Fiberglass used to be the go to insulation material we put in our walls, but nowadays more and more people and resorting to foam alternatives, such as open- and closed-cell spray foam.
In this article, we will be taking a look at how well spray foam does at soundproofing, and we will also be comparing it to fiberglass and rockwool.
But first, there’s one hugely important misconception we need to straighten out…
Table of Contents
Soundproofing vs Sound Absorption
Soundproofing is the process of isolating or blocking the sound, not allowing it to enter or to leave a room.
To do this you will need to use materials that are designed to not let sound through, like drywall.
Sound Absorption relies on materials that are good at absorbing sound, such as acoustic panels, acoustic blankets, etc. to reduce the echo inside of a room.
Sound absorption, while not being great at soundproofing, will still help with it, which is why you might want to use it in conjunction with sound-blocking materials to achieve the best results, and this is why rockwool, fiberglass, spray foam, etc., are put inside of walls since they help absorb all that sound.
Now that that’s out of the way, let’s answer the question, shall we?
Is spray foam insulation good for soundproofing?
Open-cell foam is generally used for soundproofing homes and buildings since it’s very affordable and has many air pockets inside of it which is quite good for absorbing sound.
Now, even though closed-cell foam is much denser and even forms an air barrier, open-cell is still better at absorbing sounds as well as for soundproofing because it doesn’t transmit vibrations like closed-cell does, and since it expands a lot more than closed-cell foam it also ends up covering a larger surface area, and the thicker the material, the more sound it will absorb.
Spay Foam can be very useful when soundproofing not just because of its acoustic characteristics, but also because it will expand and cover the entire surface with ease, which if you’re trying to insulate oddly-shaped spaces, doing so with Rockwool or Fiberglass will be much harder since you would have to cut it to the exact dimensions.
Additionally, rockwool and fiberglass settle over time and end up leaving a small open space at the top or at the sides, and as we know, insulating for both heat and sound requires every gap, no matter how small, to be completely sealed.
So, spray foam could be considered much more versatile and easy to use than other traditional insulation materials since it will expand and seal every possible hole with ease.
You can read my two articles where I compare spray foam to rockwool and fiberglass here:
It’s worth noting, however, that there are two types of Spray Foam available, which are; Open-Cell and Closed-Cell foam. The difference between the two is that Open-cell foam is lighter & more affordable, while closed-cell foam is heavier & more expensive, as well as a better heat insulation material because it’s better at preventing airflow.
When thinking about soundproofing, open-cell foam is generally better because of its lower cost and ease of installation.
Spray foam vs Fiberglass and Rockwool
Now, according to the Canada NRC Internal Report IRC-IR-693, there doesn’t seem to be a huge difference between fiberglass and spray foam when it comes to noise reduction, which is why open-cell spray foam may be the better alternative since it makes sealing every nook and cranny easier and you won’t have to worry about it settling over time like Rockwool or Fiberglass will, which will end up creating openings for sound to get through.
However, when comparing two 2-by-6 framed walls, one filled with open-cell foam and one with rockwool, and if the walls are properly sealed of course, then there will be a better performance achieved when using rockwool, and at a slightly cheaper price as well.
How much of a difference is there? Well, first we have to learn how sound transmission is measured:
How is Sound Deadening Performance Measured?
How efficiently sound is reduced is measured using the STC rating, or Sound transmission class, which tells us how many decibels the barrier can reduce. So, the higher the rating, the better.
In most scenarios you should be aiming for a rating of 40, which would be a 40dB reduction, since this is generally what it takes to get good enough insulation.
| STC | What can be heard |
| 25 | Normal speech can be understood |
| 30 | Loud speech can be understood |
| 35 | Loud speech audible but not intelligible |
| 40 | Loud speech audible as a murmur |
| 45 | Loud speech heard but not audible |
| 50 | Loud sounds faintly heard |
| 60+ | Good soundproofing; most sounds do not disturb neighboring residents. |
Of course, the higher the better, but if you want to get to a 65+ rating then you probably will have to build a decoupled wall to eliminate as much sound as possible.
How much does each material improve the STC Rating (sound insulation)?
A typical interior wall with ½” of drywall on either side already has an STC rating of 34.
Installing fiberglass will increase the STC rating to 39, but so will open-cell spray foam. So, you could say that both fiberglass and open-cell, not closed-cell, perform exactly the same in terms of soundproofing.
Rockwool, on the other hand, performs exceptionally well increasing the STC rating to 45, which is a huge difference.
It is worth noting that closed-cell foam does also increase the STC rating, but only to 37 and at a higher cost.
| Material | STC |
| Fiberglass | 39 |
| Spray Foam | 39 for open cell 37 for closed cell |
| Rockwool | 45 |
Potential pitfalls of Spray Foam (Fire Hazard)
Spray foam can ignite and burn if exposed to a sufficient heat source and therefore is considered to be combustible and should be handled accordingly. In addition to being flammable, this kind of spray foam has chemical properties that produce smoke when ignited, which can render occupants of a house unconscious and even lead to temporary blindness.
Spray foam, be it open- or closed cell, has a Flame Spread rating of 75, with Rockwool & fiberglass having one of 25, which means that it will spread fire to nearby combustibles much quicker, so keep that in mind as well.
Here’s a table with the flame spread rating of the different insulation materials typically used:
| Material | Flame Spread Rating | Smoke Development |
| Spray Foam | 75 | 450 |
| Mineral Wool (Rockwool) | 25 | 50 |
| Fiberglass | 25 | 50 |
| Mass Loaded Vinyl | 25 | 250 |
| Cellulose | 25 | 50 |
Additional ways of soundproofing a wall
First of all, it’s worth noting that the insulation process needs to be done during the construction of the house, otherwise you would have to either tear down parts of the wall to be able to insulate it, or add a second wall (which is an option).
Install two layers of drywall
In order to keep sound out of a room you need to block it, not absorb it, since sound absorption will only do so much.
Installing drywall is one of the easiest ways of adding more mass to the wall, effectively adding another barrier that prevents sound from getting through.
One sheet of drywall will already provide you with a significant reduction in noise, but the best way to go about it is to install two layers of 5/8 drywall.
Additionally, you should consider putting a noiseproofing compound between the two layers, which will create a slight air pocket to further increase the insulation.
Doing this should give you an approximate STC of 35 or 40. In other words, it will reduce the noise that’s allowed through the wall by about 35dB-40dB, which means that loud speech, for example, will be audible but unintelligible.
Build a decoupled 2-layer drywall
One thing you may not know is that the structures in your wall, like the studs, help at transmitting sound and vibration, and installing drywall on them will transmit all that sound to the drywall itself.
The way to approach this is by building a second wall and leaving an air gap between it and the previous wall which will then be filled with fiberglass or rockwool, and THEN installing two sheets of drywall (again, using the same noiseproofing compound between the sheets).
The way to really decouple the new wall from the original studs varies depending on the approach, for example; You could build double stud walls where the studs from one wall don’t contact the other ones, you could stagger them, and there even are systems you can buy which you install on the original studs and they will lower the noise transmission to the drywall be decoupling it.
I won’t go into any more detail because it doesn’t pertain to the topic at hand, but if you want a complete overview of how decoupling works, check out this post.
Conclusion
Open-cell spray foam is a good soundproofing material since it’s filled with air pockets, just like fiberglass and rockwool.
Not only that, but it’s very ease and quick to install, and since it expands it will cover the entire surface in no time.
The truth is that you can go either way, and if you want to use fiberglass or rockwool, you can. But open-cell foam does get the job done as well.
Last Updated on February 18, 2022 by Facundo
EN 
ES