musingsheader image
10 Helpful?

Installing Mineral Wool Insulation Over Exterior Wall Sheathing

Researchers confirm that Roxul panels and furring strips can be installed on walls as a substitute for exterior rigid foam

Posted on Aug 26 2011 by Martin Holladay

A subset of green builders have always been grumpy about foam. Such builders look at rigid foam panels and spray foam as suspect products: they are made from petroleum, laced with mysterious chemicals, and impermeable to vapor flow.

Not all green builders agree with this analysis, however. Here’s the opposing view: most new homes include many products derived from petroleum, including drain pipes, bathtubs, window frames, and the plastic sheathing on Romex wiring — so why focus only on insulation? Moreover (the argument goes), there’s nothing wrong with using petroleum-derived insulation; in fact, converting oil into insulation makes more sense than burning the stuff to drive our cars.

Regardless of whether you are a “foam is evil” builder or a “foam is a useful product” builder, you probably agree that it’s handy to have a range of available products to solve construction quandaries. That’s why it’s good news for builders that mineral wool insulation can be installed on the exterior side of wall sheathing, just like polyisoPolyisocyanurate foam is usually sold with aluminum foil facings. With an R-value of 6 to 6.5 per inch, it is the best insulator and most expensive of the three types of rigid foam. Foil-faced polyisocyanurate is almost impermeable to water vapor; a 1-in.-thick foil-faced board has a permeance of 0.05 perm. While polyisocyanurate was formerly manufactured using HCFCs as blowing agents, U.S. manufacturers have now switched to pentane. Pentane does not damage the earth’s ozone layer, although it may contribute to smog. .

Almost everyone agrees: it’s a good idea to keep wall sheathing warm

The thermal performance of a wall is greatly improved when wall sheathing is protected by an exterior layer of insulation. Exterior wall insulation keeps wood warm, keeps wood dry, reduces the condensation potential, reduces thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. , and probably increases the durability of the wall.

However, some builders worry about the use of rigid foam to keep stud bays warm — not only because “foam is evil,” but because foam does not allow the wall assembly to dry to the exterior. (Although this concern is unwarranted, it exists.) Such builders have often wondered whether rigid mineral wool panels (for example, insulation panels made by Roxul or Thermafiber) could be sandwiched between wall sheathing and vertical furring strips — in other words, whether mineral wool could be substituted for XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation., EPS, or polyiso to insulate the exterior of a wall.

Builders who insist that “a wall has to breathe” will probably prefer mineral wool insulation (which is vapor-permeable) to rigid foam insulation (which has a relatively low vapor permeance).

Mineral wool? What’s that?

Although the term “mineral wool” is sometimes used (especially in Europe) to refer to ordinary fiberglass insulation, the phrase is typically used in the U.S. to refer to rock wool or slag wool insulation. To be clear, in this article I’m talking about rock wool or slag wool products, not fiberglass.

Rock wool insulation is made from basalt, an igneous rock. Slag wool insulation is made from steel-mill slag. (In the U.S., most mineral-wool insulation is slag wool.) Both types of insulation are valued for their resistance to high temperatures.

Mineral wool insulation can be fluffy like fiberglass, and the R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. per inch of fluffy mineral wool is similar to that of fiberglass. Denser types of mineral wool have a higher R-value per inch than fluffy types; if the mineral wool is dense enough (8 lbs. or more per cubic foot), it can be formed into rectangular panels and can be installed like rigid foam. The mineral wool products discussed in this article have R-values ranging from R-4.0 to R-4.3 per inch. (Fiberglass insulation can also be formed into high-density panels, but that’s a topic for another blog. Suffice it to say that such rigid fiberglass panels are not generally available at this time.)

Early experiments with exterior mineral wool

Builders have experimented with the use of mineral wool on the outside of their wall sheathing for many years. Unfortunately, there have been several barriers to their efforts:

  • The ideal product would be very dense; however, many common mineral wool products haven’t been dense enough.
  • The ideal product would have a high R-value; however, most common mineral wool products are rather thin.
  • The ideal product would be readily available in small quantities; however, most types of mineral wool are commercial products that can only be ordered in very large quantities.

In spite of these hurdles, experimenters have bravely forged ahead. Many builders who called up Roxul’s technical help line were told that Roxul Drainboard was the densest mineral-wool product available. (Drainboard has a density of 8 pounds per cubic foot.) One downside of using Drainboard: it isn’t available in a high-R version. The thickest Drainboard product is only 2 3/8 inches thick — an oddball dimension chosen because it provides R-10.

