Mineral wool, not to be confused with sheep’s wool, is fiber-based, loose-fill insulation formed by spinning or drawing molten mineral or rock materials such as slag and ceramics.
Sometimes referred to as rock wool, slag wool, or stone wool, mineral wool is made by melting the raw material, which can be either basalt or volcanic rock (melted at around 1,600°C) or slag, a waste product of steel. The raw materials are spun to produce very thin fibers, coating those fibers with a binder to hold them together, and forming them into the insulation batt or board stock material to meet specific product needs.
High-temperature mineral wool is used primarily for insulation and lining of industrial furnaces and foundries to improve efficiency and safety. Both types of mineral wool are typically sold in batts and as loose-fill and can be used for all home insulation applications, including walls, roofs, attics, ceilings, and floors.
The use of high-temperature mineral wool enables a more lightweight construction of industrial furnaces and other technical equipment as compared to other methods such as fire bricks, due to its high heat resistance capabilities per weight.
Applications Using Mineral Wool Insulation
Mineral wool is ideal for applications that need lightweight, easy-to-install insulation with robust thermal performance.
Applications of mineral wool include thermal insulation (as both structural insulation and pipe insulation, though it is not as fire-resistant as high-temperature insulation wool) and soundproofing. Other uses are in resin bonded panels, as filler in compounds for gaskets, in brake pads, in plastics in the automotive industry, as a filtering medium, and as a hydroponic growth medium.
Types of Mineral Wool
There are several types of high-temperature mineral wool made from different types of minerals. The chosen mineral results in different material properties and classification temperatures.
Alkaline earth silicate wool (AES wool)
AES wool consists of amorphous glass fibers. Products made from AES wool are generally used in equipment that operates continuously and in domestic appliances. AES wool has the advantage of being bio-soluble—it dissolves in bodily fluids within a few weeks and is quickly cleared from the lungs.
Alumino silicate wool (ASW)
Alumino silicate wool, or refractory ceramic fiber, consists of amorphous fibers produced by melting a combination of aluminum oxide (Al2O3) and silicon dioxide (SiO2). Products made of alumino silicate wool are generally used at application temperatures of greater than 900 °C for equipment that operates intermittently and in critical application conditions.
Polycrystalline wool (PCW)
Polycrystalline wool consists of fibers that contain aluminum oxide (Al2O3) at greater than 70 percent of the total materials. The water-soluble green fibers obtained as a precursor are crystallized by heat. Polycrystalline wool is generally used at application temperatures greater than 1300 °C and in critical chemical and physical application conditions.
Kaowool is a type of high-temperature mineral wool made from the mineral kaolin. It was one of the first types of high-temperature mineral wool invented and has been used in the 21st century. It can withstand temperatures close to 1,650 °C (3,000 °F).
Benefits of Mineral Wool Insulation
Like other types of insulation, mineral wool offers thermal insulation benefits. Many builders also prefer it because it provides noise insulation, fire protection, and is energy efficient and durable.
Mineral wool is a heavier and more dense insulation material than fiberglass, giving it better sound-control properties and more effectively restricting airflow through it. When produced in board stock form, mineral wool can be rigid enough to work as insulative sheathing, like extruded polystyrene and polyisocyanurate.
It is also highly fire-resistant, which has long made it an insulation material of choice in many commercial buildings. It achieves its fire resistance without the use of any flame retardant chemicals, which are widely used in most foam-plastic insulation materials. It has an R-value of 15, significantly higher than most fiberglass insulation batts.
Though not immune to the effects of a sufficiently hot fire, the fire resistance of fiberglass, stone wool, and ceramic fibers makes them common building materials when passive fire protection is required, being used as spray fireproofing, in stud cavities in drywall assemblies, and as packing materials in firestops. Mineral wool insulation can withstand temperatures exceeding 1,800° F (1,000° C). It will not burn or release toxic gases or smoke when exposed to high heat.
Mineral wool insulation can improve energy efficiency and reduce carbon emissions, making it an excellent choice for home renovations. The actual energy efficiency benefits will depend on the thickness of the walls and other building specifications. The R-value of 15 for 2×4 stud walls and R-23 for 2×6 stud walls is significantly better than the fiberglass’ rating of R-11 or 13 and R-21, respectively.
The insulation value (R-value) of mineral wool does not change with time. As materials compact with time, typical insulation loses efficiency. The initial R-value of mineral wool will remain unchanged.
Another benefit is that mineral or stone insulation typically contains up to 90% recycled content. Instead of sending slag to landfills, the steel and ore industries recycle the material. The earth replenishes the rock through volcanic and oceanic activity.
Important Notes About Mineral Wool Insulation
In the past, there was some concern that mineral wool and fiberglass fibers might be carcinogenic, like asbestos. While those concerns have largely been dismissed, the fibers are still respiratory irritants, similar to other insulations. Installers of mineral wool should always wear quality dust masks, and the material should be adequately covered with drywall or coatings that prevent fibers from entering the indoor air in a building.
Manufacturers use phenol-formaldehyde or a urea-extended phenol-formaldehyde binder to glue the fibers together. Formaldehyde is a known human carcinogen, and if a lot of it is exposed to indoor air, can cause health complications. Fortunately, the processing drives off nearly all of the free formaldehyde in the material, so formaldehyde emissions from mineral wool have extremely low formaldehyde levels—in some cases as low as background formaldehyde levels.
As has occurred with fiberglass insulation, there is a perceived and possibly real concern with formaldehyde binders, and manufacturers are working on alternatives. In recent years, VOCs are less of a concern since there’s been a general move to using low or no VOC binders. One way to know for sure that you’re getting a mineral wool batt that has low or no VOCs is by looking for GREENGUARD Certification labels right on the packaging.