Mineral Wool Insulation Isn’t Like Fiberglass
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 my essays, 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
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: Roxul. 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 big
box home improvement outlet. 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 2x4 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 2x6 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 2x8 framed walls, and these are R-30. Fiberglass is not offered in
batts for 2x8 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 2x4 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 large
serrated knives —
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 2x4, 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 2x4 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 MemBrain
vapor retarder from CertainTeed. 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 ProClima. 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.
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 Swedish
Platform Framing article 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 beads
up and rolls off the surface of mineral wool. 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.
Fire safety
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.
ABOUTME
Energy Conservation Insulation has over 40 years of combined insulation experience ’&
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