Rockwool Thickness & Density for Soundproofing!

We all know that rockwool is great for both soundproofing and acoustic treatment. But how thick does it need to be? Is there a difference between the thicker and thinner pieces of rockwool in terms of sound absorption?
What about its density? Does it have an effect on sound absorption?

In this article, I will be going over what the ideal thickness and density for rockwool is in terms of soundproofing and acoustically treating a room, its STC rating (and what STC stands for), and I will also be covering if it’s safe to use, flammable, etc.

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

Tabla de contenidos

How thick should Rockwool be for soundproofing?

As far as sound absorption goes, the thicker the rockwool the better. Studies have shown that doubling the thickness of the rockwool used increases sound absorption by up to 400% at the 125Hz mark. If you’re trying to absorb as much sound as possible, go with the thickest rockwool you can find.

It’s worth noting that in order for rockwool to absorb sound properly it shouldn’t be compressed too much, so get the size that perfectly fits into the space you’re putting it in.

One thins that is also worth mentioning is that while thickness seems to be the biggest contributing factor to sound absorption, it’s not the only one, since density also plays an important role.

What about Rockwool Density?

Using higher-density rockwool will help absorb more sound, but doubling the density only yields about a 20-100% increase in sound absorption depending on the frequency, with the maximum increase in absorption being in the 250Hz range. The best density for absorbing the widest range of frequencies possible seems to be between 70- and 94kg/m3.

Increasing the thickness of the rockwool you use will always improve sound absorption, but with density it’s not that linear since there seems to be a cutoff at the 94kg/m3 mark where it starts performing worse and worse the higher density you go.

You can see a table with the absorption rates of rockwool with different thicknesses and densities at the bottom of the post (you can also click this link to jump straight to it).

Soundproofing for Low Frequencies

Low frequencies (125Hz and below) are the hardest ones to deal with and nearly impossible to control. Rockwool, especially really thick one, is great at absorbing frequencies at the 125Hz range and maybe a little lower, say 100Hz, but the lower the frequency the less effective it will be.

Low frequency tends to get transmitted through the structures of a home as well, which is also why it can be so hard to deal with since no amount of insulation material will keep the structures themselves from vibrating.

In this case, what you want to do is use non-resonant materials and to have air gaps between those and the existing walls, which will be filled with rockwool.

The simplest way of doing this is by installing drywall and leaving an air gap between it and the existing wall which is filled with rockwool. You should also know that depending on what kind of studs you use (wood, steel, etc.), vibrations will be transmitted with more or less ease.

For example: Putting rockwool inside a typical interior wall with ½” of drywall on either side, which has an STC (sound transmission class) rating of 34 on its own, will increase that STC rating to 45. If you install a single sheet of drywall on top of an existing wall and you fill the air gap with rockwool, the reduction in sound transmission will be quite noticeable.

¿Qué es la STC (Clase de Transmisión de Sonido)?

STC, o Clase de transmisión de sonido, es la clasificación utilizada en los EE. UU. para describir qué tan bien una partición de un edificio puede atenuar el sonido, como particiones interiores, techos, pisos, puertas, ventanas, etc.

En la mayoría de los demás países se utiliza el índice de reducción del sonido (SRI).

La clasificación STC refleja la reducción de decibelios de ruido que una partición puede proporcionar (1 STC equivale a una reducción de 1 dB), donde un número o clasificación más alto equivale a mejores resultados o atenuación general.

Aquí hay una tabla que muestra lo que representa cada calificación STC:

STC	que se puede escuchar
25	Se puede entender el habla normal.
30	Se puede entender el habla en voz alta.
35	Habla en voz alta, audible pero no inteligible.
40	Habla en voz alta, audible como un murmullo.
45	Se escucha un discurso fuerte pero no audible.
50	Sonidos fuertes que se escuchan débilmente
60+	Buena insonorización; la mayoría de los sonidos no molestan a los residentes vecinos.

Hay múltiples factores que intervienen en el cálculo de la clasificación STC, como el medio acústico, la masa, la absorción del sonido y más, de los materiales, pero no entraré en demasiados detalles sobre esto ya que no pertenece al artículo en sí.

Usually, an STC tied only to a specific material is a useless figure because the combination of materials used are more important and what give you the actual STC, which is why I previously gave you the example of drywall and rockwool used together.

How thick should Rockwool be for Acoustic Treatment?

Just like with soundproofing, the thicker the rockwool the better it is at absorbing soundwaves, and this is especially true for low frequency sounds. If you’re trying to absorb low-end sounds, then 6” Rockwool, or 8” if you can get it, will be your best bet. For general purposes, 2” should be more than enough.

