Glasswool vs. Rockwool for Soundproofing & Acoustics

I often get asked about the difference between glasswool vs. rockwool for soundproofing. After doing some in-depth research, I came up with a quick answer:

In the debate of glasswool vs. rockwool for soundproofing and acoustics, glasswool is superior. It’s better at lower frequencies, which are often a bigger problem in homes. Also, it’s the cheaper and more widely available option.

Also read: Is Rockwool Good for Acoustic Treatment?

Glasswool vs. Rockwool for Soundproofing

In this article, I’ll go through the important differences between glasswool and rockwool, and how this impacts their use for soundproofing. Hopefully, by the end, you’ll understand why one is much better than the other.

What are Glasswool and Rockwool?

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To understand their uses in soundproofing projects, it’s first helpful to understand exactly what these products are.

Glass wool is also known as fiberglass insulation. You’ve probably just realized that you’ve heard of this product, as it appears in most homes and DIY stores.

Simply put, glass wool insulation is made from glass that’s been spun into fibers and then formed into sheets using a binding agent. Thin layers are pressed together into a bigger sheet for greater strength and stability.

Rockwool, also known as mineral wool, is essentially the same product. But instead of glass, it’s made from spun stone or silica. Much like glass wool, the rock is spun into strands (like cotton candy) and then bound into sheets.

Importantly, both of these products are primarily thermal insulators. They work by trapping small pockets of air between the fibers, and this is what provides the thermal insulation itself.

Compared to solids, gasses have very poor thermal conduction, making them great insulators.

Of course, we’re not interested in their thermal conduction properties, but rather their acoustic insulation properties.

As you may already know, soundproofing works on several core principles, including:

  • Mass
  • Decoupling
  • Absorption
  • Attenuation

Glass wool and rock wool both rely, to different extents, on absorption.

Sound absorption happens when sound waves enter a material and expend some (or all) of their energy trying to make the particles vibrate.

Materials with an open structure are useful for absorbing sound waves. Acoustic foam is probably the best example of a specific soundproofing product.

Importantly, though, fiberglass is better from a soundproofing perspective. This is because its fluffy insulation is low density, meaning it’s harder for sound waves to vibrate through it.

Note, however, this is for soundproofing rather than acoustic management. In this situation, density is less of a consideration because a denser material has more mass to vibrate.

In short, both glass wool and rock wool help with soundproofing on the opposite basis of how they help with thermal insulation. Their open structure and density are fairly effective at absorbing sound waves, which is useful in wall cavities.

Comparison of Glasswool vs. Rockwool

Of course, both products are fairly similar in terms of construction and use. But there are some important differences worth considering.

Here’s a handy comparison chart so you can see these differences clearly.

MaterialGlass fiberRock or silica fiber
CostLowMedium (roughly 10% higher)
Recycled content30%Up to 70%
Ease of installationHardEasy
Fire resistanceYesYes
Density (lbs./ft.3)68

It’s worth breaking some of these down to understand why they’re relevant to soundproofing.

We’ll start with NRC, which stands for noise reduction coefficient. This is a scale from 0.0 to 1.0 that measures a material’s ability to absorb sound waves.

It translates to a percentage of sound absorbed; meaning a material with an NRC of 0.6 absorbs 60% of a sound wave’s energy. The rest either passes through or reflects back to the source.

An NRC of 1.0 means perfect sound absorption. But before you go rushing out to buy some rockwool, it’s worth understanding this in more detail.

NRC is dependent on thickness and density. To get the highest NRC, you’d need roughly a foot or more of insulation in a space.

But NRC shouldn’t be considered on its own. For example, rockwool might have a higher NRC, but it’s less effective at lower frequencies. This is where problems arise in a home, so fiberglass is generally the preferred option.

In a direct comparison of size and surface area, rockwool will always absorb more sound than glasswool. Only consider this to an extent, though.

Ease of installation is also worth considering. Fiberglass is easier to install because it comes in rolls and is easy to fit into wall cavities.

