The Ultimate Guide to Choosing Glass Wool Felt

Are you currently dealing with the question of how you can improve the acoustics in your listening room? If you are like me, then the question of the size of the absorbers should be resolved relatively quickly. Because most suppliers of mineral wool offer similar sizes in the area of ''120cm x 60cm (4' x 2').

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At Absorber Depth, you are probably struggling with two goals: deep enough to properly absorb bass, but small enough not to waste too much space in the room. Because we all have walls around us somewhere that we cannot move.

And ultimately, at the latest when you ask about the right material for the interior of your absorber, you will stumble - at least that's how I felt. There are many examples of how to build absorbers yourself for the recording studio or home theater. Simply copying it would be an option, hoping that people would think of something when choosing it. And every mineral wool absorbs better than a bare wall! So the chances are good that you will achieve an improvement in any case.

Ultimately, of course, remodeling your room will cost time and money. In the best case, the values ''such as absorber depth and absorption properties should fit your goals.

So that the values ''are in the right range even when starting up for the first time, I would like to give you a little guide today for choosing the right insulation wool.

Rock wool or glass wool - which absorbs better?

To shorten this striking question: Both can be used excellently and are also used by the most renowned recording studios and acousticians worldwide. There are also a number of other substances such as hemp, Basotect or Caruso Iso Bond, all of which have very good absorption. Much more important than the question of the material itself is a very important key figure. No other size can better determine how large the (frequency-dependent) absorption will be later:

The specific flow resistivity

Put simply, this value describes how much the speed of the air vibration decreases when passing through the material. So it is measured in front of and behind the absorber material. The lighter the material, the more it arrives after crossing. And the thicker the material, the less it gets through, but the more it is reflected. The art for us later will be to find a material that is heavy enough to absorb as much as possible, yet light enough that the sound is not reflected halfway through the material and the last cm of the absorber is no longer at all reached.

There are occasionally two values ''in data sheets. One is the flow resistance specifically related to this material thickness (for example 5 cm). It has the unit Pa * s / m. Since each manufacturer has different thicknesses in its range and we still want to determine which thickness is suitable for us, this value alone does not help us.

The second value is the length-specific flow resistance at which the previous value is divided by the material thickness. It is therefore purely dependent on the material and no longer on the thickness that was used for the measurement. It can be recognized by the unit Pa * s / m2.

With common sense it can be guessed that there is a connection between the density, i.e. the specific weight of the material, and the flow resistivity. In order to get an overview of the areas in which this value is located, I have worked through some products of common mineral wool and have written down the flow resistance and density.

On the one hand you can see that there are certain fluctuations and the values ''should only be used as a rough guide. Nevertheless, a certain linearity can be seen for each material type. Any flow resistivity can be achieved with almost all materials. Depending on the type, this requires a different material density.

The most important finding when comparing glass wool vs. rock wool: rock wool must be about 50% heavier than glass wool to achieve the same flow resistivity. For example, we achieve the value of Pa * s / m² with 35-40kg / m³ rock wool, or with 20kg / m³ glass wool.

Caruso Iso Bond, on the other hand, is very similar to rock wool. The flow resistivity of Pa * s / m² can be achieved with both materials with a material density of 40kg / m².

But now finally to the actual questions of today's article. Let's start with:

Which flow resistance is optimal for my absorber depth?

With this question you can already see how I would approach the material selection: first we determine the correct range for the flow resistivity. And then we use the material table to see which material with which weight can be used to achieve this flow resistivity.

And don't worry if you are still undecided about the absorber depth. After the following examples, we will once again address the question of how thick the material may be at the various points.

