Batts of mineral wool insulation stacked together

High-quality insulation can help keep your home comfortable and your energy bills low, all while minimizing your carbon footprint. The importance of effectively insulating your living space cannot be overstated. 

While many homeowners are familiar with fiberglass and cellulose insulation, a new type of insulating material has recently been gaining popularity among architects, builders, and homeowners alike.

Rockwool insulation is a material that offers excellent durability, high R-values, and environmental sustainability.

What is the R-Value of Rockwool Insulation?

Rockwool or mineral wool insulation offers an R-value of 3.0 to 3.3 per inch of thickness. While it costs slightly more than other common insulating materials like fiberglass and cellulose, it provides far greater thermal efficiency, heat retention, moisture resistance, and sound-dampening capabilities. 

The thermal performance does not degrade over time and will usually remain stable throughout the entire lifespan of the building. This longevity is because it is made from liquified natural rock, one of the toughest substances on earth. 

So, let’s learn some more about mineral wool and how it’s made so you can determine if you want this insulating material installed in your home. 

Rockwool Insulation – Manufacturing Process

The process by which Rockwool insulation is manufactured is quite unique and fascinating. Read on for a step-by-step breakdown of this unique manufacturing process.

Materials Used:

Rockwool insulation is made primarily from natural rock that has been heated at temperatures of over 3,000°F. To be precise, basalt rock and slag are (usually) the two main components used. 

Basalt is a common volcanic or igneous rock, while slag is essentially a recycled byproduct of steel production. Together, these components form one of the sturdiest insulating materials currently available.

Heating and Melting:

Before the basalt rock and steel byproduct can be combined to form the insulation, each material needs to be individually heated and melted. However, this is no easy task.

The rock and slag are heated in a furnace at temperatures exceeding 2,900°F until both have been melted into a lava-like liquid. The liquified materials are then mixed and transferred to a special spinning machine or chamber.  

Spinning Chamber:

The liquified slag and basalt rock take on a magma-like texture, at which point they must be exposed to a high-pressure jet of steam or air. Once treated with high-pressured steam, the liquid is blown into a giant spinning chamber.

This chamber is designed to stretch the molten rock and slag into long fiber strands by spinning the magma-like substance at super-high speeds. Just try and imagine a cotton candy machine but filled with liquified rock instead of molten sugar. 

Amazing! Right? We think so as well!

The molten rock is spun centrifugally at incredible speeds until it cools and resolidifies in the form of long fiber strands. These strands are known as rockwool. 

Collection and Compression:

As the molten rock and slag resolidify into long fiber strands, the spinning machine captures each strand and presses them together into a mat. After being removed from the spinning chamber, the strands of Rockwool are further compressed into thick, dense mats, which can then be cut into conveniently-sized batts of insulating material.

These slabs of insulation are later installed in buildings. You can customize the size and shape of each slab according to the needs of your home. 

Elimination of Organic Matter:

Heating natural rock until it has been liquified into a lava-like substance effectively eliminates all organic matter from the finished product. 

The lack of organic matter makes mineral wool a poor medium for the growth of mold and mildew since there is no source from which they can draw energy. Hence, this manufacturing process significantly increases its resistance to mold, mildew, and other such allergens. 

A man wearing a white mask and white gloves installing Rockwool mineral wool insulation in between the studs in the wall

Benefits of Rockwool Insulation

The unique composition and manufacturing process of Rockwool produces a high-performing insulating material with numerous benefits. And now you know how it’s made, it’s time to think about whether to install it in your home. 

Here are some of the advantages of mineral wool insulation in residential spaces:

High R-Value and Heat Retention

Rockwool is essentially a mass of fine, intertwined fibers, each of which has a diameter of around 3-4 micrometers. This intertwined structure allows the insulation to trap the air that passes through it.

Close-up of worker hands in white gloves insulating rock wool insulation staff in wooden frame for future walls for cold barrier.

The fibers often contain a binder and some oil to minimize dusting, slowing the heat transfer and increasing retention capacity. Binders increase the R-value of the insulation and provide it with greater insulation capabilities. 

With an average R-value of 23, mineral wool insulation can help keep your living space at a comfortable and consistent temperature without needing to adjust the thermostat so often. The thermal performance of this material has also remained stable over the years. 

Here’s a chart showing the R-values of other insulating materials compared to Rockwool:

Insulating MaterialR-Value
Rockwool3.0 to 3.3 per inch of insulating material
Fiberglass2.2 to 2.7 per inch.
CelluloseBetween 3.2 and 4.0 per inch of thickness
Foam BoardUp to 5.0 per inch

Sustainable, Recyclable Components

Rockwool insulation is constructed from sustainable and eco-friendly materials like natural rock and steam and is as good for the planet as it is for your home. Even the slag used in the manufacturing process is a recycled byproduct of the steel industry. 

