Jul. 21, 2025
Agricultural
Mineral wool insulation board, commonly known as Rockwool, is a building material used in construction to insulate walls and attic spaces. The fibers that make up mineral wool insulation come from the minerals in volcanic rock and slag waste from steel production, that are spun and pressed together into board form. Mineral wool in board form is more rigid and dense than batts, another common form of mineral wool.
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Mineral wool insulation boards come in various sizes and thicknesses, from 1” up to 5” thick. Standard sizes are limited to 24” modular increments, including:
24” x 48” (2’ x 4’)
24” x 72” (2’ x 6’)
24” x 96” (2’ x 8’)
48” x 96” (4’ x 8’)
Mineral wool insulation boards are often used in exterior wall applications where continuous insulation is desired. They are installed flush over a substrate like plywood and attached with nails and 3” washers.
Mineral wool insulation board has several physical characteristics that make it desirable for use in building construction, including:
Natural fire-resistance
High sound absorbance
R-value of 4.2 per inch
Mineral wool insulation board isn’t right for every application, however. Here are some of the problems with using mineral wool insulation boards in construction projects:
At 8 pounds per cubic foot, mineral wool insulation is dense and heavy, which makes it difficult to work with. A single 2” thick, 4’ x 8’ board weighs over 40 lbs.
The size and weight of mineral wool insulation boards present a challenge for installation without doubling labor. Often, one laborer is required to hold the sheet in place and another to secure it. The alternative is to purchase smaller sheets for easier handling, but this slows labor down by decreasing the coverage per sheet.
If you’re seeking the additional fireproofing and sound absorbance properties of mineral wool insulation, keep in mind that it will cost you more initially than standard fiberglass insulation. Expect to pay 25-50% more than fiberglass for the material, plus a higher installation cost.
Mineral wool insulation boards are only produced in specific sizes and must be cut to fit around obstacles. If you have a custom job that requires irregular insulation shapes, mineral wool will make the job very difficult because there are fewer sizes available and the material does not conform to the cavity space, and must be cut on the job.
Some building insulation materials include a built-in vapor barrier facer. Mineral wool is not one of them since it is vapor permeable and is only available unfaced. With mineral wool insulation boards, a separate vapor barrier is required in order to keep moisture out of your walls and structure.
Mineral wool insulation boards are designed to be installed over a flush substrate, such as weather-sealed plywood. That means that you’ll need additional furring strips over the top to attach your exterior finishes, like siding.
Polyiso insulation is another building material that is used in construction projects. It solves many of the challenges and downfalls of mineral wool insulation boards, including:
Higher R-Value: R6.0 per inch
Custom Facer: Polyiso adds a reflective or weather barrier option to the insulation board facer
Lightweight: Polyiso is very lightweight and easily handled by a single installer
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Competitive Pricing: Polyiso is less expensive than mineral wool
Versatile Installation: Polyiso can be installed over multiple substrates or subgirts
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Fiberglass insulation has long been a popular option for slowing the transmission of heat through the walls and ceilings of a home. While it may have the added benefit of creating a fire-resistant layer between the interior and exterior walls, fiberglass still may not measure up to the natural abilities of Rockwool. Like fiberglass, Rockwool is an insulation material that is regularly used in residential, commercial, and industrial buildings.
However, Rockwool can be differentiated from fiberglass by comparing the heat retention, fire resistance, moisture resistance, and soundproofing capabilities. Keep reading to find out more about Rockwool insulation.
The process by which Rockwool insulation is made helps to explain the true fire-resistant potential of this product. It’s composed primarily of basalt rock and a recycled steel-making byproduct known as slag. These components are superheated, allowing them to liquefy and mix together into a lava-like liquid. In order to melt these substances, the temperatures must exceed 2,900 degrees Fahrenheit.
The mixture is then blown into a large spinning chamber designed to stretch the superheated liquid into fibers. These fibers are then gathered together and compressed into a mat, which can be cut into slabs of Rockwool insulation.
By creating Rockwool through this process, all organic matter is eliminated, greatly increasing the mold- and mildew-resistance of the finished product.