On the GBA website, several builders have posted suggestions about the best techniques for installing vertical furring strips over mineral wool insulation. For example, here are two comments posted by GBA reader Thomas Jefferson:

  • Post #1: “None of the rockwool samples I’ve handled would be dense enough to install strapping on top as you would with rigid foam.”
  • Post #2: “I don’t think it would work to install vertical strapping over any thickness of rockwool without some type of squash block. For a very short span (like 1"-2" thickness) it might be sufficient to put a sleeve of plastic pipe around the screw, which would be simple to do.”

Another GBA reader, Robert Manninen, echoed Jefferson’s recommendations on the use of squash blocks: “If the ‘springy-ness’ of the rockwool board is really objectionable, cut holes in the rockwool board with a hole saw and either use a plug of wood or high load XPS to attach the nailers…”

Although many builders assume that mineral wool products are too springy, some experts have long suspected that complicated squash blocks are unnecessary. According to building scientist John Straube, “We installed vinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate). siding over 3-pound-per-cubic-foot Roxul on a number of walls, including Eric Burnett’s house, about 10 years ago with no issues.”

This year, Straube was able to provide convincing data showing that it is indeed possible to install vertical furring strips over Roxul without any squash blocks. The data came from a Building Science Corporation (BSC) test facility, where Straube and Jonathan Smegal conducted a series of furring-strip tests at the behest of Roxul.

Furring strips over Roxul work well

“We tested a range of higher density Roxul products,” Straube wrote to me in an e-mail. “The 6-pound-per-cubic-foot (pcf) and even the 4 pcf products did amazingly well, but the 8 pcf product was easier to handle for people used to the stiffness of foam and it also worked the best.”

Most of the BSC tests were performed on two thicknesses (1 1/4 inch and 3 inches) of Roxul ComfortBoard IS. “ComfortBoard IS is a new product that hasn’t been launched yet,” Straube told me. “It is essentially Roxul 80 — also sold as RockBoard 80. It has a density of 8 pounds per cubic foot.”

According to the BSC report on the testing, “All of the insulations tested showed very little deflection (less than 0.01 inch or 0.25 mm) at the loads imposed by lap siding (of wood, vinyl, or fiber cement). … Testing with various fastener embedment (in framing, in OSB, or a combination) showed no significant differences at loads less than approximately 20 pounds per square foot cladding weight. … Note that these tests were conducted to simulate some of the worst-case realistic scenarios for deflection (i.e., 24-inch-o.c. strapping, and 16 inch vertical spacing between screws). This is equivalent to only 4 fasteners per square meter. Also, the screws used were the lowest quality, length and thickness that would be reasonable for this application. Using more screws, at a closer spacing would likely decrease deflection, but more testing is required to determine the amount that the deflection could be decreased.”

The researchers unambiguously endorse the use of vertical furring over 8 pcf Roxul — even thick layers of Roxul — for most types of residential siding. The report notes, “A range of target R-values can be easily reached as similar details can be used for the design of walls that have 2, 3, 4 or even 6 inches of insulation.”

What about stucco?

At first blush, it would appear that even Portland-cement-based stucco — a relatively heavy type of siding — could be installed on furring strips screwed through Roxul. The BSC report noted, “None of the walls tested in this study exceeded 0.01 inch of deflection at 12 psf (384 lbs total), approximately equal to the typical weight of ¾-inch stucco cladding.”

However, the BSC researchers have decided that further testing is necessary before they recommend the use of stucco over Roxul: “To confirm the very favorable results achieved, it is recommended that field testing, in a test facility or on a jobsite, should be conducted to assess the potential for stucco or adhered veneer cracking over a 1-2 year test period before proceeding with wider deployment. Long-term deflection testing in a laboratory setting may give a better indication of performance with sustained loading that simulates cladding, but field testing is preferred.”

Straube elaborated, “The Building Science Corporation won’t recommend stucco over Roxul — we don’t know enough yet. We need to install some and see if it cracks. If you are talking about synthetic stucco, that’s no problem. We have been recommending and installing synthetic stucco over Roxul for 20-some years. Every major EIFS manufacturer sells a Roxul system for buildings that need the improved fire performance.”

Lighter sidings, however, shouldn’t be a problem. “Vinyl siding and fiber-cement are obviously lighter than stucco,” Straube told me. “Small movements are never going to be a problem in those systems. But with stucco, small movements can produce cracks.”

