Batts of fiberglass and Rockwool insulation are shown next to each other

There is a wide variety of materials that can be used to insulate residential basements, wall cavities, and attic spaces. Two of the most popular home insulation materials are mineral wool and fiberglass. 

Both mineral wool and fiberglass insulation have distinct benefits and drawbacks. Both are effective under different situations, and therefore, well suited to varying types of applications.

For instance, mineral wool is heavier, denser, and more rigid than fiberglass, making it harder to install. On the other hand, flexibility allows fiberglass to accommodate obstructions like wires and electrical outlets and easily conform to curved or irregular walls, unlike Rockwool. 

However, the greater density of mineral wool gives it a higher R-value (and better resistance to heat flow) than fiberglass. Rockwool insulation is also more resistant to fire, moisture, and mold than fiberglass insulation. 

Through this article, we will help you understand the essential features of both Rockwool and fiberglass insulation, including their benefits and drawbacks. 

This analysis will help you make an informed and well-considered decision about whether Rockwool or fiberglass insulation is more suitable for your home. 

Overview of Mineral Wool Insulation

Rockwool is made from a mixture of basalt (a type of volcanic rock) and steel slag (a byproduct of the steel manufacturing industry). 

Because its chief components are highly sustainable, it is one of the most eco-friendly insulating materials available.

An end-facing view of a roll of mineral wool insulation

Basalt is an igneous rock that occurs in all parts of the world. It accounts for over 90 percent of all the volcanic rock on Earth. Basalt is the most common rock found in the Earth’s oceanic crust. Therefore, it is an inexhaustible natural resource that can be extracted and used without harming the planet. 

The slag used to manufacture Rockwool is a recycled byproduct of the steel industry. It is a complex solution of oxides and silicates produced when molten steel is separated from impurities within a steelmaking furnace. If not recycled for Rockwool production, this slag would probably have ended up in landfills around the country.

The US Department of Energy estimates up to 75 percent of mineral wool might be composed of post-industrial recycled materials. 

Sustainability is one factor that makes Rockwool insulation incredibly popular among the new, environmentally-conscious generation of homebuyers. 

Manufacturing Process of Rockwool

Rockwool is manufactured through a complex and unique process. This manufacturing process gives the material many of its insulating and heat-resistant properties.

  • Melting the Raw Materials: The manufacturing process begins by heating the basalt rock and steel slag at temperatures exceeding 2,900º Fahrenheit, liquifying the raw materials so they can be combined.  
  • Spinning and Stretching: After the molten slag and basalt rock have been mixed, the compound is transferred to a specialized spinning chamber. Inside the chamber, the magma-like mixture is spun at super-high speeds until it has been stretched out into long, fibrous strands of mineral wool. 
  • Collecting the Fibers: The strands (or threads) of mineral wool are collected from the spinning chamber and flattened out using a hydraulic press until they’ve been shaped into thick, dense mats. These compressed Rockwool mats are then cut into standard-sized insulation batts before being shipped out. 

Rockwool was invented (quite by accident) in 1840, when scientists discovered that, during volcanic eruptions, strong gusts of wind could blow molten lava into fine threads which looked a lot like wool. However, Rockwool insulation did not enter mainstream commercial use until the mid-20th century. 

Characteristics of Mineral Wool Insulation

Due to its unique manufacturing process and the raw materials used, Rockwool insulation has distinctive characteristics and features.

  • Mineral wool is incredibly eco-friendly because it is largely made from a range of recycled and renewable raw materials. The main ingredients needed to make it are basalt (one of the most common volcanic rocks on the planet), recycled steel slag, and coke, a type of fuel produced when bituminous coal is heated in the absence of air. 
  • Rockwool has a reasonably high R-value ranging from 3.0 to 3.3 per inch of thickness. 
  • As mineral wool is produced by melting rock at temperatures exceeding 2,900ºFahrenheit, the end product is naturally resistant to heat and fire. As a result, this material can withstand very high temperatures (up to 1,200°F) without catching fire, melting, or releasing any toxic gases. 
  • Rockwool insulation is not prone to mold, mildew, or fungus infestations because the process of melting basalt rock in a furnace eliminates all organic matter. The inorganic Rockwool provides a poor medium for the growth of mold and mildew. 
A worker lifts a batt of rockwool insulation off a stack as he readies it to be applied to an attic ceiling
  • A binding agent made from formaldehyde, urea, and phenol holds the Rockwool fibers together. In addition, some oil is sprayed onto the surface of Rockwool batts to make them resistant to water and moisture. 
  • Mineral wool is not biodegradable, and a large amount of energy is needed to manufacture it. Click here to learn more about environmentally sustainable building materials. 

All about Fiberglass Insulation

Fiberglass is one of the most common insulation materials found in US homes. It is made from fine glass fibers and is usually sold in three different forms:

  • Loose-fill fiberglass
  • Fiberglass batts
  • Fiberglass boards

Loose-fill fiberglass and fiberglass batts are the two most common forms. 

A closeup on a piece of Loose-fill Fiberglass insulation
Loose-fill fiberglass insulation

On average, fiberglass insulation offers R-values of between 2.2 and 3.8 per inch of thickness. Loose-fill fiberglass usually has a lower R-value (around R-2.5 per inch) than fiberglass batts, which generally offer R-3.2 or higher. 

Fiberglass insulation is typically used to insulate interior walls, floors, and ceilings. It can also be fitted between beams, studs, and rim joists. 

