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What Is Mineral Wool Insulation? Is it an Effective Insulator?

removing attic insulation rock wool

If you live in Alaska, Montana, or, let’s say, North Dakota, you’re no stranger to extremely cold temps. The weather over there can get pretty rough, which is why proper insulation is a must. Now, there are lots of great options on the market, including, of course, fiberglass. That said, if you’re looking for the most reliable and durable material with market-leading insulating properties, go with mineral wool.

This product is highly resistant to heat and moisture and does a great job of soundproofing a room/house. It’s not very cheap, though, and can cause minor health issues when breathed in. So, what is mineral wool insulation? It’s time to find out!

divider 4 How Does It Work?

Along with fiberglass, mineral wool is one of the most popular materials for insulation. It was first introduced to the world back in 1871 and, over the years, has proven to be great at both acoustical and thermal control. It helps keep the warm air in and the cold air out while blocking outside noises. It’s much thicker and heavier than fiberglass (three times denser, to be exact). Thanks to its porous nature, mineral wool is highly efficient, long-lasting, and easy to install.

It traps air and “holds it hostage”, stopping it from escaping into the atmosphere. That equals exceptional performance. However, you should be careful when using it in the attic (gypsum board might not be able to hold its weight). Besides, to cut this material, you’ll need to use a serrated knife or even a wood saw. And one more thing: while mineral fiber is available in different shapes and sizes, it doesn’t offer much variety. For example, you won’t find it in a paper-faced format.

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Mineral Wool
Image Credit: RachelW1, Pixabay

What Are the Different Types of Mineral Wool Insulation?

You might’ve heard people use terms like MMMF, MMVF, mineral fiber, and cotton when talking about insulation. Well, don’t worry: all these terms refer to the same product: mineral wool. Now, there are two types of mineral wool insulation on the market—rock wool (also known as stone wool) and slag wool. At the core, they are very much alike and have similar properties. Still, they do have their unique properties. Here’s a detailed breakdown of both:

Slag Wool Insulation

The key difference between the two mineral wool varieties is the way they are produced. Slag wool, for example, comes from the waste generated by furnaces that melt and refine metal. That waste is called “slag”, hence the name for the wool. Facilities that specialize in manufacturing insulating materials spin the slag into multiple layers and then put them together to form mineral wool. Approximately 70% of this wool is recycled furnace slag.

The percentage can vary depending on the factory that makes it, but usually, it’s 70–75%, and that makes it a “green”, recycled material. Slag mineral wool is inorganic, by the way. And, since there’s no food for mold to feed on, you’ll never see mildew growing on top of it. The same is true for rock wool sheets. However, while slag wool has a pH value of 5–6, rock wool only has 4. That means rock wool is more water-resistant.

Rock Wool Insulation

As you’ve probably already guessed from the name, rock wool mostly consists of stones. It does contain black furnace slag, but the percentage is much lower (10–15%). The rest of it is diabase or basalt mixed with minerals. Both rocks are 100% natural. They are formed when magma (lava, melted rocks) cools down upon reaching the surface of the earth. The mineral composition of these rocks is almost identical.

Rock wool can be both rigid and flexible and holds tiny pockets of air inside the layers of wool. That’s how it manages to control air movement, keeping hot or cold outside air from entering the house and preventing the air trapped inside of the building from escaping. This is important: rock wool’s melting temperature goes beyond 1,000°C; slag wool, in turn, starts to melt at 500–600° C.

man installing rockwool insulation
Image Credit: brizmaker, Shutterstock

Where Is It Used?

One of the biggest pros of mineral wool is its versatility. While it’s not the most flexible material, it does have a wide range of applications. Most homeowners buy it to keep their homes warm during cold days. Others appreciate its above-average sound-blocking properties. MMMF protects countless homes both in the US and around the globe and is often put inside walls, ceilings, floors, and doors. Crawl spaces, basements, attics, and garages also benefit greatly from it.

If you’re going for continuous exterior insulation, mineral wool will help you achieve great results. Lastly, gardeners use this material as a hydroponic growth medium, along with perlite, peat, and gravel. It can be adjusted to hold large amounts of water and air and then “feed” it to plant roots. So, yes, mineral wool is not only good for thermal insulation, but also sound control, growing plants, and more. Thanks to its fire- and moisture-resistant nature, it’s highly reliable, durable, and safe.

How Big Is the Mineral Wool Market?

In 2018, MMMF was a $9.9 billion industry. It’s estimated to reach $16 billion by 2026, with a CAGR of 6.3%. The experts predict steady growth for mineral wool thanks to increased demand for safe, acoustically insulated, and eco-friendly materials. With the world recovering from the effects of the COVID-19 pandemic, urbanization is growing at a very rapid pace. And the more buildings are constructed, the more mineral fiber the companies will need.

