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Product Guide

Mineral Wool Insulation

A balanced look at an older building material’s place in present-day projects

Rockwool Comfortboard 80 installed as exterior insulation

Mineral wool—glass wool, stone wool, slag wool—has many applications, from fireproofing to sound insulation. It resembles fiberglass, but it has no glass. There are two types of mineral wool on the market: rock wool, which is made from stone, such as basalt, or diabase (volcanic rock); and slag wool fibers that are derived from iron-ore waste. The material comes in semi-rigid batts, sheets, and loose-fill, similar to fiberglass. Until the emergence of fiberglass batt insulation, mineral wool was North America’s most widely used insulation.

As a dense, fireproof material, mineral wool has always played an important role in improving the fire performance of buildings. It is not only fireproof but also protects the building by reducing heat transfer and the spread of fire within framing cavities. As a naturally incombustible material, manufacturers do not have to add fire-retardant chemicals that sometimes present a biohazard. And as thermal insulation, mineral wool has additional advantages.

Mineral wool in cavities

When compared to fiberglass batts, mineral wool offers superior insulation. It has about a 22-37% higher R-value per inch. It contains 70% recycled material, making it a greener product than fiberglass at 20-30%. Mineral wool retains its shape better than fiberglass or cellulose. It will not settle within walls, leaving cold gaps along the top plate. If there’s air infiltration, moisture will not compromise or degrade its thermal performance; and the material provides no food to support microorganisms.

With an average of nearly R-5 per inch, within 2×4 walls, the material will deliver a whopping R-17.5, and in a 6-in. cavity, R-27.5. Along with thermal insulation, it offers a true sound barrier. Two-inch-thick panels have an STC (sound transmission class) between 45 and 52 and an NRC (noise reduction coefficient) range from 0.95 to 1.09. Recording studio walls are stuffed with dense, mineral wool batts due to their sound-reducing and deadening properties. “Sound batt” fiberglass insulation does not provide a comparable benefit because this material is not as dense.

Mineral wool on the exterior

As a continuous exterior insulation board, mineral wool averages R-5 per inch, comparable to foam. Yet mineral fiber adds a fire barrier to the assembly that will not create toxic fumes if the walls should catch fire. The material maintains 90% of its insulating qualities for the product’s life and has a vapor permeance rating of roughly 50 perms. Termites don’t like it. And moisture will not compromise its thermal resistance.

Products on the market

Rockwool insulation on exterior of wall assembly

Rockwool Comfortboard 80 is a rigid stone wool board designed for continuous insulation applications. A non-structural sheathing product, it provides increased thermal performance in 4 in. and 5 in. thicknesses, providing thermal performance up to R-21 for building envelopes. Retails for $60.

Thermafiber insulation boards

With a minimum of 70% recycled content Thermafiber RainBarrier CI High Compressive (80) from Owens Corning is designed for use behind any cladding type, including combustible and open-joint assemblies. Retails for $26.

Johns Manville mineral wool

Owned by the Oracle of Omaha, Berkshire Hathaway’s Johns Manville makes mineral wool insulation for residential use. The company promotes mineral insulation to prevent spreading fire between interior rooms. Because of its resistance to rot, mold, mildew, and fungi growth, the material serves as suitable friction-fit insulation for crawlspaces. There carry two products: Sound & Fire Block, mineral wool to soundproof and delay the spread of fire between interior rooms, and TempControl, mineral wool for exterior walls, basements, and heated crawlspaces. $36 and $29, respectively for about 50 sq. ft.

Comfortbatt board

Comfortbatt is a semi-rigid stone wool insulation from ROCKWOOL for wood and steel framing. The batts include a flexible edge to compress between framing walls, joists, and rafters. The material comes in various thermal values. Retails for $45 for 60 sq. ft. of R-15.

The drawbacks

About 50% more expensive than fiberglass or foam, mineral wool is also unpleasant to work with. You must wear protective gear when installing it because the tiny mineral slivers will lodge into your skin and lungs; the particles inhaled can irritate the alveoli. Some data suggest that persistent inhalation of mineral wool may result in upper respiratory effects. The EPA has not classified rock wool or slag wool for carcinogenicity. But when installing it, wear a dust mask at minimum.

