If you are interested in green building, or call yourself a green building expert, then you should know about mineral wool insulation. If you have not seen mineral wool handled and installed, then you need to read this.
If you think that mineral wool batts are similar enough to fiberglass batts that you already know what you need to know about it, then you are a fool. And you still need to read this.
If you have already read some of , you know that I am an advocate of using mineral wool insulation to improve the energy performance of the houses we build in the U.S. There are many reasons why I think that mineral wool is the best insulation for us here. Recently I find myself making my case for this repeatedly, so I thought it would be worthwhile to get it all down in one place and just point to it in the future.
Mineral wool is different from fiberglass
So why am I constantly explaining why I like mineral wool, and what’s good about mineral wool? And why am I constantly saying, “No, that’s not what mineral wool is like; rather, it is like this”?
It is because the green building community has almost universally decided that ol’ fiberglass batt insulation is bad. There are good reasons for this, and we’ll look at those, but the more obvious question is, What does this have to do with mineral wool?
The overwhelming assumption among those interested in green building is that mineral wool is just like fiberglass batts, at least in all the ways they feel fiberglass batts are bad. So if fiberglass insulation is bad, then they are convinced that mineral wool insulation will be bad, for all the same reasons. Its an easy conclusion to reach, but a lazy misunderstanding.
Not all batts are bad
The reality is that this is just not so. In fact this simply highlights the profound misunderstanding of what mineral wool insulation is like among green building advocates. The misunderstanding centers around the form of insulation: batts. The green building community has been very quick to condemn batts, when the problems that concern them actually revolve around fiberglass batts.
Let’s make this absolutely clear: There is nothing wrong with insulation in the form of batts. Batts are a convenient way to package insulation for transport, handling, and installation, which is why it is the predominant form for insulation in the U.S.
However, there are legitimate reasons to criticize fiberglass batt insulation, the status quo in U.S. house construction. It’s a brief list of reasons, so let’s look at them.
Fiberglass batts have low R-values
First, low insulation values. That’s right: fiberglass batts sold here in the U.S. don’t provide as much insulation as they could. It’s not that you can’t make fiberglass in higher performance levels; in fact it is made and sold in Canada at higher R-values.
But the big fiberglass insulation makers are not ready to bring that high-R-value fiberglass insulation here to the U.S. How do I know? I’ve called them, spoken to their people about it, and they’ve told me they won’t bring the high-R-value insulation to the States even though I said I wanted to spec it.
I say, shame on them. We should all take our business elsewhere.
Most fiberglass batt jobs are sloppy
Second: bad installations. Bad installations mean sloppy fitting of batts into stud cavities, over-compressing the insulation or leaving gaps that allow convection, sloppy trimming around obstructions, and any number of other installation sins that spoil the effectiveness of the insulation.
The installers are only partly to blame. The material is limp and fluffy, and because of this the handling, cutting, and placement requires more care to install well. It rarely gets that care; hence it is most often installed quickly and cheaply.
Where is the air barrier?
Third: bad air sealing and a bad vapor retarder. Well, what is that about anyway? Insulation is for insulating, not for air sealing — right?
Well, somewhere along the way, somebody had the bright idea to combine a vapor retarder with a fiberglass insulation batt. It turns two construction steps into one, and in theory saves labor and so saves money. The problem is that once you’ve had a bad installation as noted above, and cut the vapor retarder around electrical boxes, what you end up with does not seal well against air leaks or vapor diffusion.
Hell, you say — I don’t need my insulation to make an air seal, because I used that good ol’ housewrap on the outside. Nope, nothing wrong with housewrap — but it provides no help with the air sealing you need at your vapor retarder.
The air seal in this case wants to be on the warm side of the wall, to prevent interior moisture from entering the wall cavity and condensing during the winter heating season.
The problem is the fiberglass, not the batt
Let’s summarize the lesson here: What most green pundits blame on batt insulation is the fault of fiberglass insulation. While mineral wool also comes in batt form, it is a completely different product with different properties.
