Rockwool vs Fiberglass: 11 Main Differences You Didn’t Know

When you’ve decided to install insulation in your home – for the first time or as a replacement for existing insulation – you normally have to choose between Rockwool or fiberglass products. You perhaps wonder what are the differences between the two types of materials and how will this influence the effectiveness of the insulation.

There are many differences between Fiberglass and Rockwool insulation. The main difference is that Rockwool has a much higher heat resistance and fire resistance than fiberglass. Rockwool also doesn’t absorb water as fiberglass does. On the other hand, fiberglass insulation is much cheaper.

In this article, we’ll make an in-depth comparison between the two types of materials and discuss how the differences influence insulation. To help you to make your decision regarding the type of insulation you want to install, we’ll first look at what they are made from and then briefly list some pros and cons of each type. Then we’ll provide you with a table summarizing the differences. For your convenience, we’ll also discuss the differences mentioned in the table in more detail thereafter. 

rockwool vs fiberglass insulation

What is Rockwool and fiberglass made from?

Rockwool

Rockwool, sometimes referred to as Mineral Wool, is wool-like man-made fiber. Its main ingredient is volcanic rock basalt. The basalt liquefies at 3,000 degrees Fahrenheit and to manufacture the insulation the liquefied basalt is mixed with slag, which is a byproduct of steel manufacturing processes.

rockwool insulation
Rockwool insulation

The mixture is then spun into threads and flattened with presses. The strands are ultimately formed into batts. Rolls and blankets with various dimensions are also manufactured. 

As Rockwood is mainly made from recycled material it is eco-friendly in that sense but is overall not as green as fiberglass as it is non-biodegradable.

Fiberglass

Fiberglass is made from thin thread-like glass fibers. The fibers are pressed at high temperatures and formed into long rolls or blankets. It only uses about 20 – 30 percent recycled material and is in this aspect not as eco-friendly as Rockwool. But as it is biodegradable it is considered greener than Rockwool.

fiberglass insulation
Fiberglass insulation

Interesting origin 

Fiberglass was discovered by accident in the 1930s. A researcher tried to vacuum seal glass blocks by using a high-pressure jet. During the process, it spewed fiber-like glass threads – and fiberglass was born!

Comparison of Fiberglass and Rockwool insulation

In the following table, you get a summary of the differences between Rockwool and fiberglass insulation. For more detail and further explanatory notes, we are including additional information regarding all the aspects mentioned in the table after this summary. 

AspectRockwoolFiberglassNotes
R-valueHigh R-value.Low R-Value. The R-value indicates the heat resistance of materials. 
Fire-resistanceHigh resistance.Lower resistance.Rockwool has 1.5 times more fire resistance than fiberglass.
SizeNot a wide range of sizes is always available.Comes in various sizes and are readily available.
Eco-friendlinessMade from 70% recycled material.Made from 20-30% recycled material.However, Rockwool is not biodegradable, while fiberglass is. 
CostMore expensive than fiberglass.Not very expensive.Fiberglass insulation is 25-50% cheaper than Rockwool.
Density High density.Low density.Rockwool keeps its insulation value longer than fiberglass
WeightHeavier than fiberglass.Lightweight.
Water resistanceHighly resistant to water and moisture.Absorbs water and moisture.Fiberglass becomes soggy when wet.
Loose-fillIt does exist but is not always available. A popular version of insulation and often used.Loose-fill is ideal for attic floors and crawl spaces.
Installing processThe batts fit friction-free and no staples are needed. Staples have to be used to keep the insulation in place.
Where to useIdeal for insulating pipes and around windows and electrical boxes. You can also fill most wall cavities with Rockwool and it will virtually leave no voids.
Commonly used to insulate interior walls, garages, and attics.

Rockwool vs fiberglass comparison table

R-value and fire-resistance

The R-value indicates the heat-resistance capabilities of materials.  The higher the value the better the resistance. According to their R-values, Rockwood is 1.5 times more effective fire-resistant than fiberglass. Because of this resistance, it is often used as a firestop.

Although fiberglass insulation is non-combustible, it is not nearly as fire-resistant as Rockwool.

Further reading: Rockwool R value

Size

Fiberglass insulation is available in a wide range of sizes. Rockwool insulations are not available in such a wide range. Although Rockwool offers rolls and blankets as well, it is commonly only available in batts.

