Cellulose and fiberglass, or glass wool as it’s actually called, are two of the most-used insulation materials in construction, with rockwool and spray foam, of course, 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 cellulose 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:
Cellulose
Cellulose insulation is a low-thermal-conductivity material that is made out of 75 to 85% recycled paper or denim and which is heavily treated with boric acid, borax, or ammonium sulfate which act as flame retardants, giving cellulose flame retardant qualities.
Cellulose can either be loose-fill or blown-in insulation and can be used both in new homes as well as in already existing ones to replace or improve the already-existing insulation.
Cellulose is a great sound absorber since it’s thick and has a lot of air pockets inside of it, but more on how it performs soundproofing-wise in a second.
Does Cellulose 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 cellulose insulation will increase that STC rating to 44, which is just slightly lower than Rockwool (STC of 45), but higher than Fiberglass and open-cell spray foam (STC 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).
As far as acoustic treatment goes, there are acoustic panels out there made out of cellulose, but contrary to fiberglass and rockwool, you can’t just make your own panels with the typically-used cellulose insulation since it’s loose-fill or blown-in insulation.
Benefits of Cellulose
- Less air leakage: While cellulose isn’t airtight and will need an air barrier to really become airtight (just like fiberglass), it actually does slow down airflow by quite a bit when compared to fiberglass.
- Ideal for tight spaces: Since cellulose is blown-in or loose-fill, this makes it a lot easier to cover an entire area with it and to really get it into the tight spots that are hard to reach.
- Boric acid, borax, or aluminum sulfate used in cellulose insulation provide resistance to mold (if it never got wet), pests, and fire.
Drawbacks of Cellulose
When compared to other insulation materials, cellulose does have a couple more drawbacks, especially in relation to water absorption:
- Cellulose must be kept dry as it absorbs up to 130 percent water by weight, and it dries very slowly after absorbing water, causing it to deteriorate and settle afterward. In addition to this, once it absorbs water, the chemical fire treatment is destroyed, and it becomes a breeding ground for mold.
- Cellulose is much heavier than fiberglass, or any other insulation for that matter, and it also absorbs water which other insulation materials don’t, which could put the structural integrity of your home at risk if not accounted for.
- Cellulose also settles about 20% over time, leaving a lot of gaps where heat and sound can get through.
- Since it’s mostly made out of recycled paper, even though it’s been heavily treated with fire-retardant chemicals and has a flame spread index of 25 or less (more on how cellulose and fiberglass compare in terms of fire safety later on in the article), it will still burn quicker than fiberglass or rockwool.
- It doesn’t absorb impact noise.
Is Cellulose considered a health risk?
Unlike other insulation materials, cellulose never has a definite list of components. Because of the complex mixture of chemicals, it is almost impossible to determine an accurate list of the components, but the primary ingredients are always ground newspapers, boric acid and borax.
Breathing shredded paper dust shed from cellulose insulation can cause some respiratory problems, and cellulose can emit gases from fire retardants, insecticides, solvents, and inks used in the manufacturing process.
Individuals with preexisting skin disorders and asthma are allergic to cellulose, because of its itch inducing particles which can be irritating to the eyes, skin, and lungs.
Lastly, once it gets wet it can become a breeding ground for mold.
Now, all that being said, cellulose doesn’t have very fine fibers which, if breathed in, are hard for the body to expel, which is actually the case with fiberglass, and this means that it shouldn’t cause any long-term damage if inhaled.
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 cellulose and rockwool.
Now, contrary to cellulose, Fiberglass, since it generally comes in batt form, can 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 (not as good of a thermal insulator as cellulose, however, since it doesn’t slow down the movement of air) and that it’s also quite affordable to purchase and install since you can do it yourself and don’t need to either hire a professional or rent equipment.
In addition to this, it’s not really flammable and has a Flame Spread index of 25 (similar to rockwool and cellulose, but it definitely keeps fire from spreading longer than cellulose because it’s just made out of glass), 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 cellulose, 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. However, modern cellulose settles by about 20%, which is not the case with fiberglass (it settles much less).
