Spray foam vs Fiberglass insulation for Soundproofing!

If you live on a busy street or if you have really noisy neighbors, then you know how important the role is that acoustic insulation plays, since the difference with and without it can be as big as night and day.

There are many different sound insulation materials, such as rockwool, spray foam, cellulose, fiberglass, denim, and more, and they all have their pros and cons.

In this article, I will be comparing spray foam, both open cell and closed cell, to fiberglass in terms of how they insulate against noise.

I will also be going over their STC rating, Flame Spread index, because you always have to consider if what you’re putting into your home can be flammable or not, their overall installation cost, if you can do it yourself or if you need to hire someone, and much more.

So, without any further ado, let’s get started!

First you need to understand the differences between Soundproofing and Acoustic Treatment

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, on the other hand, relies on materials that are good at absorbing sound, such as acoustic panels, acoustic blankets, etc. to reduce the “echo”, or reverberation, inside of a room.

Sound absorption, while not being great for soundproofing purposes, may still help with it, which is why you might want to use it in conjunction with sound-blocking materials to achieve the best results (drywall and fiberglass, for instance).

When trying to reduce outside noise, we’re talking about soundproofing, but if you want to remove the “echo” in a room, like it can typically be heard in a big & empty one, then we would need materials that absorb sound.

You can learn more about the differences between soundproofing and acoustic treatment here.

Now let’s see how spray foam and fiberglass do as far as soundproofing goes:

Spray Foam

Spray foam is becoming more and more popular because of how versatile and durable it is, since once it has been applied it will expand in size and cover a lot of surface as well as every little hole or crack that may be hard to reach otherwise.

Spray foam comes in two forms, open cell and closed cell, each with their own chemical makeup differences which gives them their unique qualities.

It’s worth noting that spray foam application/installation requires special equipment and should generally be done by a professional that you hire, since it’s not a simple procedure and it can also be dangerous (flammable & toxic), but more on this later on in the post.

Chemical Makeup of Spray foam

Spray foam is a chemical product created by two materials, isocyanate and polyol resin, which react when mixed with each other and expand up to 30-60 times its liquid volume after it is sprayed in place.

The different ratio combinations is what gives us open cell foam and closed cell foam, making one more flexible, with more air pockets as well as more expandable, and the other more rigid and dense.

Like mentioned before, installing spray foam yourself can be dangerous because of these chemical compounds used since they are highly flammable, not just the chemicals that makeup the foam but also the propellant gas used to push the foam out of the container, and they could also be hazardous to your health.

In general, I would recommend you let a professional do the work.

Open Cell

The chemical makeup used in open cell foam makes it so that as soon as it’s applied, it expands rapidly and a lot more than closed cell foam, which allows it to make its way into hard to reach spots.

Its open structure means that it will have thousands of air pockets make it more pliable and flexible, which means that it won’t break as easily as its more rigid counterpart when the structure of the house moves.

One of the major characteristics of open cell foam is that it’s not impermeable and it also doesn’t work as a vapor barrier, meaning that any liquid will be able to make its way through it (water leaking, steam, etc.).

In short; It’s cheaper, expands a lot more and faster, it’s more flexible, and it’s water and vapor permeable.

Closed Cell

Closed cell foam is much denser and harder because its cell structure is closer together. This means that it doesn’t absorb sound as well but it acts more as a sound barrier when compared to open cell foam. However, when put head-to-head with open cell foam and other sound absorbing materials like rockwool and fiberglass inside of an interior wall with ½” drywall on each side, it didn’t perform as well as those other materials because it simply doesn’t absorb sound as well.

Closed cell foam doesn’t expand as much and is much more expensive than open cell foam, but it’s much better at heat insulation and can be used in small spaces where a lot of expansion isn’t needed.

Lastly, contrary to open cell foam, it’s impermeable to water and vapor.

In short; It’s much more expensive, expands less, it’s harder, and it’s impermeable to water and vapor.

Does spray foam insulation reduce noise?

When put inside a typical interior wall with ½” of drywall on either side, which has an STC rating of 34 on its own, applying open cell foam insulation will increase that STC rating to 39, whereas applying closed cell foam will increase it to 37.

The STC Rating represents how much of a dB reduction there is, and increase of 1 STC represents a 1dB decrease of sound transmission.

As you can see, open cell foam does outperform closed cell form when applied in these conditions (in a wall, ceiling, etc.).