Rockwool is harder to install because it’s crumbly. While this makes it useful for small gaps, it can be a problem when covering large areas.

You should always cover up when installing either product because they can irritate the skin; this is true for both fiberglass and rockwool.

Finally, we have availability. Glasswool is the first choice in American homes for thermal insulation and is widely available. You can easily pick it up in your local DIY store or online.

Rockwool, however, is slightly harder to come by. Also, it’s more expensive, and you’d be better investing the difference in a better wall setup or something like Green Glue.

So, while rockwool might perform better from an absorption perspective (except at low frequencies), I recommend sticking with fiberglass.

Its cost, availability, and ease of installation more than make up for its minor failings. Also, it works better at low frequencies, which are a bigger issue in homes than higher frequencies.

How to Use Glasswool or Rockwool in Soundproofing

How to Use Glasswool or Rockwool in Soundproofing

The ways in which you can use both products in a soundproofing project are fairly similar.

However, there are a few situations in which one is better due to its unique properties.

Here are my top suggestions for using rockwool and glasswool in soundproofing.

1. Insulating a wall cavity

The most obvious suggestion is for insulating a wall cavity, as that’s what these products are for.

On their own, they won’t offer the best results. Soundproofing is most effective when you combine the strengths of several useful materials.

Along with adding insulation to a wall cavity, you’ll also want to add:

As mentioned, rockwool and glasswool absorb sound waves. But for proper soundproofing, you’ll also want to add more mass and sound deadening materials.

This will mean that the level of sound waves passing into the wall cavity is reduced in the first place, making any absorption techniques more effective.

Of course, for best results you’ll want to decouple the wall. This is a form of mechanical isolation that completely cuts off one side of a wall from the other.

But this isn’t usually practical, and that’s what the whisper clips and hat channels are for. These produce the same results without the need to build a new set of wall studs.

Either way, filling the wall cavity with glasswool insulation will be a useful addition. Its lower density is more useful from a soundproofing perspective.

2. Insulating HVAC systems

One instance where glass wool comes out on top is for insulating pipes and HVAC systems.

Commonly sold as pipe wrap (Amazon link), glass wool insulation is more flexible and so is perfect for wrapping around pipes and vents.

As mentioned, rockwool is more rigid, and so can’t be used for this kind of application.

3. Soundproofing floors and ceilings

You can use glasswool for soundproofing floors and ceilings in much the same way as for walls.

Rockwool’s rigid blocks can be helpful for insulating flat surfaces, but glasswool is still easier to install.

Also, you might find that rockwool isn’t as effective in floors and ceilings. This is because it’s more effective against airborne noises than impact noises.

The difference between the 2 is important. Airborne noises are sounds traveling through the air, such as voices, TV, or music. These are simply vibrations in the air and are much easier to absorb.

Impact noises, however, are created when an object makes direct contact with a surface, such as footsteps. They have more vibrational energy that’s passed directly into the surface rather than through the air.

The most effective ways to combat impact noises is either with decoupling or plenty of mass.

Again, you’ll still benefit from some mineral wool insulation in the cavity for some extra sound absorption. But just don’t rely on it completely as a soundproofing solution.

4. Build room dividers

Rockwool is a good addition to things like acoustic insulation panels or DIY room dividers. While these will never soundproof a space, they can be useful for managing and directing noise pollution in a large area.

Of course, if you’re planning to make a panel out of rockwool, be sure to cover all sides with drywall or a non-porous material. Rockwool is generally safer than glasswool, but you’ll still want to avoid any accidents.

Some Final Thoughts

When it comes to glasswool vs. rockwool for soundproofing, I feel glasswool is always the better choice.

It’s cheaper, easier to buy, and easier to install.

While it might not perform as well with overall sound absorption, it’s better from a soundproofing perspective because there are fewer fibers to vibrate.

Also, its lower price more than makes up for any performance issues.

My advice is to plan your project in detail and understand your needs. Only then will you be able to make the decision about which is right.

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