Eco-friendliness when manufactured

Rockwool is composed of more than 70 percent recycled content, while fiberglass insulation manufacturing only uses 20 to 30 percent recycled content. Unfortunately, Rockwood is not biodegradable. Fiberglass is to a large extent biodegradable and thus seen as greener than Rockwool.  

Cost

Rockwool is more expensive than fiberglass. Fiberglass insulation costs between 25 and 50 percent more than Rockwool.  As an example, 625 square feet fiberglass insulation will cost about $312 and Rockwool insulation might be $400 or more. Generally, Rockwool will be functioning for longer than fiberglass and the higher initial cost might be money well spent in the long run.  

Density and soundproofing

The high density of Rockwool makes it hard to compress and thus holds its insulating value for long.  As fiberglass has a lower density it will lose some of its insulating function if it’s compressed too tightly.

Regarding soundproofing, the rule of thumb is that the higher the density of the material, the better soundproofing effect it has. Rockwool has a density of 1.7 pounds per cubic foot, while fiberglass only has a density of 0.5 to 1.0. Thus, Rockwool provides better sound insulation than fiberglass. 

Weight

Although fiberglass is easy to carry because of its lightweight, it can be challenging to set the batts into place. Rockwool is heavier, but the batts are stiffer and don’t bend easily and can be installed easier. 

Water Resistance

Rockwool is highly resistant to water and moisture. Therefore Rockwool insulation is called hydrophobic.  Because of this resistance, it doesn’t absorb water and moisture and thus doesn’t promote corrosion, rot, mold, fungi, or bacterial growth. 

Because fiberglass absorbs moisture it becomes soggy when wet and then its insulation function is compromised. It can also cause bacterial growth and mold. In severe wet conditions, it can even lead to structural damage. 

Loose-Fill 

While loose-fill Rockwool does exist, it is sometimes difficult to find. Fiberglass loose-fill insulation is almost everywhere available and provides a quick, easy and economical way to insulate. The loose fill is especially ideal for attic floors and wall cavities. 

Installation process

You can install Rockwool insulation relatively easily because you get it in dense, firm batts that allow you to fit the batts friction-free into place. You don’t have to staple the material. If you use fiberglass batts you have to secure the insulation with staples. 

When you have to cut fiberglass insulation, you compress it flat with a board and slice it with a utility knife. For Rockwool, you need a handsaw or serrated bread knife. 

Whether you install Rockwool or fiberglass it is recommended that you wear protective clothing, a dust mask and goggles when cutting and handling the insulation.

Where to use

You can use either Rockwool or fiberglass insulation anywhere in your home, but Rockwool is commonly used to insulate exterior walls, heated crawl spaces and basements.   Rockwool insulation is also ideal for insulating pipes and around windows and electrical boxes. You can also fill most wall cavities with Rockwool and it will virtually leave no voids.

Fiberglass is generally used to insulate interior walls, garages, and attics.

As Rockwood is denser than fiberglass it can withstand higher temperatures than fiberglass and is ideal to use when you are living in a warm region.  Fiberglass has better heat retention and is thus good to use as the main insulation if your home is situated in colder areas.

Pros of Rockwool insulation

The most important pros of Rockwool include that it   

  • has a great water retention capacity,
  • is fire-resistant,
  • is very durable, 
  • can act as a sound barrier as well, 
  • allows good ventilation, and 
  • is highly dense. 

Pros of Fiberglass insulation

The pros of fiberglass include its 

  • low maintenance,
  • good fire-resistant capabilities,
  • anti-magnetic characteristic,
  • weatherproof abilities, and
  • inexpensiveness.

Cons of fiberglass and rockwool insulations

Neither Rockwool nor fiberglass has only pros. The cons of each type of insulation also play a role when you have to decide which insulation you are going to install.  

Rockwool‘s main cons include that it is non-biodegradable, its pH levels shift and it is relatively expensive.

Fiberglass’ cons include that it needs a gel coating every 5 years, is a hazard for asthma patients, and is non-water resistant

Conclusion

Both Rockwool and fiberglass insulation are good to use for home insulation. You can use any of the two for all the insulation in your home, but in most homes, there is a mixture of the two materials because each has specific characteristics which make it more suitable for a specific application than the other.  

By studying the differences we’ve set out in this article you will be in a better position to decide which type of insulation you need to use in your circumstances. 

Charles John

Experienced HVAC technician with 8 years of experience in the industry. Capable of handling all sorts of heating and cooling equipment as well as proficient in operational management, construction-related techniques such as preventative maintenance, electrical troubleshooting and AutoCAD

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