There’s a lot of misinformation going around regarding sound absorption and soundproofing since some people think of these terms as interchangeable, but they aren’t!
Fiberglass Insulation is generally used for both and it’s also being marketed as the end-all-be-all solution to your noise problems when in reality, it’s not.
In this article, we will go over if Fiberglass Insulation is actually effective, its cost, ways of installing it to improve the sound transmission class, and lastly, some of its drawbacks.
However, before we go any further, you should know the difference between soundproofing and sound absorption:
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 mineral wool (Rockwool), glass wool (fiberglass), 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?
Does Fiberglass Insulation work for soundproofing?
Fiberglass insulation on its own is not a suitable soundproofing material since it will only reduce the sound transmission by 4dB to 7dB max, but should rather be used in conjunction with other materials, such as drywall and mass-loaded vinyl, to further help with insulation, which is why it’s typically placed inside of walls.
This also goes for any fiber absorption product since these aren’t capable of reducing enough sound to be used for soundproofing on their own.
This doesn’t mean that Fiberglass Insulation, or any other fiber material, can’t help reduce sound transmission since it actually does offer some additional noise reduction when placed inside of a wall, ceiling, etc, but the point here is that it’s not as effective at reducing sound as they market it to be when used completely on its own, and there are other ways which are a lot more effective, such as installing two layers of drywall, for example.
But more on this in a bit!
How much does Fiberglass Insulation reduce sound?
This will mainly depend on the thickness of the Fiberglass Insulation and not so much on the density since the thickness is a much better parameter to determine overall sound absorption.
It is worth mentioning, however, that the denser the material, the more effective it will be at absorbing low frequencies, but the flip side of this is that it also becomes better at reflecting higher frequencies back into the room.
Fiberglass Insulation can improve the STC (Sound transmission class) by 4-7 points, which means that it can reduce sound by 4dB-7dB.
This isn’t a drastic improvement on its own by any means when you compare it to the STC rating of 40 that two sheets of drywall can add, but it can help further insulate a wall or a ceiling, and if you implement some of the steps that I’m going to mention later, installing fiberglass, or any other fiber material, will yield a better end result.
In fact, 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 or 40, which is a very noticeable difference.
Is Rockwool better than Fiberglass for sound insulation?
According to the Wikipedia Article I just linked about STC, both Fiberglass and Rockwool have very similar soundproofing capabilities, although some people are in favor of Rockwool and some others are in favor of Fiberglass.
While I don’t think there’s a substantial difference in noise reduction, there is a huge difference in price, and therefore, total cost-efficiency.
Rockwool is much more expensive, about twice as much as Fiberglass, which is why Fiberglass is the go-to material used to fill a wall or a ceiling when soundproofing.
Rockwoll should, however, be used near electrical outlets, or any place that could be in danger of catching fire, since it is 100% fire resistant.
Important Note: Both materials will settle over time and a gap might form at the top, which lets sound through. So, make sure to get the highest density possible to avoid this issue.
Better ways of Soundproofing
Don’t get me wrong, inserting fiberglass into a wall/ceiling will help absorb a couple of extra dB, but this method should be used in combination with others, so here are some examples;
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 noise-proofing 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.
Here’s a quick table where you can check the different STC ratings:
| 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. |
The next step is even better but requires more work;
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 noise-proofing 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 one, 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 through the drywall by 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.
Lastly, since I know that most of you are thinking about how much installing fiberglass insulation is going to cost, here’s some information:
How much does Fiberglass Insulation cost?
Fiberglass insulation typically costs $0.30 to $1.50 per square foot or about $3 to $16 per square meter depending on the thickness, density, and overall quality.
Other Factors to take into account
Aside from the cost, there are some things you should know about Fiberglass Insulation:
- It can irritate the respiratory system, eyes, and skin.
- Mold can grow on it if it gets wet.
- While it may be extremely fire resistant, it will eventually melt and burn (at around 500°C).
I wrote two additional articles covering both Fiberglasses water and fire resistance which you can find here:
Conclusion
Fiberglass insulation works really well at absorbing sound and lowering noise transmission when placed inside of a wall, ceiling, etc.
It can also be used for acoustic treatment by making acoustic panels with fiberglass batts. Just note that fiberglass insulation can be harmful to the respiratory system, eyes, and skin.
In addition to this, remember that mold can grow on it since it’s made from organic matter, which could pose a potential health threat.
Lastly, I would recommend adding layers of drywall and using it in conjunction with Fiberglass, or even better, with Rockwool.
I hope this information was useful!
Have a wonderful day!
Last Updated on September 2, 2022 by Facundo
EN 
ES