Spray Foam; Good for Soundproofing?

Last Updated on March 3, 2021 by Facundo

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…

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.

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 used for soundproofing since it’s very affordable and has many air pockets inside of it which is great for absorbing sound.

But this isn’t the case with closed-cell foam since it doesn’t have air pockets and is much more rigid, it means that it doesn’t have the same soundproofing properties.

Spay Foam can be very useful when soundproofing not just for 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 it with Rockwool or Fiberglass will be much harder since you would have to cut it to the exact size.

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.

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.

However, when thinking about soundproofing, open-cell foam is much better since it’s filled with tiny air pockets, while closed-cell, since it’s hard and doesn’t have any air in it, may couple the two walls together and transmit vibrations and noise, resulting in a lower sound insulation overall.

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 the different materials (rockwool, 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 fiberglass or rockwool, if the walls are properly sealed, then there may be a slightly better performance achieved when using fiberglass and rockwool, and at a slightly cheaper price as well.

Just note that this will be hardly noticeable, and installing spray foam is much simpler.

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.

STCWhat can be heard
25Normal speech can be understood
30Loud speech can be understood
35Loud speech audible but not intelligible
40Loud speech audible as a murmur
45Loud speech heard but not audible
50Loud 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 and rockwool will increase the STC rating to 39 or 40, but so will open-cell spray foam. So, you could say that both fiberglass, rockwool and open-cell, not closed-cell, perform similarly.

However, rockwool usually does perform slightly better, but it also costs twice as much.

It is worth noting that closed-cell foam does also increase the STC rating, but only to 37 and at a higher cost.

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.

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