For more information on Straube's recommended details for installing mineral wool insulation on the exterior side of wall sheathing, see Insulating Sheathing for Residential Construction.

Many builders are unfamiliar with mineral wool

Of course, conservative builders are likely to be wary of product they aren’t familiar with. If builders get a chance to handle dense mineral wool, however, they are usually impressed. The Roxul products in question — those with a density of 8 pcf — have a compressive strength of between 4 and 5 pounds per square inch. “They sell it for roof board, and people walk on it,” says Straube.

Roxul is unharmed by water. “It’s coated with an oil-based product,” says Straube. “It’s hydrophobic. Water beads up on it, and it doesn’t absorb water. If you put it in a tub of water and put bricks on top of it to keep it underwater, it will pick up water. But as soon as you take it out of the water, the moisture evaporates in no time flat. It dries out like crazy the moment you have a sunny day.”

What about insects? “Insects don’t eat rocks,” says Straube, “so I’m not too concerned about insects.”

Damp sheathing and drying to the exterior

Builders who install rigid foam on the exterior of walls have learned that thin foam is riskier than thick foam. (Exterior rigid foam should always be thick enough to keep the wall sheathing above the dew point in winter; for more information, see Calculating the Minimum Thickness of Rigid Foam Sheathing.) Does the same concern arise when using Roxul?

I posed the question to Dr. Straube, who said, “If air is presumed to leak through the inner stud cavity, it can reach the sheathing, where it cools down and dumps its moisture load. But thinner Roxul insulation is not as risky as thin rigid foam, because when the OSB is wet, it can dry much more quickly to the outside through Roxul than through foam. But if you switch the OSB to fiberboard, it would truly change the story, because fiberboard can dry out at a rate that is 10 times faster than OSB.”

Builders excited about building walls that dry in both directions should be cautious, however, because vapor-open walls can perform poorly during the summer if the building is air-conditioned. “If you have a very vapor-open assembly, it’s good and it’s also bad,” Straube said. “In the summer, when rain wets wood siding, these vapor-open walls are the type of walls that taught us about inward solar vapor drive. But knowing that inward vapor drive is a risk, we should make sure there is a gap between the siding and the sheathing, and we should ventilate the gap and back-prime the siding, so water isn’t stored in the siding. Those steps are probably enough to remove the summertime risk from vapor-open assemblies.”

What about housewrap? “I wouldn’t put the housewrap on top of the Roxul — I would put it behind the Roxul,” said Straube. “For one thing, installing the housewrap between the Roxul and the furring strips would be a pain. Anyway, one of the reasons for choosing high-density rockwool is that it is dense enough to resist wind-washing.”

OK — I want some

According to Jonathan Ram, technical product specialist at Roxul, “We hope to launch ComfortBoard IS this fall — we’re aiming for early October.” The product has the following specifications:

  • Density: 8 pounds per cubic foot
  • Available thicknesses: 1 1/2 inch and 3 inches
  • Available panel sizes: 3 ft. x 4 ft. and 4 ft. x 6 ft.

Ram said that the price of ComfortBoard IS “will be around the same ballpark as rigid foam.”

Ram is aware that builders looking for Roxul products have often faced problems with availability. “We’re trying to build up our distribution system,” Ram told me. “We’re working on it. We hope you will be able to buy the product in smaller amounts — not just entire truckloads.”

According to Straube, Roxul is serious about improving the availability of their insulation products. “Three years ago Roxul built a new factory,” Straube said. “Until recently, Roxul didn’t have enough capacity to meet the demand. It was embarrassing for us, because we told people about mineral wool, and they would say, ‘That sounds perfect.’ Then when they tried to buy some, they found out it was hard to get a hold of. But a new plant came online about 2 or 3 years ago, allowing Roxul to triple their output.”

If Roxul is able to develop an efficient distribution system for ComfortBoard IS — and especially if Roxul begins distributing a 6-inch-thick version of the product — I predict that many builders will be eager to try it.

Last week’s blog: “A Bold Attempt to Slay R-Value.”

Tags: , , , , ,

Image Credits:

  1. Building Science Corporation

Aug 26, 2011 7:10 AM ET

Edited Aug 26, 2011 8:46 AM ET.

I have some Drainboard
by Dan Kolbert

If anyone in northern New England is interested in trying it, I have some surplus Drainboard from our current job in southern Maine I can sell at a reduced price. dan at kolbertbuilding dotcom

Aug 26, 2011 9:56 AM ET

And per Roxul's MSDSs, panels
by mike eliason

And per Roxul's MSDSs, panels do contain cured urea extended phenolic formaldehyde binder (1-6%).