It does not suffer from any of the above problems of fiberglass, yet retains the best part: it’s easy to handle, easy to install, and best of all, your labor force already knows how to do it. That is no small point.
Roxul is going after the residential market
OK, let’s talk about mineral wool. I’m going to refer generally to “mineral wool,” but as of this writing I have in mind the products of one manufacturer: . This is the only mineral wool manufacturer taking the residential market seriously in the U.S. right now.
Roxul sells its mineral wool in consumer-friendly packages, just as you would expect in a . They are making their batts in sizes made to fit stud walls, clearly aimed at the market for building houses of wood, the very same market that fiberglass is sold to. This is important. Same kind of packaging, same kind of expectations, same kind of product experience.
They are putting their products into long-standing material distribution streams that every builder in the U.S. understands. It’s available from the same sources, ready to be installed by the same people.
There are other mineral wool manufacturers out there, but they are not proactively pursuing the residential market. I’m not talking about them or their products. If your criticism of mineral wool revolves around your experience with some other product from some other time, then your concerns don’t apply here and now. You should catch up.
Mineral wool batts have a higher R-value
So let’s talk about insulation value first. In batts offered for 2×4 stud walls, mineral wool comes in R-15, while fiberglass comes in R-11 or R-13 (although it can be special ordered in R-15 in the U.S.). In batts offered for 2×6 walls, mineral wool comes in R-23, fiberglass comes in R-19 (and again, you can special order higher R-value, but only R-21).
Furthermore, mineral wool is available in batts that fit 2×8 framed walls, and these are R-30. Fiberglass is not offered in batts for 2×8 stud walls.
These are the products available now, today, in mineral wool. If you are interested in maximizing the energy performance of your walls, it’s a no-brainer. Even if you changed nothing else about the way you build, you can improve the performance of your walls by switching to mineral wool.
Mineral wool batts are easier to install
Second, installations. You might think, “How can mineral wool batts be easier to install than fiberglass batts?” It’s simple. Fiberglass batts are limp, soft blankets. They have to be hung and stapled into place or they will slump and leave gaps.
Mineral wool is dense and firm, and it friction fits into the stud space. People don’t generally understand this, and this is why I am including photos. Mineral wool has form; it has a shape. If you pick it up it does not drape or fold; it retains its shape. (See Image #2, below) It is a block. This block, when shoved into a wall cavity — and I say shoved because it literally must be pushed into place — when shoved into place, fills the entire wall cavity (see Image #3, below). No gaps, no sags, no spaces. More importantly, no convection, no drafts, and no stapling.
You still have to cut the batts to fit
There is one caveat here: Just as with fiberglass, mineral wool must be cut to fit odd-spaced studs and triangular corners that may exist between framing members. However, this is much easier to do with mineral wool.
With fiberglass batts, you are actually told by experts to use a 2×4 stud as a straightedge while you cut the fiberglass batts with a sheetrock knife. You are expected to compress the fiberglass enough to cut through it with the short blade of the razor knife. And if you are doing it correctly, you trim the batt and kraft paper to different widths, to leave a tab so you can staple it up. Good luck.
Mineral wool is different. You can actually measure and cut mineral wool to size, like cutting a big block of wood. In fact, for carpenters this comes as second nature, because as you might expect they are quite good at measuring, and then cutting something to fit, when that something has a shape and can be understood like a piece of wood.
The cuts are made with — imagine a giant bread knife. (See Image #4, below.) So there is no compressing the insulation flat so you can cut through it. The mineral wool retains its volume while you cut through it with the long knife.
You don’t cut against a 2×4, but rather you cut it on a purpose-made cutting table, which is just like a very large carpenter’s miter box (see Image #5). It adjusts for the thickness of the insulation, and allows you to set precise angles for cutting to fit those odd spaces. (.)
Insulation cutting tables
The mineral wool cutting table, for me, is the eye-opener: the cold water in the face that made me realize that we never took insulation very seriously here in the U.S. Here we pull a 2×4 out of the dumpster, and use a razor blade holder to cut it on the floor deck. That’s the best we can do.