In fact, according to figures released by the US Department of Energy, some varieties may contain as much as 75 percent post-industrial recycled material. Click here to learn more about the sustainable building materials currently changing the real estate market.

All forms of mineral wool will contain anywhere between 16 and 75 percent recycled material, but the exact amount might vary depending on the specific product. Consult your architect or builder if you want to purchase mineral wool insulation with a higher level of recycled content. 

Safety and Fire Resistance

Rockwool insulation is non-combustible and incredibly heat-resistant. The components of mineral wool – such as basalt rock and steel slag – need to be heated beyond 2,900°F before they can be melted and mixed to create the finished product. 

It makes sense, therefore, that mineral wool can resist fire and heat up to a temperature of 1,400°F. In other words, the material won’t melt or catch on fire until the temperature exceeds 1,400°F, although some varieties are known to be heat-resistant up to 2,150°F.

The fire-resistant qualities of mineral wool have made it the ideal insulating material to be used as a firestop between the different floors of a house. Click here to learn more about the fire-resistant properties of various insulating materials. 

Using mineral wool as a firestop between floors is required by the building code of many zoning districts in the US. You can ask your local building inspector to help you identify the parts of your house that most require mineral wool firestops. 

Moisture-Resistant and Hypoallergenic

As a material, Rockwool is highly water repellent. It is also quite vapor-permeable, meaning that it does not trap moisture and allows it to escape into the air. This makes it the perfect insulation material for the parts of the house that are prone to extreme humidity, such as the kitchen and the bathroom. 

Any liquid water that comes into contact with the material drains away without being absorbed into the material. Water vapor can also easily pass through mineral wool insulation without dampening the material. Moreover, the manufacturing process of rockwool insulation effectively eliminates any organic matter. 

These factors ensure rockwool insulation does not promote the growth of mold, mildew, fungus, and other such allergens.

Most mineral wool products are extensively tested for their resistance to fungal growth. Hence, it is one of the safest and most hypoallergenic insulation materials currently available. 

Exceptional Durability

Apart from being great for the environment and an effective form of insulation, rockwool is also a good investment because it has the longevity to remain in your home for as long as it stands, with minimal corrosion and no decreases in insulating capability. 

In other words, the R-value of the insulation remains constant, even many decades after installation. Natural rock and steel slag are both sturdy materials and are relatively resistant to corrosion and decay.

Furthermore, as mentioned above, rockwool is a non-organic, vapor-permeable material that does not absorb and retain moisture in humid environments. As a result, it is not susceptible to the problems of rusting, corrosion, and premature degradation that is often caused by moisture. 

Ease of Installation

Rockwool insulation is straightforward to work with. Firm, dense batts of Rockwool can be easily cut with an insulation saw or a long, serrated knife rented from your local DIY outlet.

Once cut into the required shapes and sizes, batts can be friction-fit snugly into place, without the need for staples. 

Mineral wool fibers are compressed so tightly together that the material can never slump down or shift out of position, which might have lowered its insulating value. Products made from the material can also be used to quickly fill all holes and gaps in your building envelope, leaving no opportunities for air leakage. 

The batts will not be affected by weather conditions if stored outside. Even if they get wet, the water beads will simply roll off the fibers instead of soaked in.

However, the insulation does not come with any foil or kraft paper to act as a vapor barrier, so an independent permeable membrane might need to be installed for maximum efficiency. 

Great Soundproofing Ability

Rockwool insulation is thicker, denser, and heavier than other common types of insulating material, such as fiberglass. While this might cause slight inconvenience during transportation, it also results in excellent, built-in soundproofing that few other materials can offer. 

The insulation batts can effectively slow the transmission of soundwaves, reducing (or sometimes wholly blocking) the unwanted noises of traffic, construction, or other activities taking place outside your home.  

With a density of about 1.7 pounds per cubic foot, Rockwool can dampen sound by 10 to 15 decibels. The exceptional sound-deadening capability makes it the perfect insulating material for bedrooms, music studios, and media rooms. 

Where Do You buy Rockwool (Or Mineral Wool Insulation) Anyway?

Mineral wool is pretty bulky, so it’s probably best to pick it up locally at a high-volume distributor or store like Home Depot or Lowes. At least around me, they usually have it in stock.

The main two options that I can see are the Safe’ n’ Sound Rockwool product, which is a 3-inch soundproofing and fire-resistant version, and the R-15 Comfortbatt.

Amazon does have some options, but due to the shipping costs, you’ll definitely want to compare prices to the above options.

Here is the link to Amazon’s main options, which look to be the brands ProRox and Roxul mostly.

Final Thoughts

The R-value of Rockwool insulation can vary between 3.0 and 3.3 per inch of thickness. It also offers other useful features like fire resistance, moisture resistance, and soundproofing, which have made it one of the most popular types of insulating material in the US.

However, before installing Rockwool insulation in your home, you should research and consult with home improvement experts and architects in your area. 

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