Fiberglass might be the best choice of material if you are trying to insulate your home on a budget. It is affordable, widely available, and easy to install, which has made it popular with builders, architects, and homeowners alike. 

Fiberglass Insulation – Manufacturing Process

The raw materials used to manufacture fiberglass include silica sand, soda ash, limestone, borax, feldspar, and magnesite. Recycled glass is also frequently used. 

When the manufacturing process begins, these ingredients are mixed and then fed into a furnace for melting. The furnace may use either electricity or fossil fuel for its operation, but the molten glass must be kept at temperatures of around 2,500ºF at all times. 

The super-heated, molten glass is then fed into a perforated metal container that is constantly spinning. This container will have thousands of tiny orifices through which it can escape. 

After passing through these orifices, the molten glass comes out as fine strands or filaments, which are subjected to a downward blast of hot gas or air. The glass is converted into fine strands of fiberglass through this process. 

The fiberglass strands then fall into a conveyor belt, where they are layered and interlaced together until they form a fleecy mat. This mat is then sprayed with a binder fluid and compressed for greater density and thickness. 

The finished product can then be cut into batts or boards of various sizes. These batts are quite durable and capable of trapping air pockets, making them highly heat flow-resistant. 

Important Features of Fiberglass

Like all insulation materials, fiberglass insulation comes with a handful of features and characteristics that are unique to it. 

  • Fiberglass is naturally brown or white. However, fiberglass batts are sprayed with pink polymer glue during the manufacturing process, giving them the distinctive color we are familiar with. 
A picture of un-faced pink fiberglass batts in the wall, with a man installing it
Loose-fill fiberglass insulation
  • Fiberglass is light, pliable, and easy to shape. It can easily conform to all surfaces, including curved and irregular walls, arches, domes, etc. It is, therefore, a suitable insulation material for buildings that are not perfectly rectilinear in shape. 
  • The lightness and flexibility have also made it relatively easy to install. In fact, if you have any experience with home upgrades and repairs, then you can install fiberglass insulation in your room over a DIY weekend. 
  • Loose-fill fiberglass can be blown into a wall cavity (or an attic space) with the help of a blowing machine. It is an excellent insulating material for the less accessible parts of your home.
  • Fiberglass insulation might require a gel coating every five years or so. If not properly maintained, fiberglass batts can settle and lose shape over time, leading to a deterioration in R-value. 

Mineral Wool vs. Fiberglass – Which One Should You Choose?

Let’s dive in and see how fiberglass and mineral wool’s features compare with one another. 

Thermal Performance

Rockwool insulation typically offers better thermal resistance than fiberglass. Something demonstrated by the fact that Rockwool is usually assigned a higher R-value than fiberglass.

R-value is a measure of the heat-resistance capabilities of any given material. 

Rockwool insulation has a minimum R-value of 3.0 per inch, while the minimum R-value of fiberglass is 2.2 per inch of thickness. Click here to learn more about the R-value of mineral wool.

Furthermore, Rockwool insulation is not prone to sagging or settling, so its R-value does not deteriorate over time. However, the R-value of fiberglass insulation often decreases over time, particularly when exposed to moisture or when not maintained correctly.  

Installation Process

Both fiberglass and mineral wool insulation are easy to install. The batts of fiberglass and mineral wool can be easily cut and trimmed to fit into any wall panel or cavity. 

However, mineral wool is denser and heavier than fiberglass, so you will probably need more manpower to install it, meaning higher labor costs.

The enhanced weight of mineral wool makes it unsuitable for attic applications, where the insulation material will be resting directly on top of the gypsum board. 

Having a relatively lower density means that fiberglass insulation is more flexible, making it easier to install in buildings with curved and irregular walls. Loose-fill fiberglass can be used to easily cover up small gaps or holes in a building envelope. You do not need to use up a whole batt of insulating material to insulate a small or remote space.

Fire Resistance

Both fiberglass and mineral wool insulation are non-combustible, so they can both help delay the spread of a housefire to a certain extent. 

However, Rockwool is made primarily from basalt rock, which will only melt at temperatures exceeding 2,900ºF. The relatively thinner fiberglass insulation might start melting when the temperature touches 1,100ºF. 

Because Rockwool is denser than fiberglass and has a much higher melting point, it is considered more fire-resistant.

Usually, the density of a material is directly correlated with its fire-resistant capabilities, which is why Rockwool batts are widely used as a firestop between the different floors of a house. 

Price Considerations

Mineral wool insulation is about 25 percent more expensive than fiberglass insulation. 

Fiberglass insulation typically costs about 50 cents per square foot, while Rockwool costs around 72 cents. This seemingly small difference can add up very quickly when you’re trying to insulate large sections of your home, such as an entire basement or attic.

However, Rockwool insulation is more durable than fiberglass and is less likely to settle over time. Therefore, by paying more upfront to install Rockwool, you might actually be saving yourself some significant expenses a few years down the line. 

Mineral wool also has better heat-resistant capabilities, which may help reduce your energy bills in the years to come. 

Wrapping Up

As you can see, both Rockwool and fiberglass insulation have their advantages and drawbacks. The good thing is that you can take advantage of both types (while avoiding their pitfalls) by installing them in the respective areas for which they are best suited. 

For instance, using Rockwool to insulate exterior walls, crawl spaces, and basements and installing fiberglass in attics, interior walls, and residential floors. 

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