Rockwool and fiberglass, or glass wool as it’s actually called, are the two most-used insulation materials in construction and they both perform extremely well. However, is there one that outperforms the other? Is one more affordable, easier to install, etc.?
In this article, I will be comparing fiberglass to rockwool 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 costs, 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!
Table of Contents
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 materials, while not being great for soundproofing purposes on their own, 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. For example, drywall and rockwool used together.
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 fiberglass and rockwool do as far as soundproofing goes:
Rockwool
Also known as mineral wool, rockwool is literally what the word implies: Rock fibers (volcanic rock and slag) which are spun and then given a specific shape (batt, pipe and board forms).
Rockwool is a great sound absorber since it’s thick and has a lot of air pockets inside of it, and it’s also worth mentioning that you can get different densities and thicknesses depending on your needs (the thicker it is, the more sound it will absorb).
It’s not impermeable to water or vapor, and although it doesn’t absorb water like other types of wool, if it gets wet it won’t be able to insulate as well as when it’s dry. Luckily, since it’s made of rock, once it dries it’s as good as new.
Does rockwool 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 rockwool insulation will increase that STC rating to 45, which is much higher than any of the alternatives out there (spray foam and fiberglass have a rating of 39).
The STC Rating refers to the amount of noise that is being reduced in decibels; One point increase in the STC rating represents a 1dB noise reduction. So, the higher the STC rating, the better (but more on this later on).
Rockwool can also be used for acoustic treatment and, in fact, it’s what most people, including myself, use for treating the acoustics of their home studio since it’s fantastic at absorbing sound, much more so than acoustic foam.
Of course, fiberglass is also being used for making acoustic panels, but that’s beside the point.
Benefits of Rockwool
The benefit of rockwool is that not only does it have the highest sound insulation capabilities of the typically used insulation materials because of how dense it is (spray foam, fiberglass, cellulose, etc.), but it’s also extremely affordable and you can install it yourself since it’s not toxic, won’t harm your skin like spray foam or fiberglass, and it’s not flammable.
In fact, it could be considered fire-blocking since, well, it’s rock, so when you install it in your home at least you know that it won’t help spread a fire.
Drawbacks of Rockwool
The biggest drawback would be that it’s not that great of a heat insulator since air can easily pass through it, which means that humidity will as well.
In addition to this, rockwool tends to settle once installed and a small air gap will form on top of it allowing sound, heat and cold to get through.
Lastly, it easily breaks apart, which means that it won’t last as long other insulation materials unless properly installed.
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 (it’s worth noting, however, that additional vapor barriers are generally needed).
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.
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 (better thermal insulator than rockwool but worse sound insulator) and that it’s also quite affordable to purchase and install, and 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), which is why I always recommend it over something like spray foam since that one is much more flammable and, therefore, more dangerous.
Drawbacks of Fiberglass insulation
Similar to rockwool, 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. So, my recommendation would be to wear gloves, a face mask and some safety goggles to be on the safe side.
Lastly, even though it’s quite good at heat insulation, it’s nowhere near as good as closed cell spray foam , which means that in more extreme climates it won’t be as effective.
Is Fiberglass dangerous?
Some studies have shown that fiberglass, or glass wool, can release some small particles in the air which when breathed in can cause irritation and that could be considered carcinogenic, which is why it should be covered with a really thick and tight cloth once installed.
In addition to this, when handled, small glass shards can hurt your skin, eyes and make their way into the respiratory system causing breathing difficulties, which is why you should always wear gloves, have as little exposed skin as possible, wear safety goggles and a face mask.
This is not the case with rockwool, however, since it’s not carcinogenic and even though when handled some particles may become airborne, these are much heavier than the fiberglass particles and will drop to the floor in no time. Also, breathing in rockwool particles isn’t a health risk because the body can easily expel them.
Now, I know I already went over which material reduces sound transmission best by providing you with their STC values, but what does STC stand for?