Advantages of Mineral Wool Insulation
  • Easily handles 1,000 degrees Celsius. As far as the best fire-retardant materials go, mineral wool is right on top of that list. First, it can withstand 1000+ degrees C (that’s 1,800+ degrees F). Second, it won’t burn or release toxic fumes when exposed to heat/fire. On top of that, mineral wool does not conduct heat. That’s why it’s considered a safe and reliable product.
  • Highly resistant to moisture. If you live in an area with above-average humidity levels, investing in mineral wool for insulation will be a great choice. That’s because it’s naturally resistant to high levels of moisture. Next, this material doesn’t lose its thermal or acoustic insulation properties no matter how wet it gets.
  • Impressive insulation properties. This one’s self-explanatory: mineral wool is better at insulation than most similar products on the market. It’s fibrous, dense, and specifically designed for maximum efficiency. Mineral cotton boasts a market-leading R-value per inch (R-15 and higher) which makes it an energy-efficient option.
  • An eco-friendly product. Despite the unappealing look and irritating nature, both rock wool and slag wool are environmentally-friendly materials. As mentioned earlier, mineral wool is mostly made from reused waste, which can’t be said about the vast majority of insulation materials available on the market.
Disadvantages of Mineral Wool Insulation
  • Slightly on the expensive side. If you’re looking for a cheap insulation material to cover your walls, ceiling, and floor, mineral wool won’t be an ideal pick. While it does, indeed, deliver excellent performance and lasts for a very long time, it’s a little bit pricey for someone on a tight budget. For example, fiberglass is 25–50% cheaper.
  • Heavier than you might think. Mineral wool is an incredibly dense product. It’s also quite heavy (at least, for a material that looks lightweight) because it’s crafted from slag and rocks. Also, MMMF is hard to compress and lacks flexibility. That’s well compensated by its thermal control, though.
  • Tends to irritate the skin and lungs. When our skin is exposed to mineral wool fibers, that causes irritation. And when it gets into our lungs, the symptoms include a sore throat, coughing, and nasal congestion, along with a “stuffy” nose. Fortunately, all these side effects stop once you move away from the wool or cover it.
  • Not a biodegradable product. Although MMMF is eco-friendly, it won’t degrade when put in a compost pile or a landfill. Again, this has to do with the fact that it’s made of stone. That said, stone wool CAN be recycled at a factory, as long as it has the proper equipment to reprocess and reuse it.

divider 1 Frequently Asked Questions (FAQs)

Mineral Wool vs Fiberglass: What’s the Difference?

At a glance, these two materials are very similar. They look alike and have the same goal: to keep warm air in and loud noises out. However, mineral wool is much denser and thicker than fiberglass (1.7 pounds vs. 0.5–1 pounds). As a result, it’s significantly better at insulation. Also, fiberglass is rarely (if ever) used for continuous exterior insulation. Besides, if you go with slag wool, you’ll be working with a 70% recycled material; fiberglass is only 20–60% reused.

The R-value per inch is higher as well—there’s a 25–37% difference (R-15 vs. R-13), with mineral wool taking the lead. More importantly, mineral fiber has a higher melting point and is practically immune to fire. On the downside, MMMF is more expensive. It’s heavier, too, and less flexible. Now, when exposed to fiberglass, you’ll get red eyes, a sore throat, and irritated skin (yes, much like with MMMF). It’s biodegradable, though. Still, mineral wool is a better pick compared to fiberglass.

How Long Does Mineral Wool Last?

MMMF is a long-lasting material—that’s what makes it so special. It’s made from minerals, not glass, and isn’t “afraid” of fire, water, mold, rust/corrosion, or temperature extremes. Mineral wool is also thick, multi-layered, doesn’t deteriorate over time, and, when installed properly, will last for the lifetime of your house. Most companies that produce mineral wool guarantee perfect performance for 30–50 years, which goes to show just how reliable and long-lasting this insulation material is.

To put things into perspective, fiberglass has a maximum lifespan of 80 years; cellulose has an even shorter life cycle: 20–30 years. Looking for an alternative that lasts a lifetime? Then go with spray foam insulation. Now, severe physical damage is the only thing that can ruin an otherwise perfectly fine layer of MMMF. Mostly, we’re talking about critters or insects chewing through it. This rarely happens, of course, but it can still be an issue.

When to use mineral wool insulation When not to use mineral wool insulation
Heat/moisture resistance is a must The budget for the project is limited
You’re ready to pay for superior insulation You want to avoid skin/lung irritation
Eco-friendliness is important to you You’re looking for a biodegradable product
The project requires dense insulation Lightweight materials are a top priority

divider 4 Conclusion

Are you ready to pay a bit extra for premium-quality insulation for your home? Then mineral wool will be a great purchase. First, it’s highly resistant to fire, moisture, mold, and mildew. Next, it does a decent job of insulating sounds. On top of that, both rock and slag wool are environmentally-friendly products. This is a heavy material, however, and won’t degrade in a recycling bin.

More importantly, mineral wool is dangerous both to our skin and lungs. On the bright side, it lasts for a long time, and, despite the slightly steep price tag, MMMF is well worth the investment. The impressive insulating properties, dense, thick nature, and ease of use turn it into a must-have material that can keep your house nice and warm!

Featured Image Credit: LianeM, Shutterstock


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