The confusion about the amount of recycled material used to make Rockwool insulation can mostly be attributed to the statistics about mineral wool insulation in general. Rockwool is a brand-specific type of mineral wool insulation that is so popular the name became synonymous with the appropriate term, which is mineral wool. The brand-specific Rockwool insulation is typically made of between 16 to 40 percent recycled materials, according to the manufacturer. However, the U.S. Department of Energy has stated mineral wool insulation contains an average of 75 percent of recycled materials.
This estimate is hard to back up because the Department of Energy makes the distinction between standard ‘rock wool’ insulation and ‘slag wool’ insulation, but doesn’t note the difference in the amount of recycled material for each product. Also, this is a perfect example of the ‘Rockwool’ brand name being used in place of the generic material name, blurring the lines between products.
In general, it can be derived that the amount of recycled material in Rockwool insulation is not precise because it ultimately depends on the specific product. Standard Rockwool insulation may only have between 16 to 40 percent recycled material, while slag Rockwool insulation can be made with up to 75 percent recycled material.
Both fiberglass and Rockwool are effective at keeping a home cool in the summer and warm in the winter, but the specific thermal efficiency of these materials favors Rockwool. While fiberglass insulation is capable of offering an R-value of about 2.2 to 2.7 per inch of insulation, Rockwool has an R-value between 3.0 to 3.3 per inch of insulation.
Fiberglass insulation also tends to lose its thermal efficiency over several years as it begins to degrade. Due to the method of construction and the materials used to make Rockwool insulation, the thermal performance of this insulation remains stable over the lifetime of the building. However, Rockwool tends to cost more per square foot than fiberglass insulation.
As noted previously, Rockwool insulation is formed from literal rocks and steel slag that must be heated beyond 2,900 degrees Fahrenheit in order to mix the component materials and create this highly effective insulation. With this in mind, it makes sense that mineral wool products in general can resist fire, flames, and heat up to 1,400 degrees Fahrenheit, while some Rockwool products are capable of resisting temperatures up to 2,150 degrees Fahrenheit without melting, smoking, or catching on fire.
This impressive heat-resistance is ideal for building homes because the insulation forms a fire-resistant barrier between the interior and exterior of the home, between rooms, and even between floors, slowing the spread of fire. It should be noted that fiberglass insulation is also highly heat-resistant, though it begins to melt at about 1,100 degrees Fahrenheit.
The durability of this insulation material is difficult to dispute, given that it is capable of retaining its thermal efficiency over several decades with minimal degradation in its R-value. This is primarily due to the materials used to make Rockwool insulation, including rock and steel slag, which are known for having a high level of durability and natural resistance to decay and corrosion.
However, the durability of Rockwool isn’t limited to the heat retention quality of the material. The impressive water-resistance, mold-resistance, mildew-resistance, and fire-resistance also contribute to the durability and capability of Rockwool insulation. This is especially true of the material’s moisture-resistant qualities, without which the insulation could absorb and retain water from highly humid air, causing the premature breakdown of the material.
Some people may not appreciate the heftiness of Rockwool insulation because it does tend to be thicker than fiberglass insulation, but this helps to slow the transmission of heat and it has the added effect of slowing sound waves. As sound waves attempt to move through the material, they are slowed and sometimes completely blocked, creating built-in soundproofing.
While the thickness of the insulation helps to block noise, it is the density of Rockwool insulation that provides the soundproofing. Fiberglass insulation has a density of about 0.5 to 1.0 pounds per cubic foot, allowing it to reduce sound by 4 to 10 decibels. Rockwool insulation has a density of around 1.7 pounds per cubic foot, which is capable of consistently dampening sound by 10 to 15 decibels.
The construction and composition of Rockwool makes it ideal for rooms that are prone to high levels of humidity, like the bathroom or kitchen. Rockwool insulation’s moisture-resistant and vapor-permeable qualities mean that any liquid water will drain away from the insulation instead of soaking into it, while gaseous water vapor will pass through without dampening the material.
Additionally, Rockwool insulation is inorganic, so it makes a poor medium for mold and mildew to grow as there is nothing for them to use for energy. In fact, Rockwool products are also tested and certified as resistant to fungal growth, reducing the chance that users will open up the wall and find a dangerous biological problem waiting for them.
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