Tools to simulate the degree of absorption

Since it is very time-consuming to impossible to acoustically measure all materials and all combinations at home, I have come to appreciate a (free!) Online tool. Of course, every simulation is only an approximation. But to get a feeling for the effects of different absorber depths and flow resistances, I know of no better and easier way than this calculator: http://www.acousticmodelling.com/porous.php

There are some limitations to keep in mind when using this tool. For one thing, I assumed an angle of 0 degrees for the simulation, i.e. we assume a vertical angle when the sound hits the wall. This is the case, for example, when we think of the wall behind the speakers. In practice, this angle changes somewhat for absorbers on the side walls, depending on how large the width of the room is and how large the listening distance is. In my experience, the 0 degree bends are the 'worst', i.e. a low degree of absorption is displayed. If we can increase the angle in practice, the values ''should always be better than the simulation for 0 degrees, since the sound travels more through the absorber at an oblique angle of incidence and is therefore better damped.

The tool only calculates with the absorber depth and the flow resistance. There is no way that the material density is taken into account. In this respect, the values ''are to be treated with caution and can differ slightly in reality.

Another assumption of the tool is the use of an infinitely large absorber wall. The output values ''are only achieved if a sufficient number of absorbers are placed side by side without gaps. I think it makes sense that we cannot conquer a 100 Hz wave (with a wavelength of 3.40 m) with a single absorber in the size of 1.20 m x 0.60 m. At low frequencies, we should be aware that we have to apply large areas. At high frequencies, i.e. if the absorber is larger than the wavelength (for example, 1 kHz has a wavelength of 34 cm), we can already achieve good absorption with a single absorber.

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For the sake of completeness, the technical parameters that I used for my curves: air temperature: 20 degrees Celsius, air pressure: Pa, Angle of Incidence: 0 degrees, Porous Model: Allard and Champoux ().

To classify how well the curves of the free tool compare with professional software, I performed the same simulation with the Soundflow software from AFMG (second graphic). The specific weight is also taken into account in the calculation. I used the density of the respective rock wool here. At low frequencies, the degree of absorption is somewhat higher compared to the calculation without weight.

With Soundflow I also used an angle of incidence of 0 degrees and an infinitely large area as parameters. Bies was used as a model.

Examples of different absorber depths

Using the following examples, I would like to give you a small guideline to find a reasonable flow resistance.

5cm without gap

Even if our 20cm absorber depth has remained, the curve extends significantly further down to low frequencies due to an additional wall distance of 10cm. We now reach the absorption level of 0.5 not only at 90 Hz, but already at around 60 Hz.

I personally would also adjust the flow resistivity with an additional wall clearance and choose a slightly lighter material. Since I want to operate my wall absorbers pretty exactly in this constellation (20cm depth and 10cm wall distance), I took a closer look at this case. I would choose a flow resistivity of Pa * s / m².

Since I want to try Caruso Iso Bond because of the cleaner processing, I have already ordered some packages with WLG 040. There are not exactly Pa * s / m², but the Pa * s / m² that the manufacturer promises are close enough. This lower value also has advantages if I want to increase the absorber depth to 30cm on my back wall.

The difference between my current absorbers with Pa * s / m² rock wool and the Pa * s / m² Iso Bond will not be huge. Because in addition to the flow resistivity, it is ultimately of great importance for the overall effect how many absorbers I use and where I position them.

But I'm still curious! I will try to find out to what extent the forecast of this calculator is also noticeable in measurement in the real living room by comparing absorbers of the same size.

30cm without wall distance

If we adapt the flow resistivity to the absorber depth in this way, we see the linear relationship between the absorber depth and the lower end of the effective range. If you are wondering how deep your absorber should be, this is the graph that is most likely to give you an answer.

When it comes to speech or singing, you could make good improvements with 10cm. The human voice extends down to 100Hz and the 10cm are not yet ideal.

As soon as it comes to music with drums and bass, frequencies below 100 Hz will probably also play an important role. And an absorption in this frequency range can be achieved with 20cm.

What is the best material for me?

If, on the one hand, we now know how thick our absorber should be and then we use the example curves to read the area in which our flow resistivity should be, we can go to the last question about the material.

For the absorption properties it doesn't matter whether rock wool or glass wool or a special acoustic foam like Basotect or Caruso Iso Bond. You can use the material table at the beginning of this article to find some common types and approximate prices as a first orientation. With a little luck, you will find one of the fabrics in your local hardware store. But it doesn't have to be exactly the same type. Any other manufacturer can be used for glass wool and rock wool as long as the density (in kg / m³) is in the corresponding range.