Mineral wool is cut precisely to fill every void — quickly, accurately — providing complete fill of the wall cavity. Installations are easier, faster, and better than with fiberglass.
I’ll say with some confidence that no other insulation product can fill a stud space so completely. No spray, no blown-in product, no blanket can fill a wall void as well as a proper installation of mineral wool insulation.
Smart vapor retarders
And third, air sealing. Mineral wool only comes in unfaced batts. No foil or kraft paper vapor retarders are offered. This means an independent vapor retarder must be installed. Simply said, this is the best way to create an airtight envelope for the house.
My favorite product in this regard is a variable-permeability membrane, a so-called “smart” membrane, because the permeability self-adjusts to suit conditions. An example is the . This is their branding of the smart membrane product made by their European owner, Saint-Gobain.
Another high-quality variable-permeability membrane is Intello Plus and DB+ by . This vapor retarder membrane has a low permeability level in dry conditions, but if the humidity level within the wall gets high, the material will open up to allow the moisture to dry to the other side. The ProClima Intello membrane is notable because it is well reinforced. It will not tear or split from stapling, and this tolerance of handling makes it easier to work with.
Include a service chase
Now, if you wish to increase the likelihood that you won’t need to make any punctures in this airtight layer, then you should plan on a wall with an interior wiring chase, such as the U.S.A. New Wall that we’ve elaborated on .
Your best chance for an airtight wall is if you don’t puncture your air barrier with electrical work, outlet boxes, and switches. If you’ve not studied it before, when you are done here go read the U.S.A. New Wall article, and the to see how all these come together to make a simple but high-performance wall.
That covers insulation value, installation, and air sealing issues. Mineral wool goes on to excel in other ways that contribute to my preference for this material.
It won’t stay wet
Mineral wool is hydrophobic. From the dictionary: “Tending to repel or fail to mix with water.” If fiberglass insulation becomes wet, you end up with a wet lump of glass lint, with no insulation value to boot. Mineral wool, on the other hand, will not become wet.
In fact, water . This promotes water draining and drying if the wall cavity becomes wet, rather than holding water like a sponge. Which would you rather have in your wall?
Addressing thermal bridging
One of the big issues with improving the performance of our walls is thermal bridging through the wall studs. The most popular way to overcome this has been to install insulation on the exterior of the wall, continuously, to insulate the studs from the cold.
The issue here is that rigid foam insulation has been the most common way to do this. The problem, however, is that the foam creates a vapor retarder, and the last thing you want is a wall with a vapor retarder on both faces.
So the practice has been to make walls with exterior foam insulation without interior vapor barriers. The foam must be thick enough to keep the dew point within the depth of the foam in order to prevent condensation within the wall cavity.
The problem here is that only the most general recommendations for these configurations can be made in the building code. Weather outside of the design limits can result in condensation. Highly humid interior conditions can cause condensation.
My opinion is that these walls are not resilient designs and are poor practice. A wall with a traditional configuration — with a vapor retarder on the interior — works in all conditions, even when the temperature or interior humidity goes beyond the design values.
My only caution is, if the home is to have air-conditioning, then it is very important to use a smart vapor control sheet, as mentioned above. These variable-permeability membranes will ensure the wall performs well during the cooling season when the vapor profile of the wall is reversed.
The good news in this story is that mineral wool comes in configurations that can be used as exterior insulation in place of foam. These are very dense fiber panels that are strong enough to support siding and cladding material mounted over them in a manner similar to foam. The difference: mineral wool is vapor permeable, which means you can insulate on the exterior without trapping moisture in the wall.
Furthermore, the insulation value of these dense mineral wool panels approaches the performance of foam. Typically for XPS foam you would get in the neighborhood of R-5 per inch. Mineral wool will provide R-4.6 per inch.
And because of mineral wool’s hydrophobic properties, it will not absorb moisture in this location, and in fact will promote the drainage and drying of rainscreen siding cavities.