More on STC ratings and how spray foam compares to fiberglass insulation in a second.

Dangers of Spray Foam

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 harm occupants of the house since it’s toxic.

Almost all spray foam insulation available for purchase contains, or should contain, flame retardant, because untreated foam is a fire accelerant.

Most of the foam insulation products coming to the market are either polyurethane or expanded polystyrene foam, both of which are manufactured from petroleum derivatives. Untreated (treated ones as well but will take longer to burn) and exposed to elevated temperatures or flame, these foam products will burn vigorously, producing high quantities of smoke, and spreading fire.

Not only that, but the propellant gas used to dispense of the foam (methylene-diphenyl-diisocyanate 4,4) is also highly flammable, plus it’s heavier than air and will form invisible clouds which could burst into flames or even explode in the presence of an ignition source.

This goes back to my previous point; Let a professional do the work.

Fiberglass Insulation

Fiberglass insulation, or properly named glass wool (fiberglass is actually the one used for boats, etc., which is rigid), is an insulation material made from fibers of glass arranged into a texture similar to wool.

The process traps many small pockets of air between the glass, and these small air pockets result in high thermal and acoustical insulation properties.

Glass wool generally comes in the form of rolls or in slabs, with different thermal and mechanical properties, but it is also produced as a material that can be sprayed or applied in place, making it quite versatile.

On the back you can generally see an adhesive aluminum foil or paper which serves as a vapor barrier and also to keep the fibers in place.

Since it’s such a commonly used material for insulating a home, the batts you can get generally have the size to fit in standard spacing studs in walls, which makes installation quite simple, but if  it doesn’t fit where you need it to, it’s quite easy to cut (just make sure that it fits snugly since any gap left unsealed will let heat and sound through).

Does fiberglass insulation reduce noise?

When put inside a typical interior wall with ½” of drywall on either side, which has an STC rating of 34 on its own, adding fiberglass insulation will increase that STC rating to 39, which is identical to the insulation provided by open cell spray foam but lower than rockwool.

Fiberglass, just like rockwool, can also be used for acoustic treatment and a lot of people use it for treating the acoustics of their home studio since it’s fantastic at absorbing sound, much more so than acoustic foam.

However, it’s worth noting that you will need to cover those panels with a very thick cloth since fiberglass can release tiny particles which you may breathe in and that can be bad for your health (which is why I generally recommend rockwool for acoustic panels).

Benefits of Fiberglass insulation

The benefits of fiberglass insulation are that it’s an excellent thermal and sound insulator and that it’s also quite affordable to purchase and install when compared to spray foam since you can do it yourself (you will need to use gloves since the tiny glass shards will hurt your hands).

In addition to this, it’s not really flammable and has a Flame Spread index of 25 (similar to rockwool), whereas spray foam is much more flammable and, therefore, more dangerous.

Drawbacks of Fiberglass insulation

Once fiberglass is installed it will begin to settle, which will cause it to leave a gap of air on top of it that lets sound and heat through.

You can’t install it with your bare hands since the tiny glass shards will hurt you, and it releases microscopic glass particles which can harm your respiratory system.

Lastly, even though it’s quite good at heat insulation, it’s nowhere near as good as closed cell foam, which means that in more extreme climates it won’t be as effective.

STC (Sound Transmission class)

STC, or Sound transmission class, is the rating used in the US to describe how well a building partition can attenuate sound, such as interior partitions, ceilings, floors, doors, windows, etc.

In most other countries, the Sound Reduction index is used (SRI).

The STC rating reflects the decibel reduction of noise that a partition can provide (1 STC is equivalent to 1dB reduction), where a higher number, or rating, equals better results, or overall attenuation.

Here’s a table showing what each STC rating represents:

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.

There are multiple factors that go into calculating the STC rating, such as the acoustic medium, mass, sound absorption, and more, of the materials, but I won’t go into too much detail about this since it doesn’t pertain to the article itself.

STC Rating of Spray Foam vs Fiberglass insulation

Note: Here I will be going over which type of insulation material works best when put inside a typical interior wall with ½” of drywall on either side, which has an STC rating of 34 on its own, and how it increases this rating.

Of course, the higher the STC rating, the better.

Spray Foam39 for open cell 37 for closed cell

Fire Spread Rating

While this article focuses mainly on sound insulation, I think that it’s extremely important to address if these materials can be considered a fire hazard or not, which is determined by the flame spread index.