Any word on pending ComfortBoard IS MSDS data?

Aug 26, 2011 11:40 AM ET

Fireproof sheathing
by K Willets

This seems like a good exterior insulation for wildfire areas.

Aug 26, 2011 11:54 AM ET

by Philipp Gross

Being from Europe and now working in the US on super insulated houses I am very exited about this. I knew it works because it is done in Europe all the time. Question I have are:
Is it feasible to instal 2 or even 3 layers of 3" Roxul ComfortBoard IS the same way until the thicker versions are available?
How about fastening them to concrete (cast in place new construction) or brick (retrofit)?

Aug 26, 2011 2:12 PM ET

by Ian Brown

No need to mind the gap, eh? If you use mineral wool as your rigid insulation, you wouldn’t need a draining house wrap between it and the OSB, would you? Any water that makes it to the WRB would just drain through the mineral wool, right? Any reason to prefer one kind of WRB over another in this particular context?

Aug 26, 2011 2:28 PM ET

by Ian Brown

Thermafiber also has 8 pcf board products, Versaboard 80 and Firespan 90. They should work as well for this, shouldn't they?

Aug 26, 2011 3:14 PM ET

Response to Philipp Gross
by Martin Holladay

Q. "Is it feasible to install 2 or even 3 layers of 3 inches of Roxul ComfortBoard IS the same way until the thicker versions are available?"

A. Certainly it's possible to install 2 layers of 3 inches, because the BSC researchers specifically advise using this system for up to 6 inches of rock wool. Beyond 6 inches, I'm not sure.

Q. "How about fastening them to concrete (cast in place new construction) or brick (retrofit)?"

A. Sure -- people install furring strips onto concrete or brick all the time. It's a pain, but the fasteners exist. Most people use Tapcon fasteners.

Aug 26, 2011 3:17 PM ET

Response to Ian Brown
by Martin Holladay

Yes, mineral wool insulation drains well.

Opinions on the best WRB vary widely. My own opinion: installation details matter more than the material chosen. As usual, don't forget your air barrier (for example, taped plywood or taped OSB).

Aug 26, 2011 3:18 PM ET

Thermafiber versus Roxul
by Martin Holladay

I don't know of any reason why 8 pound per cubic foot Thermafiber wouldn't work just as well as Roxul.

Aug 26, 2011 4:28 PM ET

by harold welch

Is there a link to this study? Any reason this couldn't also be rigid fiberglass?

Aug 27, 2011 3:36 AM ET

by bob coleman

while not using nearly as much oil i guess, not impressed they spray the product with it
i don't think the current forms of board are sprayed with oil except maybe the drainage board?

also note the weight of the insulation changes quite a bit, from 1 - 2 lbs per cubic foot to 8 lbs

Aug 27, 2011 5:52 AM ET

Resonse to Brett Welch
by Martin Holladay

I think there is no reason why fiberglass manufacturers couldn't come up with an 8-pcf density panel that would work just as well as the Roxul panels under discussion here.

Aug 27, 2011 5:54 AM ET

Edited Aug 27, 2011 6:01 AM ET.

Response to Bob Coleman
by Martin Holladay

I'm pretty sure that Roxul doesn't "spray" their products with oil. Once this hurricane passes through, and everyone is back in their offices, I'll get in touch with Roxul and get a better description of why their product is hydrophobic.

I'm not sure what you meant when you wrote, "The weight of the insulation changes quite a bit, from 1 - 2 lbs per cubic foot to 8 lbs." It sounds like you are talking about density, not weight. And I don't think you mean that the density "changes," unless you are referring to differences in density between dry mineral wool and wet mineral wool. I think you are saying that the product is available in many different densities. If that's what you're saying, you're right. Choose the density that works best for the application under consideration.

Aug 27, 2011 3:40 PM ET

Thermafiber VersaBoard 80
by TJ Elder

This BSC test proves the value of building and testing full scale mockups. I want to add that the type or amount of resin used in different rockwool products affects the compressibility, as well as moisture absorption. The 8# sample that I suggested would still need squash blocks was VersaBoard 80, seen in the photo. It is much more compliant than EPS foam, for example.

I would suggest that the type of screws used to affix furring strips would also affect bearing capacity on the furring, because a #10 screw will bend before a #12 or a 1/4" SIP screw. But this is the type of thing that needs BSC test methods to explore.


