What’s the Difference: Fiberglass vs. Mineral Wool Insulation

By Joseph Truini

Fiberglass insulation installation
The kraft paper on fiberglass insulation acts as a vapor barrier. Be sure the paper faces the warm side of the room.

For nearly 80 years, fiberglass insulation has been the most popular type of home insulation, and it’s easy to see why: Fiberglass is affordable, easy to install, readily available in a wide variety of sizes and, most importantly, an excellent insulator.

Fiberglass insulation is composed of fine glass fibers that are woven, lightly compressed and then cut into long rolls or batts. It is available faced with kraft paper or aluminum foil as well as unfaced. Fiberglass also comes as loose-fill insulation, which can be spread by hand or blown into place by a mechanical blower.

Interestingly fiberglass insulation was invented quite accidentally. In the early 1930’s, a researcher was trying to create a vacuum seal between glass blocks when a jet of high-pressure air blew a stream of molten glass into whispery-fine fibers. That accidental discovery led to the first large-scale production of fiberglass strands, which were eventually used to create fiberglass insulation.

Today, fiberglass remains the undisputed king of insulation, but its crown is being threatened by a relatively new insulating material: mineral wool.

Mineral wool, which is also commonly called rock wool, comes in easy-to-install batts, similar to fiberglass. But instead of being composed of fluffy glass fibers, rock wool is made from volcanic rock, primarily basalt. It, too, was invented by chance when it was discovered that strong winds routinely blew molten lava into fine threads that resembled wool during volcanic eruptions. That revelation eventually led to the manufacture to rock wool insulation.

Mineral Wool Production

To make mineral wool insulation, basalt and industrial slag are melted in a 3,000° F furnace. (Slag is a by-product of steel production that usually ends up in landfills.) Next, the super-heated liquid is exposed to a high-pressure stream of air and then spun into long fiber strands. The strands are compressed into thick, dense mats and then cut into batts of insulation.

Now that you have a basic understanding of fiberglass and mineral wool, let’s take a look at the differences between these two popular types of insulation.

Installing a mineral wool insulation batt
Mineral wool insulation comes in dense, unfaced batts that are pressed into place.
Fiberglass vs. Mineral Wool: How They Stack Up

R-Value: The thermal resistance of insulation is measured by what’s commonly known as the R-value, and the higher the R-value, the better. Fiberglass has an R-value of approximately 2.2 to 2.7 per inch of thickness. Mineral wool has a slightly higher R-value, ranging between 3.0 and 3.3 per inch.

Size: Fiberglass insulation is available in a wider range of sizes and types than mineral wool. Mineral wool insulation is typically only available in unfaced batts.

Sustainability: Mineral wool is composed of 70 percent or more recycled content. Fiberglass insulation typically contains 20 to 30 percent recycled content.

Cost: Fiberglass insulation costs 25 to 50 percent less than mineral wool. Fiberglass insulation for a 2×6 wall costs between 57 cents and 72 cents per square foot. Mineral wool insulation for the same wall runs about $1 to $1.10 per square foot.

Density: Mineral wool insulation has superior sound-deadening properties. It has a density of 1.7 pounds per cubic foot, as compared to 0.5 to 1.0 for fiberglass. Because of its density, mineral wool is hard to compress. Fiberglass, on the other hand, will lose some of its insulating value if it’s compressed too tightly.

Weight: Fiberglass is lightweight and easy to carry, but the batts are rather limp and can be challenging to set into place. Mineral wool is heavier than fiberglass, but the batts are also stiffer, so they don’t bend or flop over as easily.

Water Resistance: Mineral wool insulation is hydrophobic, meaning it’s highly resistant to moisture and water. Since it doesn’t absorb moisture, mineral wool doesn’t promote rot, corrosion, fungi, mold, mildew or bacterial growth. If fiberglass insulation gets wet, it becomes soggy, and its insulating value drops significantly.

Loose-Fill: Loose-fill fiberglass insulation provides a quick, easy and economical way to insulate attic floors and wall cavities. Loose-fill mineral wool does exist, but it’s difficult to find.

Installation: Mineral wool comes in dense, firm batts that are friction-fit into place; no stapling required. Fiberglass batts must be secured with staples or wire. To cut fiberglass insulation, compress it flat with a board or metal straightedge, then slice it with a utility knife. Use a serrated bread knife or woodcutting handsaw to cut mineral wool insulation. It’s recommended that you wear a dust mask when cutting and handling any type of insulation, including fiberglass and mineral wool.

Fire Resistance: Mineral wool is extremely fire resistant and can be used as a firestop. Fiberglass insulation is noncombustible, but not nearly as fire resistant as mineral wool.

Cutting fiberglass insulation
To cut thick fiberglass insulation, first compress it with a straightedge and then slice it with a utility knife.

 

Final Word

When properly installed, fiberglass and mineral wool are both excellent insulators and each will keep your home warmer in winter and cooler in summer. In fact, it’s not uncommon to find homes insulated with both types: economical fiberglass insulation installed throughout most of the house, and mineral wool used as firestops and in places where a little extra R-value is required, such as north-facing walls. When insulating your home, be sure to ask the local building inspector to specify the exact type and thickness of insulation required, and to point out any areas that need mineral-wool firestops.

Joe Truini is a home improvement expert who writes about a variety of topics related to carpentry and plumbing. Joe is also the author of numerous DIY books, including the best-selling Building a Shed. To learn more about choosing the right insulation and see a wide variety of options, please visit the Home Depot website.

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