the underlayment installation methods are a glue-down application. Another method of installation is as a floating floor. 

Loose-lay underlayment installations, which are common for floating floors, require only a small amount of adhesive around the perimeter of the room, reducing VOC emissions to a minimum.

As discussed, mass is another component in the acoustic assembly that helps with airborne noise. Should the system that you choose require a new subfloor, you might consider “Green Glue Compound” which acts as a damping glue between the existing sub-floor and the new layer of sub-floor. This will only work with an existing wood sub-floor as you need to screw the new sub-floor to the existing for the glue to perform properly. 

Do you have a question about greening your home? GreenHomeGuide invites you to Ask A Pro. Let our network of experienced green building professionals – architects, designers, contractors, electricians, energy experts, landscapers, tile & stone specialists, and more – help you find the right solution.