Aug 27, 2011 4:40 PM ET

Edited Aug 27, 2011 4:42 PM ET.

I like this much better than foam or fiberglass
by Gregory La Vardera

FIberglass manufacturers could make similar products to Roxul, but they don't want to. At least that's what I was told when I spoke to them. They feel it won't sell. Certainteed makes R24 5.5" batts and sells them in Canada. They won't bring them here. I suppose if they did see the money in it then perhaps they could make the 8pcf panels, but there is no guarantee that it would behave as Mineral Wool does when its wet. We all know fiberglass batts soak up water like a sponge. Water rolls off mineral wool.

The oil coating is part of the process of manufacturing the fibers - its not something simply sprayed on the outside of a finished batt. Its something that is present all through the product. It does not just repel water at the surface, but all the way through the material. This is very different behavior than glass batts. I suspect they are using a mineral oil which is a by-product of gasoline production and other refining, FWIW its not a primary consumption of oil, and not at nearly the rate that petroleum goes into making foam.

I've asked Roxul directly about the formaldehyde content. I was told it was very small and disperses before the product reaches consumers. There would be no significant off-gassing of the binder in place. I'd like to see info otherwise to be sure.

Its great that Roxul has asked BSC to test this, because now perhaps green building enthusiasts will believe what has been done in Europe for years.

I think this is the way to add insulation to the exterior of a wall system. I've recommended this in the Better and Best versions of my USA New Wall. I also agree with Martins recommendation for the location of the house-wrap or building paper(non AC house) - it should be behind the mineral wool, which if you are keen on the Zip sheathing is where it will be as well.

Aug 28, 2011 9:11 AM ET

Response to Thomas Jefferson
by Martin Holladay

I think we can all agree that XPS has greater compressive strength than 8 pcf mineral wool. The relevant question for this application, however, is simply, "Is the compressive strength adequate for installation under furring strips without squash blocks?" The answer appears to be, "Yes, it's adequate" -- at least for siding with a weight per square foot that is less than stucco.

Even if you can squish it with the palm of your hand, it may still have enough compressive strength for the application we're discussing.

Aug 28, 2011 3:48 PM ET

Edited Aug 28, 2011 3:53 PM ET.

For Phillip Gross
by albert rooks

Here is an interesting detail that I've been working on. The design goal is to adapt the "extra thick" exterior mineral wool applications used on CLT's or other massive assemblies and fit them on Standard US 2x6 walls. I'm with you in that I'd like to see a quick easy way to apply thick amounts onto a standard frame. The problem that I see is that the 1.5" face of a 2x6 is really to thin to reliably hit with a long fastener.

The wall design in the attached pictures is based on Roxul Fab Rock HT. It's a 10" thick commercial product from Roxul that is used to fabricate into shapes for plumbing systems. It's the 6.5 lb density which is just enough to not compress provided that you use the right fastener. (More on fasteners in a later post.) With 10" of Fab Rock HT at about R 4.1/inch it yields R 41 in a single layer. Placed on top of a standard 2x6 R 21 wall and you do the math… :)

As you can see, I took the FabRock and created a special profile on top and bottom, and then created a shop built I beam to join them. The goal is to solve three issues: 1, lack of reliability of long screws hitting the framing O.C.
2, Providing a ledger that to carry the weight of the Fab Rock. and 3, Keep the outer flange of the I beam 2" back from the outer surface to avoid thermal bridging.

The I beam is nailed or screwed the framing at the wall flange. The Rainscreen strapping is screwed to the outer flange. Since the rockwool at this point is only 2" thick, all that is needed is a 5" screw. The load bearing wall can be advanced framed at 24" OC but the rainscreen straps can be applied at 16"OC. Both the rain screen and the inner flange fasteners are set into framing lumber, not sheeting. Good solid connections.

I like running the I beams horizontally. Others have looked at it and thought of running them vertically. Horizontally the process seems more straight forward because I think of rainscreen strapping as vertical and I don't want to think of applying a cross-batten.

I think the design has some pretty good possibilities, I started down this road because I feel that mineral wool is superior to exterior XPS, EPS foam for many reasons. I thought that a simple installation method might be one solution to the question of a super insulated wall that doesn't suffer from low, or near zero permeable cold sheathing. With all of the wood framing behind 10" of pretty indestructible rock wool, a standard frame wall with taped seams would last in the 100's of years.

If anyone in the western US would like to explore it further on a project, I'd be more than happy to setup and cut the joinery in the FAB Rock and make the I beams (or not). That said, please feel free to take this idea and improve it on your own too. It appears to me that rock wool can make a really good exterior insulation. Much better than any of the styrenes from the insect standpoint alone.

