Mineral wool, also known as stone wool and rock wool, is a very durable, effective form of insulation made from stone slag and other materials. Mineral wool is available as loose fill, batts, and rigid boards, also referred to as mineral wool rigid panels.
Rigid mineral wool boards are commonly used to insulate walls, ceilings, floors, HVAC ductwork, masonry walls, and areas subject to high heat. The melting point of mineral wool is in excess of 2000 degrees Fahrenheit making it ideal around furnaces and electric heating elements.
However, working with rigid mineral wool requires PPE, or personal protective equipment. Mineral wool fibers are caustic to eyes, skin, and lungs just like fiberglass. Be sure and follow the directions that come with your rigid mineral wool boards.
Here we will discuss rigid mineral wool boards in greater detail and describe their most common uses. Likely the most well known mineral wool manufacturer, Rock Wool, offers innovative, unique attachment methods and specialty products made from mineral wool.
Are Rigid Mineral Wool Boards Better Than Foam Insulation Boards?
Rigid mineral wool boards are generally superior to rigid foam panels in durability and resistance to heat. Rigid mineral wool insulation is produced from slag and other rock material, so its fire resistance is unequaled compared to fiberglass and foam insulation boards.
However, rigid mineral wool should not be attached with tapes or adhesives because most rigid mineral wool does not include a vapor barrier. Rigid mineral wool is designed to breathe, so adhesives and tapes that would block this ventilation should be avoided.
Most rigid mineral wool boards do not provide a mounting tab either, so mechanical attachment with corrosion resistant fasteners is common. Some rigid mineral wool manufacturers provide standoff hardware that allow the boards to be sandwiched between other materials, like drywall, without using any fasteners.
Rigid Mineral Wool Boards
Both rigid mineral wool and rigid foam boards are ideal for most insulation projects. However, rigid mineral wool will usually be more expensive to purchase than rigid foam boards, so since the performance is similar, rigid foam is used when either material is appropriate.
Rigid mineral wool boards are perfect around HVAC air handlers, furnaces, ovens, ranges, and other potential sources of ignition. Rigid foam boards should never be used near sources of open flame, like a pilot light or furnace door as they may burn and emit toxic fumes.
Rigid mineral wool provides about R-4 per inch and is available in both faced and unfaced forms, although unfaced is most common. Unlike other forms of insulation, mineral wool insulation is typically described by its weight per cubic foot instead of its R value.
For example, #8 (or eight pound) mineral wool will be more dense (and effective) than #6 mineral wool. For reference, a rigid mineral wool board made from #6 mineral wool will be about 2” thick, 48” long, and 24” wide. Two inches of rigid mineral wool provides about R-7 of insulation value.
Rigid Foam Boards
Rigid foam insulation boards offer about the same R value per inch as rigid mineral wool, but at a considerably lower cost. The most common rigid foam boards are molded from some form of polystyrene, a very moldable plastic.
EPS, or expanded polystyrene, is a lightweight rigid foam board that provides about R-3 per inch on average. XPS, or extruded polystyrene, is a more dense version of rigid foam board that provides closer to R-7 per inch of insulation.
What Other Advantages Does Rigid Mineral Wool Offer?
Rigid mineral wool has a couple of advantages that make it more appropriate than other forms of insulation in some situations. Rigid mineral wool boards are excellent sound absorbers, so they are very common between interior walls and sound recording spaces.
Rigid mineral wool also does a great job of preventing the spread of fire. Mineral wool lwill not burn, but it will melt. Extremely high temperatures (over 2000 degrees F) are required to melt mineral wool, so it is commonly used as a fire wall between multi-family buildings, like apartments or condominiums.
Does Rigid Mineral Wool Have Any Disadvantages?
Rigid mineral wool boards do a great job of insulating while protecting from fire, but they do have a few limitations compared to more popular materials. Rigid fiberglass boards, for example, are usually less expensive than rigid mineral wool.
Unless increased heat resistance or sound deadening is required fiberglass batts, rigid foam boards, and cellulose batts will often provide a more cost effective option.
How Are Rigid Mineral Wool Boards Installed?
When rigid mineral wool boards are surface mounted vertically they commonly use mechanical fasteners, as opposed to tapes or adhesives. When used as cavity insulation, rigid mineral wool boards are typically held in place by compression, rather than fasteners.
When surface mounted vertically, rigid mineral wool usually requires mechanical fasteners that provide a mounting pin. The pin extends outward from the wall and the rigid mineral wool is impaled upon the pins. An additional 2” corrosion resistant washer is typically added for better holding power and snapped over the pin to secure the board permanently.
Because rigid mineral wool is not typically sealed, it requires the addition of a vapor barrier. Although the mineral wool will not retain moisture itself, it will allow moisture to pass through to any nearby surface. Wood rot is common when a vapor barrier is not installed.
Rigid mineral wool is also more expensive than rigid foam panels, so it is typically used only where high resistance to heat is required. However, rigid mineral wool boards may outlive other forms of insulation, providing a higher return on investment over time.
Vertical Rigid Mineral Wool
A common application for rigid mineral wool boards is to insulate masonry, like a basement wall. Fiberglass batts would require some form of stud wall to attach to, but rigid mineral wool can be attached to a masonry wall if a vapor barrier is installed in between the insulation and the wall.
Another application for rigid mineral wool boards is to fill a void, such as the one created between two wall studs. The space is usually 3 ½” deep, 14 ½” wide, and 96” high, which conveniently matches common rigid mineral wool board sizes.
Rigid Mineral Wool Used As Cavity Insulation
Rigid foam board insulation is easier to install in a cavity than rigid mineral wool. Rigid mineral wool uses compression to stay in place when used inside a cavity (like a wall), so the boards must be trimmed precisely.
Most rigid mineral wool manufacturers do not recommend using adhesives or tapes to secure the material, as they will likely detach and could block air flow. Rigid mineral wool boards are flexible, so they can be cut to fit the cavity snuggly without spaces or compressing the insulation. Each board is carefully measured and trimmed to fit the space it will occupy.