The flame spread index is determined by the distance that a flame travels along a test substrate in a specific timeframe to determine its propensity to burn and how rapidly it can spread flames.

It’s divided into these 3 classes:

ClassFlame Spread Index
  1. Class A fire ratings indicate a flame spread rating between zero and 25. Materials that fall into Class A or Class 1 include; brick, gypsum wallboard, and fiber cement exterior materials. These materials do not burn well and are very unlikely to contribute fuel to a fire.
  2. With a Class B or Class 2 fire rating, the flame spread rating would fall between 26 and 75. This rating is typical for slower-burning whole wood materials, such as planks that are in the same form as they were when they were cut from the tree.
  3. A Class C or Class 3 fire rating has a flame spread rating between 76 and 200, which incorporates building materials like plywood, fiberboard, and hardboard siding panels, as well as any of the faster burning whole woods.

What is a good flame spread rating?

The lower the Flame Spread index the better, since this means that the flame won’t travel and spread as rapidly. Class A materials, which have a rating of 0-25, do not burn well and are very unlikely to contribute fuel to a fire, which makes them the safest ones.

Flame Spread Rating and smoke development of Rockwool vs Spray Foam Insulation

I will be including other popular insulation materials as well so that you don’t have to go scouring the internet for that info as well.

Of course, the lower the number, the better:

MaterialFlame Spread RatingSmoke Development
Spray Foam75450
Mineral Wool (Rockwool)2550
Mass Loaded Vinyl25250

Cost of Fiberglass insulation vs Spray Foam

Closed Cell Spray foam insulation costs $0.25 to $50 per board foot (a square foot that is covered by one inch of spray foam insulation), and expanding foam insulation costs from about $0.90 to $1.50 per square foot, but that’s not accounting for the cost of labor since a professional needs to do the installation.

Fiberglass insulation typically costs $0.30 to $1.50 per square foot, but considering that you can easily install it yourself it’s definitely not as high of an initial investment as Spray foam.

Which one will last longer?

Once spray foam has been applied and if that’s been done properly, it will last for the life of your home (between 80 and 100 years). Similarly, fiberglass has the same longevity if install properly of around 80 to 100 years.

Which one is easier to install yourself?

Definitely fiberglass is easier to install yourself since it’s not dangerous to do so as long as you use gloves and a face mask. With spray foam, since it uses a highly flammable gas propellant to push the foam out of the vessel, the risk or fire is quite high if the proper precautions aren’t taken.

In addition to this, you also need to wear a mask when applying the foam as well as a host of other protective gear, which is why I recommend that a professional do it since they already have all of that equipment.

Lastly, if you spray the insulation foam on a surface that you weren’t supposed to, it will take some work to remove it, while this can’t really happen with fiberglass since it doesn’t adhere to any surface.

Spray foam and Fiberglass insulation Comparison Table

Here’s a quick table with all the differences between fiberglass insulation and Spray Foam to make things easier for you:

 FiberglassSpray Foam
Sound Transmission Class3937 for closed cell 39 for open cell
Blocking air effectivenessNoneClosed Cell blocks air
Effective in extreme temperaturesMediumOpen Cell Medium-High Closed Cell Really High
Sound DampeningVery highClosed cell low Open Cell high
CostLowMuch higher
Vapor BarrierNoOpen Cell No Closed Cell Yes
Longevity80-100 years (depending if installed properly or not)80-100 years
FlammabilityExtremely low (Flame spread rating below 25)Medium-High (Flame spread rating above 75)
Installation ProcessDIYProfessional Installation


I would suggest you go with fiberglass over spray foam most of the times, and rockwool over fiberglass if I’m being honest, mainly because it’s a material that you can install yourself, it’s way less flammable, which is something that you should always consider when installing insulation, it’s more affordable and also more effective for soundproofing.

If you live in a place where there’s more extreme weather conditions (extremely hot during the summer but freezing cold in winter), then closed cell foam might be your best bet.

Lastly, you can always use rockwool for making extremely good and affordable acoustic panels.


Which insulation is best for soundproofing? Out of all the materials used for soundproofing, rockwool offers the highest STC, or Sound transmission class, rating of all at about 45 when put inside a typical interior wall with ½” of drywall on either side, which means that it reduces noise coming from the other side of the wall by 45dB.

Last Updated on May 11, 2022 by Facundo

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