Mineral wool is made from stone melted in furnaces. The manufacturing process involves high-heat combustion using gas and coal in furnaces, boilers, and dryers. Operations run 24 hours a day year-round.

While mineral wool contains recycled content, it is, in turn, difficult to recycle with few reuse applications (although the industry is making an effort to reduce landfilling and improve “take back” services). It does not decompose or break down when disposed of.

Consider the ongoing case of community opposition to mineral wool manufacturing that is unfolding in Jefferson County, West Virginia. Neighbors are asking: “Who bears the pollution costs of manufacturing ‘eco-friendly’ products?” The story was published in March of 2021 in VICE World News. (For a dispassionate, scientific review of the balance between effective insulating characteristics that make the mineral wool green in buildings but an energy-intensive and polluting material to produce, refer to “Environmental assessment of mineral wool manufacturing.” The researchers conclude that “manufacturing optimization should take place within this industry in order to improve the efficiency and mitigate negative connotations of the material production.” Moreover, they recommend the development of new insulation materials with lower carbon footprint and less energy-demanding manufacturing to further minimize the negative environmental implications.)

Life Cycle Assessments (LCAs), which consider the environmental impact of a product over its lifetime—from manufacturing to end-of-life use—provide a clearer assessment of building insulation levels. A Danish study concluded that the production of mineral wool offset the environmental benefits of super-insulated buildings. However, “a shift away from the dominating use of coke to a greener source of energy for thermal energy in the production of mineral wool could change the results of this study entirely, allowing for greater levels of insulation while maintaining climate change impact benefit.”

On the other hand, according to this Rockwool Sustainability Report, the industry claims mineral wool can become “net carbon positive” within 200 days, “meaning the carbon emitted during the production of building insulation is offset by the avoided emissions of the insulation’s use in buildings.” The same report concludes that as an energy-intensive business, Rockwool needs to “develop our melting technologies, which are currently predominantly based on foundry coke.”

Overall, mineral wool is an excellent building material with good performance measures but it has a heavy manufacturing impact.

________________________________________________________________________

Fernando Pagés Ruiz is a builder and an ICC-certified residential building inspector active in code development.

11 Comments

  1. Expert Member
    Michael Maines | | #1

    Are there specific products claiming R-5/in? The rigid mineral wool products I'm familiar with are around R-4.2/in.

    Rockwool's sustainability logic is flawed; using any type of insulation will offset its carbon emissions eventually, compared to not using any insulation at all. The comparison should be between different types of insulation at the same R-value. There are things mineral wool can do that other insulation can't do; the same is true of spray foam, and when those characteristics are required, I use both products without guilt. But in many cases there are other insulation choices that have much less environmental impact.

    1. charlie_sullivan | | #3

      Well put. The discussion about whether the emissions associated with insulation production are worth it should be framed as a comparison to other insulation types, not as a comparison to the straw-man of not using insulation or using too little insulation.

  2. kurtgranroth | | #2

    Mineral wool clearly has a lot of compelling benefits but with all that it has, I'm baffled why they beat the drum of its acoustic benefits so hard as if its special or so much better than the competition. In the end, mineral wool and fiberglass have COMPARABLE acoustic characteristics and are far more similar than they are different.

    The article mentions an STC rating between 45 and 52 for a two-inch-thick panel of mineral wool. That is just shy of being meaningless. STC is all about blocking sound transfer so what matters is the "assembly", not its individual components. It's notable, then, that most lab-tested high-STC wall assemblies are designed with fiberglass batts rather than mineral wool. The reason is that while mineral wool has a slight edge over fiberglass in acoustic transfer in a head-to-head comparison, the instant you install either into a wall, it becomes the least important part of that assembly and that slight edge is erased.

    Roxul, in particular, likes to create these entire "rooms" made of rigid Safe'N'Sound batts at trade shows just to show how well they work... but of course, you'd never leave the batts like that in the real world. Create the same room with rigid fiberglass batts and there's not one person out of a thousand that would be able to detect a difference. Add drywall to either and nobody can.

    Then we get to the NRC rating. That rating is all about absorption, so I see it mostly in the context of acoustic control rather than sound attenuation/blocking. It is, indeed, common to use mineral wool batts to create inexpensive acoustic control panels... but it's just as common to use rigid fiberglass batts, since they perform almost identically. Two-inches of OC 705 has an NRC of 1.05, which is essentially the same as the same thickness of mineral wool. They have similar octave absorption curves, as well. Really, just use whatever you have on hand or what costs the least in your market.