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 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 into 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, or completely sealed off.
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, and wear safety goggles and a face mask.
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 Cellulose (Which one soundproofs 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 |
| Cellulose | 44 |
| Fiberglass | 39 |
| Spray Foam | 39 for open cell 37 for closed cell |
As you can see, from a purely acoustic insulation standpoint, rockwool outperforms all other insulation materials, with cellulose being a close second.
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 Cellulose 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. 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 cellulose and fiberglass have a very low flame spread index and smoke development rating. Just remember that cellulose is made out of paper, and while the fire retardant chemicals may help it not burn as fast, at some point it will.
Rockwool and fiberglass, on the other hand, can withstand temperatures of up to 1000°C and are considered “fire-blocking”.
It’s worth noting that no matter what material it is you install in your home, 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 Cellulose vs Fiberglass
Loose-fill cellulose typically costs about $0.50 to $1 per square foot, damp-spray cellulose costs about $0.60 to $1.80 per square foot of wall space, and Dense-packed cellulose often costs $2 to $2.25 per square foot, whereas Fiberglass insulation typically costs $0.30 to $1.50 per square foot.
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. Cellulose, on the other hand, will only last 20 to 30 years, with degradation beginning as early as 15 years after installation.
Which one is easier to install yourself?
Definitely fiberglass is easier to install yourself since you don’t need to hire a professional or need to rent additional machinery, whereas cellulose, since it’s blown-in or loose-fill, will require the use of additional equipment, which is also why professionals who already own that equipment generally do it.
Fiberglass and Cellulose Comparison Table
Here’s a quick table with all the differences between cellulose and fiberglass to make things easier for you:
| Cellulose | Fiberglass | |
| Sound Transmission Class | 44 | 39 |
| Blocking air effectiveness | Very low | None |
| Effective in extreme temperatures | Medium | Medium |
| Sound Dampening | Very High | Very High |
| Cost | Low | Low |
| Vapor Barrier | No | No |
| Longevity | 20-30 years and will start to degrade after 15. | 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 | Generally, hire a professional. | DIY with more caution |
Conclusion
Fiberglass seems to be the better alternative, especially for anyone who wants to do the installation themselves, since you won’t need to rent out any additional equipment.
Also, cellulose absorbs a lot of water which is not the case with fiberglass, and this could cause a potential mold issue and also add a lot of weight to the structure of the home if it were to get wet at some point.
Lastly, fiberglass lasts like 3 to 4 times longer.
Now, you could always use cellulose to fill the tiny gaps that were left exposed after installing fiberglass, since it’s so much easier to seal every nook and cranny with cellulose. But you could also do this with spray foam and not have to deal with the settling and water absorption problems (just know that spray foam is much more flammable than both fiberglass and cellulose, so I wouldn’t recommend insulation the entire home with it if you can).
FAQs
Does cellulose insulation settle over time? Cellulose insulation is known for settling about 20% after it’s been installed, leaving a lot of open gaps for air, and therefore hear and sound, to get in or out.
How long will cellulose last? Contrary to most other insulation materials which last 80-100 years, cellulose insulation will only last about 20 to 30, and it may also begin degrading as soon as 15 years after being installed, and since it also absorbs a lot of water, once it gets we it will degrade even faster.
Is cellulose or fiberglass insulation better for soundproofing? Cellulose offers better sound insulation than fiberglass, but this comes at the cost of cellulose not being water resistant and absorbing up to 130 percent water by weight, which decreases its insulation capabilities, it becomes a breeding ground for mold, and it starts degrading much sooner.
Sources:
https://en.wikipedia.org/wiki/Cellulose_insulation
https://www.retrofoamofmichigan.com/blog/what-is-cellulose-insulation-material
https://www.kompareit.com/homeandgarden/insulation-compare-cellulose-vs-fiberglass.html
https://www.homelogic.co.uk/is-cellulose-insulation-safe-to-breathe
https://www.energy.gov/energysaver/insulation-materials
Last Updated on May 11, 2022 by Facundo
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