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:
| 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. |
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 Fiberglass vs Rockwool (Which one insulates the best)
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.
| Material | STC |
| Mineral Wool | 45 |
| Fiberglass | 39 |
| Spray Foam | 39 for open cell 37 for closed cell |
As you can see, from a purely acoustic insulation standpoint, rockwool outperforms both spray foam and fiberglass by quite a lot.
But what about safety? Which one is more or less flammable?
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:
| Class | Flame Spread Index |
| A | 0-25 |
| B | 26-75 |
| C | 76-200 |
- 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.
- 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.
- 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 Fiberglass 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:
| Material | Flame Spread Rating | Smoke Development |
| Fiberglass | 25 | 50 |
| Mineral Wool (Rockwool) | 25 | 50 |
| Spray Foam | 75 | 450 |
| Mass Loaded Vinyl | 25 | 250 |
| Cellulose | 25 | 50 |
Both rockwool and fiberglass have a very low flame spread rating and smoke development which makes them ideal insulation material and they should help slow down the spread of fire.
It’s worth noting that no matter what material it is, if you give it enough time and heat it will burn at some point, but the lower the flame spread rating, the more time you have before the fire spreads.
Cost of Rockwool vs Fiberglass
Rockwool typically costs $0.50 to $2.30 per square foot depending on the thickness and density, whereas Fiberglass insulation typically costs $0.30 to $1.50 per square foot. It’s also worth noting that fiberglass comes compressed, so when you buy a pack that’s the same size as the rockwool pack it actually has twice as much fiberglass in it.
Which one will last longer?
Once fiberglass has been installed it will last for the life of your home (between 80 and 100 years), that is, if it has been installed properly. Rockwool can also last for the lifetime of your home if properly installed, but since it settles and also breaks apart a little more than fiberglass, you could expect it to last anywhere from 30 years to 100.
Which one is easier to install yourself?
Definitely rockwool is easier to install yourself since it’s not dangerous to do so. Fiberglass isn’t really that dangerous to install yourself, but you need to take more precautions to keep your skin, eyes and respiratory system from getting irritated.
Fiberglass and Rockwool Comparison Table
Here’s a quick table with all the differences between rockwool and Fiberglass to make things easier for you:
| Rockwool | Fiberglass | |
| Sound Transmission Class | 45 | 39 |
| Blocking air effectiveness | None | None |
| Effective in extreme temperatures | Medium-low | Medium |
| Sound Dampening | Extremely high | Very High |
| Cost | Low | Low |
| Vapor Barrier | No | No |
| Longevity | 30-80 years (depending if installed properly or not) | 80-100 years (also depends on the installation) |
| Flammability | Extremely low (Flame spread rating below 25) | Extremely low (Flame spread rating below 25) |
| Installation Process | DIY | DIY with more caution |
Conclusion
I would suggest you go with rockwool over fiberglass simply because it performs better as far as soundproofing goes, it’s just as affordable, just as easy to work with, and you don’t need to take as many precautions when installing it. Plus, it’s safer for your health overall.
FAQs
Is Rockwool better than fiberglass for soundproofing? Rockwool has a sound transmission class of 45, which is higher than fiberglass’ rating of 39, meaning that Rockwool can lower noise by 6dB more than fiberglass.
Is Fiberglass good for soundproofing? Fiberglass is a great soundproofing material since it’s thick, dense, and has a lot of air pockets inside of it to trap sound, and when put in a typical interior wall with ½” of drywall on either side, it helps can help reduce sound transmission by up to 39dB.
Is Rockwool good for soundproofing? Rockwool on its own is not good for soundproofing since it’s designed to absorb and dampen sound instead of blocking it, but it works extremely well when used in conjunction with drywall or when put inside of a wall, lowering the noise that is allowed through that wall by an average of 45dB.
How thick should Rockwool be for soundproofing? The thicker the batt of rockwool the better for sound absorption since doubling the thickness results in an almost 400% increase in absorption at 125Hz. Increasing the density can also help, but when doubling the density, the difference in sound absorption won’t be anywhere near as much (only 20% increase) as when compared to doubling the thickness.
Last Updated on May 11, 2022 by Facundo
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