What Is Mineral Wool Insulation(Rockwool)?

Mineral wool, also known as Rockwool, is a type of insulation made from volcanic rock such as basalt or dolomite and combined with waste products like steel slag or iron ore waste.

Mineral wool is a thermal insulation material many contractors use for walls, floors, roofs, attics, pipes, and ducting. Building codes and architects often specify it for soundproofing and fire retardant locations.

Rock Wool

How is Mineral Wool Insulation Made?

Slag wool originated in Wales around 1840 and was patented in the US in 1870. High-temperature wool became commercially available around 1953, with other applications appearing in the 1970s and 1980s.

To make mineral wool, manufacturers heat crushed volcanic basalt rock to 1600 degrees, turning it back into lava. They then pour the liquified rock into a spinning machine which creates long thin strands of mineral wool. 

Producers add oils and resins to hold the strands together and act as a water-repellant. Another machine layers the fleece in a zig-zag pattern, and rollers compress the product to increase density. As a final step, manufacturers heat it in an oven to solidify the resins so the insulation holds its shape. They then cut it to size and package it. 

Mineral Wool Uses

Builders use mineral wool to insulate all areas of a house. Mineral wool is more expensive than fiberglass but provides better R-values. As a blanket insulation, mineral wool comes in multiple sizes, thicknesses, and R-values.

Manufacturers also chop mineral wool into small cotton-like bits to act as blown-in insulation for attics. Blown-in insulation does a better job of insulating the small cavities around ducts and wires and is easier to apply than batt insulation.

Installers may also use blown-in mineral wool as wall insulation for new construction and retrofits. It doesn’t have to be dense-packed like cellulose insulation making for an easier install. The trade-off is in R-value – mineral wool’s R-value is about R-3.1 per inch compared to cellulose at R-3.6.

Mineral wool batts provide much better soundproofing than fiberglass. Since mineral wool is very dense, it provides the mass necessary to deaden and absorb sound waves. Architects and designers often require mineral wool insulation in multi-family buildings and noisy locations. Installers can use it to isolate bedrooms and offices from incoming noise and to prevent sound from escaping home theaters and media rooms.

Types of Mineral Wool

Mineral wool is manufactured in batt form, blown-in form, or duct insulation. All products share some common characteristics.

  • Non-combustible. Melting point 2150 degrees F (1177 degrees C).
  • Moisture Resistant. Excellent for interior and exterior applications, for use above grade and below grade.
  • Vapor Permeable.
  • UV Resistant. Will not deteriorate like polystyrene if left exposed to sunlight for a time.
  • Sound Absorbing. High sound absorption qualities.
  • Long-Term R-value. Stable product. R-value does not deteriorate over time.

Mineral Wool Batt Insulation

Most mineral wool manufacturers make several types of products for different applications.

Roof Insulation

Buildings can lose up to 25% of heat through poorly insulated roofs. In hotter climates, a well-insulated roof prevents heat gain, and insulation R-value requirements regularly exceed R-40.

Mineral wool insulation is manufactured in multiple sizes, thicknesses, and R-values. It is also made into cant strips and other roofing products that need to remain stable–eliminating the need for wood substrates.

Floor Insulation

Mineral wool insulation provides excellent insulation under floors.

  • Under Slab Floors. Concrete slabs have almost no R-value. Underslab insulation prevents heat conduction through the concrete into the soil below, which is effective for heated floors.
  • Separating Floors. Floors between living areas in multi-story buildings transfer heat and noise. Mineral wool reduces heat and sound transfer.
  • Exposed Floors. Mineral wool keeps floors over areas like crawl spaces warm and is insect and pest resistant.

Interior Wall Insulation

Insulating interior walls with mineral wool adds comfort and quiet to workspaces and living areas. It prevents heat transfer and noise penetration and, in case of fire, slows flame.

Exterior Wall Insulation

Mineral wool batts can replace fiberglass batt insulation in stud cavities of exterior walls. They provide a better R-value and are more rigid–eliminating insulation sag. Wool batts are available in multiple sizes and R-values. The downside is that they are more expensive than fiberglass.

Contractors can also use mineral wool batts on a building’s exterior to add insulation under new siding, stucco, or EIFS applications. Adding insulation to the outside of walls makes the entire building envelope more energy efficient and prevents thermal bridging of wall studs.

HVAC Insulation

HVAC systems carry warm and cool air throughout a building. Uninsulated ducting loses a significant amount of warm and cool air into the surrounding space. A normal house has hundreds of feet of ducting. Commercial buildings like office towers and airport terminals can have thousands of feet.

Effective insulation maintains the optimal operating temperature of the system. It provides a comfortable working climate, prolongs equipment life, and saves money.

Mineral Wool Blown-In Insulation

Installers can use mineral wool blown-in insulation in stud cavities, just like cellulose blown-in insulation. There are two methods of application. For this first method, installers attach netting over the inside of the studs and blow the cavities through hose-sized holes about three-quarters of the way up the cavity.

Installers can also combine mineral wool with adhesive and blow it into bare stud cavities without the need for netting. The insulation will stick to the sheathing and studs. Once dry, they can apply a vapor barrier and drywall.

There is no need to dense-pack mineral wool. It’s a relatively large, coarse fluffy product. Mineral wool conforms to services inside the stud cavities–such as electrical wires, plumbing, and HVAC services.

Blowing R-40 into an attic requires a thickness of about 13” mineral wool. It weighs slightly more than two pounds per square foot, so a 2,000-square-foot attic ends up with over two tons of insulation resting on the drywall.