    In the end, mineral wool is absolutely a fine component in many acoustic use cases, but so is fiberglass!

    1. Expert Member
      MALCOLM TAYLOR | | #4

      Kurt,

      I just had this conversation with a consultant for a recording studio I'm building. As far as I can see it's like buying a brand, which however indifferent the performance benefits are, makes it look like you know what you are doing.

  3. gstan | | #5

    I was really proud of myself - going through 4 years of college and paying attention to
    pump prices, I had managed the entire 4 years without ever paying more than 18 cents
    a gallon for gas (the early 1960's). This brings up what has happened to energy prices,
    lumber and other materials since those halcyon years. I don't think anybody will argue,
    they have all gone up! Way up! Now! Let's think about the economics of fiber glass vs
    mineral wool in the longer term. What do you think will happen to the cost of heating
    (or cooling) your house over that longer term? Is it worth it to pay 50% more per
    sq. ft. for mineral wool to obtain a 20% to 30% reduction in heat transfer through
    the walls? Keep in mind that the total additional cost when building the house is a
    tiny fraction of the total construction cost but the total cost of the energy to heat
    and cool over a life span is enormous. Anybody who choses fiberglass (if they have
    a choice) isn't thinking too clearly! If there is any place in the construction process
    where "overkill" is justified it's in the amounts and quality of the insulation - it's
    actually hard to have too much in the longer term.

    1. Expert Member
      MALCOLM TAYLOR | | #6

      gstan,

      I agree in part, but I'm not sure it's that simple.

      FIrst, even if you agree with the difference in performance Fernando has cited, the benefit of mineral wool doesn't translate into anything near that amount of improvement in the whole wall or roof R-values. If instead you use the difference in price between the two types of batts, there may well be other places to spend that money which would yield better results.

      But a larger problem is that using your assertion that energy prices will inevitably increase as a justification leaves you with no mechanism for deciding what insulation levels are appropriate. More is always better. So why not Passive House? Why not Passive House + 50%?

      No matter how high energy prices get there will always be a point where more insulation doesn't give you an appreciable reduction in energy consumption. That's why we typically use modelling to see what makes sense.

  4. irene3 | | #7

    I didn't find mineral wool at all unpleasant to work with. I am not a terrifically handy person and I installed mineral wool batts all around the top of my basement walls (after air sealing), cutting and cobbling as needed to fit odd-shaped bays. I would far sooner work with it than with floppy fiberglass batts, and being able to do it myself meant that it cost very little. I would expect even for professionals it would be faster to install. The fire performance and longevity are both huge plusses to me, not to mention that it doesn't absorb water.

  5. chimewind | | #8

    I have heard there are slight differences between mineral wool manufacturers (I've only used Rockwool mineral wool) and that some are not as nice to work with as others (dustier, harder to cut, etc).

    1. Expert Member
      MALCOLM TAYLOR | | #9

      I've never found installing any batt insulation much fun, but considering how little time the job takes, I wouldn't make a decision on which type to use based on my comfort.

  6. user-1112741717 | | #10

    Another factor is ready-availability at retail outlets. To obtain 2" thick batts that I needed as exterior insulation my choices were to travel 50 miles round trip to a specialty retailer (minimum order 56 square feet) or order online. One big online retailer asked dear product prices to obtain free shipping. My walls have horizontal blocking in the stud bays and having 1.5" and 4" on hand would be useful for a 2x6 wall vs notching 5.5" 2' OC for all that blocking. Not to mention buying a big box store's shrink-wrapped bundle when it a small project. It's sometimes hard to argue with fiberglass' ready availability. Plan ahead.

  7. user-5454807 | | #11

    Mineral wool used to be 50% more than fiberglass but it seems that now - even though both have increased significantly in the last couple of years - the former is closer to double the price ( R13 vs. R15: $.66 vs. 1.20 per sq. ft). It should be noted that mineral wool will apparently last much longer than fiberglass.
    The establishment of the new Rockwool plant in a coal energy sourced area is disappointing: hopefully they will improve their process to reduce their usage.
    Despite these deficits, we're still using Rockwool because of its water and - particularly - fire resistance.

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