I personally started my first attempts at building absorbers with rock wool because it is cheap and available everywhere. One disadvantage is certainly the health risk that you are exposed to during processing. If you approach the matter with suitable clothing, gloves and, if necessary, a respirator and wrap the rock wool in such a way that later no flakes can enter the room, then I think rock wool is a great material for acoustic purposes.

Glass wool should be very similar, although it is said to have even higher health risks.

On the one hand, it is really low-risk with natural products such as hemp or sheep's wool. As you can see in the table, the flow resistances are in the very low range, so that we can only use these two substances optimally for thick absorbers.

In its natural state, sheep's wool is even far below what we wanted to use for our purposes. By compressing more material into a smaller space, however, the flow resistance can be increased significantly.

Now that I have extensive experience with rock wool, my next level will be Caruso Iso Bond. The most important argument for this building material is certainly that it is less health-critical and cleaner to process than mineral wool. On the other hand, you get Caruso Iso Bond with all relevant densities and flow resistances, both for thick and thin absorbers. The only disadvantage: it is considerably more expensive than rock wool or glass wool.

[Addendum: You can find my comparison between Caruso Iso Bond and Rockwool here.]

From an acoustic point of view, most materials can be used. It is more a question of your wallet and your personal demand for a clean environment, whether you will be happy with rock wool, with hemp or with Caruso Iso Bond. Your ear will definitely be happy if you dedicate yourself to the topic of room acoustics and build your first absorber!

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When it comes to insulation materials, glass wool and rockwool are two of the most commonly used products. Both materials are highly effective at insulating against heat and sound, but they are made from different substances, have unique properties, and are suited for different applications. In this article, we’ll explore the key differences between glass wool and rockwool, covering their composition, manufacturing processes, thermal and acoustic properties, fire resistance, and environmental impact.

1. Composition and Manufacturing

Glass Wool

Composition: Glass wool is made from recycled glass and sand, which are melted together at high temperatures and then spun into fine fibers. The primary ingredients are silica, soda, and lime, with other minor materials used to improve the product's performance. The fibers are then treated with a binder to help them hold together.

Manufacturing Process: The process begins with heating the glass and other raw materials in a furnace until they melt. This molten mixture is then extruded through a spinning mechanism, creating fibers. These fibers are collected and compressed into mats or rolls, which are then cut and packaged for use in various applications.

Glass Wool Felt

Rockwool

Composition: Rockwool, also known as mineral wool, is made from natural rocks like basalt or dolomite, along with recycled slag from steel production. These raw materials are heated to extremely high temperatures (over 1,400°C) and then spun into fibers, much like the process used for glass wool.

Manufacturing Process: The raw materials are melted in a furnace and then rapidly spun into fine, wool-like fibers. These fibers are then compressed into mats or batts, similar to the way glass wool is manufactured. Rockwool is also treated with a binder, which gives the insulation its stability and strength.

2. Thermal Insulation Performance

Both glass wool and rockwool are excellent thermal insulators, but they perform slightly differently depending on the application.

Glass Wool

Glass wool has good thermal insulation properties, with a typical thermal conductivity of around 0.030 to 0.035 W/m·K. This makes it a popular choice for residential and commercial applications, where controlling heat loss or gain is important.

Its insulating effectiveness is due to the air pockets trapped between the fibers, which help to reduce heat transfer. Glass wool is most commonly used in walls, roofs, and floors to maintain comfortable indoor temperatures and improve energy efficiency.

Glass Wool Tube

Rockwool

Rockwool generally has a slightly better thermal conductivity compared to glass wool, with typical values ranging from 0.034 to 0.040 W/m·K. While not drastically different, this makes rockwool slightly less effective at insulating against heat loss in some applications.

However, its ability to handle higher temperatures and provide thermal protection in more extreme environments (like industrial applications) makes it a preferred choice in those situations.

3. Acoustic Insulation

Both materials are also excellent at soundproofing, but their performance varies in different scenarios.

