Last Updated: Nov 4, 2021
Besides an occasional trip into an unfinished attic to store Christmas decorations or look through old family albums, most people never interact with their home's insulation. Insulation is generally hidden away between 2x4s, plywood sheathing, and drywall. This seldom considered aspect of home construction plays a significant role in the energy efficiency and thermal performance of the buildings we inhabit.
Poorly insulated homes are not always easy to spot. Behind many luxurious facades covering 3,500 square feet of living space, many houses have inadequate insulation that leads to high levels of heat transfer. This heat loss will lead to significantly higher energy bills, negatively impact air quality, increase noise pollution, and increase your home's carbon footprint.
Finding more sustainable insulation options are an essential item on the agenda for every homeowner. Rise has written extensively about the different types of insulation options on the market and the pros and cons of some environmentally friendlier insulation types.
In this review, we take a thorough look at the benefits and challenges of mineral wool (also referred to as stone or rock wool). Mineral wool insulation is growing in popularity among builders and homeowners interested in more sustainable home solutions.
For many decades, fiberglass insulation was the standard for home construction. On the plus side, that pink cotton candy-looking material was relatively inexpensive, easily sourced, and moderately efficient at insulating your home. It also came in pre-sized rolls of batts or blankets that fit between standard building code-sized joists and studs. For that reason, 90 percent of cavity insulation in new homes in 1994 relied on fiberglass insulation.
So what are the main drawbacks? First, it is sourced from non-renewable materials, specifically plastic reinforced with glass fibers. Projections of global demand for insulation materials expect a rise of 8.2% over four years to $8.9 billion in 2024. As sustainable home construction methods gain popularity, energy-efficiency targets and goals are being added to building codes. Many existing homes will also undergo major renovations, such as low carbon retrofits. With the average-sized home well over 2,600 square feet, the thought of coming up with billions of dollars worth of non-renewable insulation material certainly has its environmental implications.
Second, fiberglass insulation has been linked to adverse health impacts, including irritating the skin, eyes, and respiratory system. According to one study, inhaling the microscopic fibers released by fiberglass insulation could potentially slice DNA and cause cell mutation, leading to cancer. Recommendations for anyone handling fiberglass insulation include wearing a dust mask, gloves, goggles, clothing, and a hat covering the entire body. Following work with fiberglass insulation, wash all clothes in a separate load.
Mineral wool—not to be confused with sheep's wool—is fiber-based, loose-fill insulation. It has a similar appearance to fiberglass insulation, though it is a bit stiffer and denser. In general, there are two main types of mineral wool insulation.
The first type of wool insulation is mainly made from natural stone fibers such as molten basalt or diabase. This type of mineral wool is sourced from volcanic rock, melted at around 1,600°C. This melted rock is subsequently spun into "wool" and bound together by different resins and oils.
The second is made from spinning slag. Slag is a waste product in the production of steel. Manufacturers can also make it from natural rock or a combination of molten slag and natural rock and even glass. Typically, mineral or stone insulation contains up to 90% recycled content.
Both types of mineral wool are typically sold in batts and as loose-fill and can be used for all home insulation applications, including walls, roofs, attics, ceilings, and floors. Like other types of insulation, mineral wool offers thermal insulation benefits. Many builders also prefer it because it provides noise insulation and fire protection as added side benefits. In terms of thermal performance, mineral wool batts made for traditional 2×4 walls achieve an impressive R-value of 15. These R-values are significantly higher than the R-11 to R-13 values that characterize most fiberglass insulation batts.
There are several practical advantages and environmental benefits that come with mineral wool insulation. Below, list a few of the most important advantages.
Mineral wool insulation can improve the energy efficiency of homes, helping to cut back on carbon emissions. The actual energy efficiency benefits will depend on the thickness of the walls and other building specifications. The R-value of 15 for 2x4 stud walls and R-23 for 2x6 stud walls is significantly better than the fiberglass' rating of R-11 or 13 and R-21, respectively. For this reason, mineral wool is an excellent choice for home renovations.
Mineral wool is a very resilient insulation material. The insulation value (R-value) of mineral wool remains unchanged over time. While other standard insulation options will decrease over time as materials compact, the initial R-value of mineral wool will remain unchanged.
Mineral wool insulation can protect buildings from the spread of fire, thus reducing vulnerability to household fires. Tests have shown that Mineral wool insulation can withstand temperatures exceeding 1,800º F (1,000º C). It will not burn or release toxic gases or smoke when exposed to high heat. This fire rating helps delay the spread of fire in a home, thus improving a home's resiliency.
Mineral wool insulation is similar in appearance and function to the more commonly used fiberglass loose-fill insulation. This allows mineral wool insulation to be easily swapped into conventionally constructed homes as a more sustainable option than fiberglass for major home renovations.
