Q. Are houses insulated with open- or closed-cell spray foam quieter than homes insulated with standard fiberglass batts? In general, what is the best insulation to use in terms of sound control, and does the choice of insulation play a large role in how quiet a wall is?
A. Bonnie Schnitta, president of SoundSense, a company that specializes in acoustic engineering and consulting in Wainscott, N.Y., responds: Here at SoundSense, our engineers are often asked about the acoustic properties of different types of insulation, specifically as they relate to how much sound the insulation will stop. While we have a list of preferred types and brands of insulation that we recommend to our clients, it’s impossible to make informed choices about a specific product and method of installation without a firm understanding of acoustic fundamentals. Just how effective any acoustic material will be is dependent on the construction of the walls where it will be installed. If a wall is improperly configured, it may render any acoustic treatment ineffective.
STC AND DB
The Sound Transmission Class rating, or STC, of a wall assembly is a single-number rating calculated in a controlled ASTM-certified lab environment to quantify the ability of a material to block sound with strict control of any potential flanking, or acoustic leakage. When making recommendations for any project, we start with a target STC in order to select the appropriate materials and configurations to achieve any acoustic goals. (For more information about STC-rated wall assemblies, see Noise Control Manual for Residential Buildings by David A. Harris, or Architectural Acoustics by Marshall Long).
Generally, STC ratings can be interpreted as follows:
- 25 Normal speech can be understood quite clearly
- 30 Loud speech can be understood fairly well
- 35 Loud speech is audible but not intelligible
- 45 Loud speech is very faint
- 48 Some loud speech is barely audible
- 50 Normal speech is not audible, but amplified sound will be audible
- 60 Minimum requirement for amplified sound
When a field test is performed to determine the transmission loss of the partition, this test is referred to as ASTC (Apparent Sound Transmission Class). Viewing the numbers above, it is easy to see that no matter how much sound a specific type of insulation will stop, if the STC when the insulation is added does not exceed 35, then there is a good chance that the client will not be pleased with the extra expense of the insulation. And although the STC or ASTC is a single number, the amount of sound that the wall stops can vary significantly in different frequencies, even if the STC is the same. As an example, walls with the same STC with only one layer of 1/2-inch gypsum board perform better in most of the higher frequencies, while that same configuration with 5/8-inch drywall will perform better in the lower frequencies. Sound or noise (unwanted sound) is measured by decibels (dB), which is a logarithmic function, much like the Richter scale for earthquakes. One rarely hears of a 2 or 3 earthquake, but an earthquake of 5 or more makes the news. As sound increases or decreases, decibels increase or decrease logarithmically, not arithmetically; thus, a doubling of the volume of a sound shows only a 10-point increase in dB. For example, one TV set at a normal conversational level is about 60 dB. Ten TV sets at the same volume will sound twice as loud but only register another 10 dB, or about 70 dB.
The standard acoustic rule of thumb is that any sound that exceeds the background noise level by 5 dB(A) or more has the potential to be an annoyance. By the same token, if a sound reduction measure does not provide at least a 4 dB improvement, there will not be a perceivable difference. SoundSense typically engineers walls with the guidelines shown in the table “STC Requirements and Recommendations,” on the next page.
FLANKING
Sound will travel through any weak or incomplete junctions of a partition. For example, the STC rating of a wall may be reduced by as much as 10 points with something as seemingly minor as a 1-inch-square gap where the wall meets the floor. Even a wall on which the space between two sheets of drywall has not yet been taped and spackled has an STC rating that is up to 10 points lower than that same wall after finishing.
Like water, sound will take the path of least resistance. It will pass through even the smallest gaps, such as where drywall is not properly caulked at seams or a floor-wall intersection, the perimeter of doors, and recessed lights without an insulated can or a high-hat muffler. For this reason, acoustic leakage paths such as doors, outlets, plumbing penetrations, shared chases, conduits, ceiling-wall or floor-wall points of intersection, and the like in construction should be addressed so the wall or other partitions perform up to their designed STC ratings (see “Innovations in Sound Control,” JLC, Mar/06).
REVERBERATION
To determine what, if any, insulation should be used in a wall, it’s important to understand the phenomenon of reverberation amplification. Reverberation is sound persistence due to repeated boundary reflections, such as between two reflective parallel surfaces, even after the source of the sound has stopped. Most people are familiar with this phenomenon, as it is experienced in a noisy restaurant or in an echoey room.
When reverberation amplification occurs, sounds may be amplified to the point of disturbance. Putting insulation in a wall will inhibit any sound entering the cavity of a wall from having a direct reflection and help prevent reverberation amplification within the wall cavity.
INSULATION
The range of improvement for an uninsulated wall versus a wall with a single layer of standard fiberglass batt insulation will be between 3 and 11 STC points. The greater improvement is from such wall configurations as a double stud wall or a wall with added resiliency, such as from resilient clips, but it is also a result of the absorptive ability of insulation, its thickness, and its ability to disrupt reverberation amplification.
The speed of a sound wave is dependent on the properties of the material through which it passes; the more energy required to pass through a material, the less energy remains in the wave, also known as transmission loss. This means that the overall density of the insulation—whether from the thickness of the insulation or from its density—will improve transmission loss. This is on average about a 4 dB improvement, which is significant and perceivable.
Finally, considering how acoustic leakage can significantly degrade the ASTC of any wall, fully filling the wall cavity with a thick or dense insulation can potentially improve the transmission loss by at least 5 dB. Based on mass and density, we typically recommend mineral wool insulation, such as Safe’n’Sound and ComfortBatt from Rockwool, Johns Manville Mineral Wool Insulation, and Thermafiber from Owens Corning for the most effective sound control in an STC-rated assembly. Spray foam can solve acoustic leakage problems by filling the small holes previously discussed. If a high-STC wall is being used that is achieving its high STC by including resiliency in its configuration, then spray foam that hardens will short-circuit this configuration, unless the spray foam does not connect the two portions of the wall that have a resilient connection. If the spray foam is not rigid, then it has the benefit of filling holes and helps with the high STC design of a wall as long as the spray foam has the same density as that of sound batt.
For these reasons, we generally don’t recommend either closed-cell or open-cell spray foam insulation for sound control, unless there is attention to preventing a short circuit. As you can see from the chart above, neither insulation has a significant impact on the STC rating of a 2x4 partition wall. Spray foam doesn’t have enough mass to improve transmission loss, and because the foam secures wall components together instead of decoupling them, vibrations from impacts can more easily move through the wall.