Glass Wool

Glass wool’s fibrous structure allows it to absorb sound effectively, making it ideal for soundproofing walls, ceilings, and floors in residential and commercial buildings. It is particularly useful in reducing noise between rooms and controlling airborne sound.

It is particularly effective at mid to high frequencies of sound, making it an excellent choice for general noise reduction in offices, schools, and homes.

Rockwool

Rockwool generally provides better soundproofing performance, especially for low-frequency sounds such as traffic noise or heavy machinery. This is due to its denser structure and higher mass.

Rockwool’s superior sound absorption makes it a top choice for industrial buildings, soundproofing studios, and music venues, where low-frequency noise needs to be mitigated.

4. Fire Resistance

One of the most significant differences between glass wool and rockwool is their fire resistance.

Glass Wool

Glass wool is naturally fire-resistant due to its composition of glass fibers, but it is not fireproof. It can withstand temperatures of up to 250°C to 300°C before it begins to lose its insulating properties. However, if exposed to higher temperatures or direct flames for extended periods, the material can degrade and lose its effectiveness.

Glass wool is often treated with fire-retardant chemicals to improve its fire resistance, but it is not suitable for high-temperature applications like fire protection or furnace insulation.

Rockwool

Rockwool is much more fire-resistant than glass wool, as it can withstand temperatures of up to 1,000°C or more. This makes it an excellent choice for fire-resistant walls, ceilings, and floors in buildings, as well as for applications where high heat is a concern, such as industrial settings, boilers, and furnaces.

Rockwool does not burn and can provide critical protection in the event of a fire, preventing the spread of flames and providing valuable time for evacuation or fire suppression.

5. Moisture Resistance

Glass Wool

Glass wool is susceptible to moisture and can lose its insulating properties if it absorbs water. Moisture can also cause the fibers to clump together, reducing the material’s effectiveness. For this reason, glass wool should be installed in areas that are dry or where it can be protected from water exposure.

Some glass wool products are treated with water-repellent coatings, but they still remain less resistant to water compared to rockwool.

Rockwool

Rockwool is highly moisture-resistant and does not absorb water in the same way that glass wool does. It will maintain its insulating properties even in wet conditions, which makes it ideal for use in areas that may be exposed to high humidity, such as basements, bathrooms, and industrial applications.

This resistance to moisture makes rockwool a better option for areas prone to condensation or water damage.

6. Environmental Impact and Sustainability

Glass Wool

Glass wool is made from recycled glass, which makes it a relatively eco-friendly option. However, the manufacturing process can produce emissions, and the use of chemical binders may affect its sustainability.

Glass wool is fully recyclable, and many manufacturers focus on producing more sustainable, low-impact products.

Rockwool

Rockwool is also an environmentally friendly material, as it is made from natural, abundant materials like basalt rock and slag. The production process has a higher environmental impact compared to glass wool, but rockwool is typically recyclable and can be reused in future production.

It is considered a sustainable material, with many manufacturers focused on reducing energy use and emissions during production.

7. Cost

Glass Wool is generally less expensive than rockwool, making it a cost-effective choice for residential applications where heat insulation and soundproofing are required.

Rockwool tends to be more expensive, but its superior fire resistance, moisture resistance, and soundproofing capabilities often justify the higher cost in commercial and industrial applications.

Conclusion: Which Is Better for You?

Both glass wool and rockwool offer significant advantages for different applications. Here’s a quick summary to help you choose the right material for your project:

Choose Glass Wool if:

You’re working on a residential insulation project where cost-efficiency is important.

You need soundproofing for mid to high-frequency sounds.

Moisture resistance isn’t a primary concern, and the environment is dry.

Choose Rockwool if:

You need superior fire resistance for high-heat environments.

You’re looking for better soundproofing for low-frequency noise.

The insulation will be exposed to moisture or high humidity.

In conclusion, both materials have their strengths, but rockwool offers superior performance in extreme conditions, while glass wool is a more affordable and versatile option for everyday use. The choice between the two ultimately depends on your specific needs and the environment in which the material will be installed.