Foam insulation brings other problems, such as dilemmas in flashing practices, and the dubious reliance on adhesive tape as a long term weather barrier.
Mineral wool simply makes wood stud construction safer. This is the same material that is used to fireproof steel members in commercial construction. Mineral wool will stand up to temperatures that will reduce fiberglass to a puddle of molten glass.
Mineral wool will increase the length of time that a wood-framed house will stand during a fire. It gives the occupants more time to exit safely, and firefighters a safer window of time to enter a burning home.
Let’s just go over some broad conclusions now. Mineral wool is a completely different material from fiberglass. They both just happen to fall in the broad classification of batts.
Mineral wool is available in higher insulation values than fiberglass. Mineral wool’s rigid shape and ability to be measured and cut accurately enables it to fill stud voids more completely than any other insulation product, with less effort, and more speed.
Mineral wool fits into the building practices of 99.9% of America’s builders with no need for new processes, extensive retraining, or changes to new subcontractors, new suppliers, and new business relationships.
Mineral wool is the easiest way for the vast majority of builders to step up their game and start building better.
Appendix 1: Concerns of green builders
Those focused on green building often immediately point to the presence of formaldehyde in binders used in mineral wool. Formaldehyde-based resins have long been used in the “glues” that prevent fibrous insulation from coming apart. It’s long been used in fiberglass, and is still used in many fiberglass products today. But it is clearly on the way out. Fiberglass batts from Knauf and Owens Corning are now available with alternate binders.
Mineral wool still contains a very small amount of formaldehyde-based binders. The wall insulation products from Roxul meet the standard (formerly known as GreenGuard Children and School certification), which means they are consistent with products that are used in schools, day-care centers, or other environments where children spend significant periods of time. What this means is if you are a builder shifting from traditional fiberglass batts, this mineral wool will likely have lower formaldehyde emissions than what you used before.
If you are a green builder seeking to eliminate any formaldehyde, then you have other options.
On the matter of embodied energy, the Roxul products now use from 75% to 93% recycled content, depending on the availability of the raw materials, and their facilities are “zero waste to landfill.”
If you are a green builder who can get the same performance from a material with less embodied energy, then great. If you are a builder considering shifting from fiberglass, the energy savings from the lifetime performance of the mineral wool will greatly exceed any difference in manufacturing energy of the fiberglass you use now.
Appendix 2: Predictions
Higher R-value insulation will eventually own the market here. CertainTeed, one of the biggest U.S. makers of fiberglass, is now owned by Saint-Gobain, a large producer of mineral wool in Europe. CertainTeed now has access to mineral wool, and could begin to sell it in North America as it gains market share.
Owens Corning, the other large American insulation maker which is highly vested in fiberglass, has purchased an American mineral wool manufacturer, Thermafiber, widening its offerings to include both mineral wool and fiberglass.
The third large American manufacturer of fiberglass insulation is Johns Manville, which has recently acquired an American mineral wool manufacturer, IIG (Industrial Insulation Group).
Not all of these companies are actively promoting mineral wool for residential building insulation, but it is remarkable that we find all of the major American insulation manufacturers vested in mineral wool.
Fiberglass has also been dogged by health questions. In 1984, fiberglass was classified as a “reasonably anticipated” carcinogen. This did not proceed to the next level, “known carcinogen,” and in 2011, following more research, it was removed from this list.
Currently fiberglass is classified similarly to mineral wool, which is considered what is called “biosoluble.” This means that these mineral fibers dissolve in contact with tissues, leaving no fibers to trigger disease. Whether fiberglass can shed its unhealthy reputation, or if it will be overtaken by mineral wool with its higher performance levels, has yet to be determined.
Editor’s note: This article was adapted from a blog written by Greg La Vardera in 2012.
Greg La Vardera is an architect practicing in Merchantville, New Jersey, just outside of Philadelphia. His work has included the development of several off-site building methods and progressive design in catalog house plan publishing. He is now focusing on creating energy-efficient prototypes for production building based on Swedish precedents, as well as posting blog entries on .