This is why bass traps are generally so thick, because it’s the only way of dealing with those low-end frequencies.

How dense should Rockwool be for Acoustic Treatment?

For sound absorption, what’s most important is thickness and not so much density since doubling the thickness increases sound absorption by 400%, whereas doubling the density only increases it between 20-100%. The best density for absorbing the widest range of frequencies possible seems to be between 70 and 94kg/m3.

One also very important factor that I want to cover is safety, specifically how rockwool affects fire insulation since I think that it’s really important that you consider this:

Índice de propagación de llamas

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

El índice de propagación de la llama se determina por la distancia que recorre una llama a lo largo de un sustrato de prueba en un período de tiempo específico para determinar su propensión a arder y la rapidez con la que puede propagar las llamas.

Se divide en estas 3 clases:

Clase	Índice de propagación de llamas
A	0-25
B	26-75
C	76-200

Clasificación de fuego Clase A indicar una clasificación de propagación de llama entre cero y 25. Los materiales que pertenecen a la Clase A o Clase 1 incluyen; Materiales exteriores de ladrillo, paneles de yeso y fibrocemento. Estos materiales no arden bien y es muy poco probable que aporten combustible a un incendio.
Con clasificación de fuego Clase B o Clase 2, el índice de propagación de la llama disminuiría entre 26 y 75. Esta clasificación es típica de materiales de madera entera de combustión más lenta, como tablones que tienen la misma forma que tenían cuando se cortaron del árbol.
Una clasificación contra incendios Clase C o Clase 3 tiene una clasificación de propagación de llama entre 76 y 200, que incorpora materiales de construcción como madera contrachapada, tableros de fibra y paneles de revestimiento de madera dura, así como cualquiera de las maderas enteras que queman más rápido.

¿Qué es un buen índice de propagación de llama?

Cuanto menor sea el índice de propagación de la llama, mejor, ya que esto significa que la llama no viajará ni se propagará tan rápidamente. Los materiales de Clase A, que tienen una clasificación de 0 a 25, no se queman bien y es muy poco probable que aporten combustible a un incendio, lo que los convierte en los más seguros.

Is Rockwool Fireproof?

Rockwool has a flame spread index below 25 and a smoke development rating of 50, which is as low as any insulation material can be. This means that rockwool is not considered a fire hazard and that it’s an insulation material that will probably never burn or contribute to the spread of a fire.

This is the reason that I always recommend rockwool over something like spray foam because, even though spray foam is an excellent insulation material, it’s highly flammable and dangerous.

Sound absorption coefficient of rockwool depending on thickness and density

Here’s some information I found where they did measurements are done according to a standard test method such as ASTM C423 or ISO, where;

0.00 = no absorption.
0.50 = 50% absorption.
1.00 = 100% absorption.

It’s worth noting that in some cases, the measured sound absorption coefficient is greater than 1.00.

As recommended by the test method, these values are reported as measured and not adjusted and differences in coefficients of less than 0.15 are not significant.

Thickness	Density	125Hz	250Hz	500Hz	1000Hz	2000Hz	4000Hz
1″ (25mm)	3.0 pcf (48 kg/m3)	0.14	0.25	0.65	0.90	1.01	1.01
1″ (25mm)	7.4 pcf (118 kg/m3)	0.07	0.32	0.77	1.04	1.05	1.05
1.5″ (25mm)	3.5 pcf (56 kg/m3)	0.15	0.47	0.98	1.06	1.02	1.02
1.5″ (25mm)	5.9 pcf (94 kg/m3)	0.17	0.58	1.06	1.07	1.00	0.99
2″ (51mm)	2.5 pcf (40 kg/m3)	0.28	0.60	1.09	1.09	1.07	1.07
2″ (51mm)	6.0 pcf (96 kg/m3)	0.32	0.81	1.06	1.02	0.99	1.04
3″ (75mm)	2.5 pcf (40 kg/m3)	0.52	0.96	1.18	1.07	1.05	1.05
3″ (75mm)	5.9 pcf (94 kg/m3)	0.6	0.9	1.0	1.0	1.0	1.0
4″ (100mm)	2.5 pcf (40 kg/m3)	0.8	1.1	1.2	1.0	1.0	1.0
4″ (100mm)	5.9 pcf (94 kg/m3)	1.0	0.9	1.0	1.0	1.0	1.0

Conclusión

The thicker the rockwool, the better it is at absorbing sound, and this is also typically the case the more dense it is.

As you can see on the table, the thicker and denser the rockwool the more it will absorb, especially in the 125-250Hz range, since thinner rockwool already does a pretty good job at absorbing higher-frequency sounds.

I hope this information was useful!

Have a great day!