What Is Mineral Wool Insulation(Rockwool)?

Mineral wool is also known as Rockwool insulation. Stone wool and slag wool are also used but describe specific products. Mineral wool is a type of insulation made from volcanic rock such as basalt or dolomite. The rock is combined with waste products like steel slag or iron ore waste.

Mineral wool is used as a thermal insulation material for many construction applications such as walls, floors, roofs, attics, pipes, and ducting. It is also used as a filtration product and hydroponic growth medium. Building codes and architects often specify that mineral wool is to be used in soundproofing and fire retardant locations.

How is Mineral Wool Insulation Made?

Slag wool was first made in Wales around 1840. It was patented in the US in 1870. High-temperature wool became commercially available around 1953 with various other types of applications appearing in the 1970s and 1980s.

Crushed volcanic basalt rock is heated to 1600 degrees and turned back into lava. The liquified rock is poured into a spinning machine which creates long thin strands of mineral wool. It has been compared to the making of cotton candy at circuses and exhibitions.

Oils and resins are added to hold the strands together and act as a water repellant. Another machine layers the fleece together in a zig-zag pattern. Rollers compress the product to increase density. Heating it in an oven solidifies the resins so the insulation holds its shape. It is then cut to size and packaged.

Mineral Wool Uses

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

Mineral wool is also chopped into small cotton-like bits. These are blown into attics to provide excellent insulation value. Blown-in insulation does a much better job of insulating all the small cavities and around ducts and wires. It is much easier to apply than batt insulation.

Blown-in mineral wool is used as wall insulation for new construction and retrofits. It does not 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. Cellulose is R-3.6.

Mineral wool batts provide much better soundproofing insulation than fiberglass. Mineral wool is very dense. It provides the mass necessary to deaden and absorb sound waves. Architects and designers often require mineral wool insulation to be used in multi-family buildings and noisy locations. It is used 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 or blown-in form or as 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. Can be installed 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. R-value measures the thermal resistance of a material–or how well it prevents heat conduction. Warm or hot air flows to cooler areas.

Building codes are updated regularly to require higher R-values from the insulation products used during construction and retrofits. Not only does mineral wool provide high R-values, but it is also non-combustible.

Roof Insulation

Buildings can lose up to 25% of heat through poorly insulated roofs. In hotter climates, a well-insulated roof prevents heat gain through the roof. Roof 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. Very effective for heated floors.
• Separating Floors. Floors between living areas in multi-story buildings transfer heat and noise. Mineral wool reduces heat transfer. It is also one of the most used products for soundproofing applications.
• Exposed Floors. Keeps floors over areas like crawl spaces warm. 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 are often used to replace fiberglass batt insulation in stud cavities of exterior walls. They provide a better R-value. They are more rigid–eliminating insulation sag. Wool batts are available in multiple sizes and R-values. They are more expensive than fiberglass.

Mineral wool batts can also be used on the building’s exterior to add insulation under new siding, or stucco, or EIFS applications. Adding insulation to the outside of walls makes the entire building envelope more energy efficient. It 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

Mineral wool blown-in insulation can be used in stud cavities just like cellulose blown-in insulation. There are two methods of application. Attach netting over the inside of the studs and blow the cavities full through hose-sized holes cut into the netting about three-quarters of the way up the cavity.

Mineral wool can also be combined with adhesive and blown into bare stud cavities without the need for netting. The insulation will stick to the sheathing and studs. Once dry, a vapor barrier and drywall can be applied.

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

Blown-in mineral wool provides slightly lower R-values than cellulose–both dry or wet. R-3.1 compared to R-3.6 for cellulose. It is also heavier than cellulose and fiberglass blown-in insulation.

Blowing R-40 into an attic requires a thickness of about 13”. It weighs slightly more than two pounds per square foot. A 2000 square foot attic ends up with over two tons of insulation resting on the drywall.