Spray foam insulation can and does effectively reduce noise transfer between spaces, but a lot depends upon where spray foam is installed and what your expectations of sound proofing are.
Different spray foam insulation products have distinct sound deadening qualities too. Either way foam is incredibly good at finding and closing up the cracks and fissures you find in a wall, or a roof structure, where noise could be transferred.
How far do you need to go to enjoy a quiet life? Is spray foam enough on its own? Are there other strategies you can use to reduce noise nuisance?
Here’s our guide to reducing noise with spray foam.
First Principles of Noise Reduction
What is the difference between sound deadening and sound proofing?
Sound Proofing
Sound proofing is when all external noise is completely excluded from a space.
Making a space soundproof cannot be achieved through insulation alone. Sounds such as speech, road traffic, animal noises and music travel readily through the air, reflecting off hard surfaces while being absorbed by others. A glass and concrete office building will bounce noise back at you, while a conifer hedge will dampen it.
Percussive impact noise, such as footfall on an overhead floor, or vibration from a large appliance, is transferred through the solid structural parts of a building, and the only way to completely eliminate this sound transmission is to isolate the structure. You could do this by separating the floor above from the beams below using a proprietary isolating system.
Sound deadening
Sound deadening is when noise is cut down, but not excluded entirely from a space
For most projects, unless you are building a media studio, which might demand more complex specialist attention, simple noise reduction work will be all that is required. What you need to know is how effective these sound deadening strategies are.
The good news is that spray foam insulation can help to reduce noise, deaden sound, and deliver a level of comfort as well as privacy in many settings, without being complicated.
Is Spray Foam Rated Highly as a Sound Insulating Material?
How can you be certain that spray foam is any good as a sound insulator? The answer to that is there is a rating system, the STC or Sound Transmission Classification, for example, a standard uninsulated sheetrock wall between two internal rooms is rated 25.
The number indicates the level of decibels that can transfer easily through the wall in test conditions. Open cell spray foam is especially good at dampening sound levels due, in part, to its high expansion rate, which means it can get into all of those small spaces that other insulators just can’t reach.
Open cell spray foam is less dense than closed cell, its larger structure traps more air, interrupting noise transfer. Closed cell is a denser product, while it is better at stopping heat transfer, it has a slightly lower STC rating, but just by a couple of points.
Coupled with the STC rating is another acronym, the NRC, or Noise Reduction Coefficient, which can tell us how much noise a material can absorb. A rating of 1 tells us a material will absorb 100% of transmitted sound. Open cell spray foam can achieve an NRC rating of 0.75 at a depth of three inches, which is an excellent result.
What Rating Do I Need?
A human voice has a wide range, starting with a whisper at about 30 decibels. Normal speech comes in at about fifty-five to sixty decibels, which means that an STC rated wall should reduce conversation in an adjoining room to a murmur.
| STC Rating | What Can be Heard |
| 25 | Normal speech can be heard clearly and understood easily through a wall structure |
| 30 | Loud speech can be understood reasonably well, normal speech heard but not understood |
| 35 | Loud speech is audible, but difficult to understand |
| 40 | The onset of privacy |
| 45 | Loud speech inaudible and up to 90% of population not disturbed |
| 50 | Very loud sounds, i.e. musical instruments, a music speaker, etc can be faintly heard, 99% of population not disturbed |
| 60+ | Superior sound proofing with most sounds inaudible |
Unfortunately, there are many issues that can cause a wall’s STC rating to fail, and these are due, in the most part, to penetrations in the drywall that disrupt the insulator and the air barrier; think electrical outlets, light fittings, that sort of thing.
In any setting, paying attention to these areas can maintain and enhance the noise reduction rating of your walls. For most homes a rating of fifty between internal bedroom walls should be sufficient to guarantee an undisturbed night.
A motorcycle engine runs at approximately 95 decibels, and when accelerating this figure can climb rapidly. A lot of airborne road noise can be transferred through the roof space and down through the ceiling into the bedrooms.
Which is Better, Open or Closed Cell Spray Foam?
Open cell spray foam has sound transmission classification (an STC rating) of 39 at 3” deep, which is a direct tie with 4” of regular fiberglass. Mineral wool does better at three and a half inches, with a top STC rating of 45 for an insulation product, however, what foam does so well is fill any and all gaps in a structure that might contribute to sound transference.
Closed cell spray foam is generally two to four times denser than open cell, and for this reason it doesn’t do as well in SCT or NRC ratings, but its air sealing ability is never considered in lab-based tests. In the real world of renovation and construction this benefit counts for so much more.
Noise Nuisance: Road Noise
Living close to a major highway, or on a well-used local commuter or school route can be tiresome with vehicles passing by almost constantly. Road noise is mostly airborne, and it can often be heard from miles away, but stopping it from intruding on your home life is not impossible.
This is where spray foam insulation wins. Open cell spray foam used in your wall construction not only air seals, but a three-inch deep application of Handi Foam Sound Barrier can deliver an STC rating of 35, NRC rating of 0.7 and an R-value of 12.9 (4.3/inch).
Filling an external wall cavity with up to five inches of low-density spray foam will give you an R-value of 21.5, meeting code in all climate zones. In addition, it will also push the STC of the wall up to fifty, helping to deaden most road noise to acceptable levels.
Noise Nuisance: Garage Noise
In many modern houses with a garage, there is oftentimes a living space directly above, usually a bedroom. The ceiling structure of the garage should be insulated and protected from fire. If the insulation is a competent sound deadening material then so much the better.
Open or closed cell spray foam both perform well in this location, although impact noise, such as hammering, or drumming, if the garage doubles up as a space for band practice, is almost impossible to eliminate entirely.
A lot of the time it’s the noise made by a garage door opening and closing that can cause most disturbance. To combat this it’s worth considering upgrading the drywall from a half inch standard thickness to ⅝”. Not only will it raise the STC rating, but it will also add fire protection in a vulnerable location.
If you need to completely soundproof a space, then that will require additional construction details that any insulation material on its own cannot deliver.
Noise Nuisance: Privacy
Partition walls between bedrooms can be less than four inches total thickness, and if they are older, or the rooms share a common HVAC system, then it is likely that there will be no insulation between the wall studs. Oftentimes it would be most practical to remove the drywall from one side of the wall and insulate the space.
Spray foam is good in that it will penetrate all the tiny airspaces in the wall. Wrap masking tape around any electrical outlets to protect the internal workings but close up the airways that noise can travel through.
When reboarding use two layers of 5/8” drywall and stagger the joints to seal up the wall and increase density. Using a low-density foam such as open cell should deliver an STC rating higher than fifty. Adding a tube of Green Glue between the boards creates a tiny air gap that will add to the sound deadening.
In a new build it might well be worth considering insulating all internal walls with open cell spray foam at the same time that the external walls are having insulation installed. Once the services have been mounted in the walls, and one side has been boarded to provide a substrate the work can be done very quickly.
Time is money on any construction project, so this strategy could shorten the duration of the works and still deliver the privacy required between bedrooms and common living areas.
Best Practices for Noise Reduction
It might be tempting to completely fill the depth of a wall void with spray foam, but in actual fact, in dividing bedroom walls it would be of benefit to leave a minimum air gap of approximately one inch between the face of the foam and the backside of the drywall. This would add a 1” barrier and reduce impact noises because one side of the wall does not come into contact with the other.