Mineral wool insulation can improve the acoustic performance of a home. It's an excellent option for multifamily residences and single-family homes located near interstates or other noisy areas.
Mineral wool insulation is mainly sourced from natural, renewable, or recyclable materials. The steel and iron ore industry produce millions of pounds of slag each year. Instead of sending that material to landfills, it can be recycled into high-performance insulation. When made from stone, mineral wool is dependent on a virtually inexhaustible natural resource. The earth makes 38,000 times more rock every year through volcanic and oceanic activity.
Mineral wool insulation is about 10 percent more expensive than fiberglass insulation options. R-15 fiberglass insulation to fit a 4-inch wall will cost roughly $0.51 per square foot. R-15 mineral wool batt to fit a 4-inch wall will cost approximately $0.64 per square foot.
Installers should follow safety procedures when working with mineral wool insulation. While mineral wool doesn't cause much skin irritation, the tiny slivers can be inhaled during installation. If inhaled, it could lead to respiratory problems. So, when installing, it is advised that you wear a dust mask and gloves, and clothing to protect your skin.
One of the historical pitfalls of mineral wool batts has been that it typically has a binder to hold the fibers together. These chemical binders historically contained volatile organic compounds (or volatile organic compounds) like formaldehyde. The insulation would then off-gas these chemicals. This off-gassing is dangerous for anyone installing the product and negatively impacts the homeowners' indoor air quality.
VOCs have been less of a concern in recent years since there's been a general move to using low or no VOC binders, mitigating those pitfalls. Unfortunately, they're still around, so you need to pay attention when picking your mineral wool.
One way to know for sure that you're getting a mineral wool batt that has low or no VOCs is by looking for GREENGUARD Certification labels right on the packaging.
Another environmental concern associated with mineral wool insulation is the amount of energy utilized during manufacture. As we mentioned above, the process of melting volcanic rock requires exceptionally high temperatures to be maintained in furnaces for extended periods. The embodied energy footprint of this material is relatively high compared to other insulation options. Mineral wool sourced from steel or iron ore slag can reduce this carbon footprint as it comes from recycled materials.
Mineral wool batts are impervious to moisture, which means you don't need to worry about mold or mildew or throwing out the insulation if it gets wet. Perhaps the most significant health implications are present during installation. Even though formaldehyde has been removed from most newer versions, it is a good idea to wear proper eyewear, masks, and gloves during the installation process.
The high density and composition of mineral wool insulation make it an excellent sound dampening and absorption product. Mineral wool has a sound absorption rating of 95% and NRC of 0.95 - 1.05, depending on density and thickness.
Mineral wool offers high performance with a relatively low environmental impact. However, if you have budgetary constraints, fiberglass insulation will likely continue to be your cheapest option. On the other hand, Blown-in insulation might be a better choice for homes with intricate roofs and attics that contain many nooks and crannies. This insulation option tends to do the best job of filling in hard-to-reach gaps that might be hard to insulate otherwise.
Spray foam insulation, like mineral wool, offers a high R-value and is also great and fits into tight spaces. Unfortunately, it is also one of the pricier insulation options and can negatively affect the atmosphere if you aren't careful to choose a low global warming potential (GWP) option.
Rigid board insulation, another alternative, comes in panels that can easily be cut to size. They are made from polystyrene, polyisocyanurate, or polyurethane, which have a sizeable environmental impact.
Structural Insulated Panels (SIPs) are usually a quality insulation option for homes built to the international passive house standard as they are airtight structural elements. SIPs, however, are not easily integrated into home remodels.
As with all sustainable home construction elements, there is no "one size fits all" solution. With insulation, the best option will depend on several factors, including the home's location and orientation, the building standard of the house, a homeowner's budget, and other contextual and climatic considerations.
Mineral wool insulation is not difficult-to-find with its rise in popularity as an alternative to fiberglass batt insulation. A few of the largest manufacturers and suppliers include:
ROCKWOOL: Offers several insulation products made from mineral wool.
Thermafiber is a company under Owen's Corning banner. They make a variety of residential and commercial insulation, sound, and fireproofing products that are widely available.
Knauf Insulation: This company offers mineral wool slabs and has manufacturing sites in 16 different countries.
Mineral wool batt insulation is easily sourced from building supply companies such as Home Depot and Lowe's. Rock wool is worth checking out as an alternative to conventional fiberglass batt insulation if you are building or renovating.
Back to our original question of whether this building material gets a pass or fail on the sustainability side — from us at Rise, it gets a pass! And this is a good thing as it is one of the most widely available insulation materials out there, perhaps second only to fiberglass insulation.
Tobias runs an agroecology farm and a natural building collective in the mountains of El Salvador. He specializes in earthen construction methods and uses permaculture design methods to integrate structures into the sustainability of the landscape.