Btw... The upload changed the orientation of the pics and they show sideways instead of upright. I don't know how to change them... Sorry!

Full Veiw.JPG Joint veiw.JPG RS Strap.JPG

Aug 28, 2011 3:59 PM ET

Response to Martin
by TJ Elder

No disagreement here, I plan to be among the first to demonstrate the wall assembly in the BSC illustration. After considering numerous iterations on rockwool with squash blocks, none of those wall systems seemed user friendly enough to actually use. A rockwool panel just dense enough to install the same as rigid foam would have to be the most viable way for someone to substitute wool for foam.

Aug 28, 2011 4:42 PM ET

albert, difficulty of
by mike eliason


difficulty of cutting the mineral wool around the 'i-joist'? the europeans have a thermally broken 'stud' (called a box-traeger) that is 2 pieces of lumber with wood fiber insulation between them. like the r-stud, only more effective/environmentally friendlier. as i recall, they're wider than a 2x (maybe closer to a 3x).

compared to what you're showing, potentially better r-values and easier to fabricate...
detail here:
pic similar to box-traeger here:

Aug 28, 2011 4:50 PM ET

Reply to Mike
by albert rooks

Well Yes, the box studs are of course great. but the idea here is to hang vast amounts of rockwool onto a standard frame for those that want to approach super insulation from regular assemblies. If you drop the standard framing of course now there are many ways!

The Fab Rock arrives at the site with joinery. It's not site cut. This is not a product I'm marketing in this case. Just an idea that I like. :)

Aug 28, 2011 5:32 PM ET

Edited Aug 28, 2011 5:38 PM ET.

Change up the fastener!
by albert rooks

The timing on this post is funny. I have just been working on adding new fasteners aimed exactly at the compression issues of exterior insulation boards. Below are pictures of a small trial that I did in the shop today. All that it illustrates is that if you use a "broken thread" fastener, you can apply good compression to the strap, sink the counter sunk head, all without creating distortion aross the plane of the wall.

Most of the screws that are available to screw down the strapping are a 3/4 thread. They are designed to be a clamp: as you set the screw, the head is forcing the substrate towards the material that the thread is in. The more torque on the head the more the compression.

In the attached pictures is the Heco Topix Therm. It's a German made screw made to apply exterior insulation. The way the broken thread works is: You set the screw as any other. There are two sets of synchronized threads. One at the tip and one at the head. When the head has reached the range of the strap, the second thread is in the strap. As you turn, the top set of threads are "setting" the C/S Head into the strap rather than the head being "set" by compression from the threads at the tip.

The result is that the counter sunk head is set flush to the rainscreen face without enough compression to the insulation to cause distortion and the "wavy wall syndrome". The diameters that I'm looking at is 6 & 8mm which are about 1/4" 5/16". For adding 2 - 3" of exterior insulation the same can be done with a full thread rather than a broken thread.

I should have these in the online store in a week or two. If you'd like to experiment and see if they really work, feel free to drop me a line at I'd be happy to send out a few samples when the stock arrives.

Btw… If you want to look for good fasteners closer to home, try a full thread in the right length. A 3/4 thread will give you trouble...

Hope the pictures don't turn sideways again!

Heco Topix Therm GBA.jpg Heco Set GBA.jpg Heco no distrotion GBA.jpg

Aug 28, 2011 7:19 PM ET

Edited Aug 28, 2011 7:41 PM ET.

by John Brooks

Albert, I think you have made a lot of very good comments here at GBA

I wonder what happens when you add windows to the wall.
I think windows are the big pain with all of the outside insulation stratagems.

Aug 28, 2011 7:47 PM ET

Edited Aug 28, 2011 7:49 PM ET.

Reply to John
by albert rooks

Yes... I agree. The I beam orientation is un-natural. But it does receive the strapping easily. Stop the I beam at the window R/O. Put the windows in a box. I'd guess they'd be "innie windows" in a box with that sort of HP wall anyway.

There are all sorts of problems: Hanging porch overhangs and anything that requires blocking.

Good point about the picture orientation telling me something. It's probably mostly about my own internal orientation!... or lack there of :)

Gee John, I didn't say that it was a good idea. Just that it showed promise. :) You have to admit that it makes one heck of a blanket. It would certainly keep out the cold for those 15 minutes in Texas when you get below 60 deg.


Oh geez! Now you edit your comment and my reply and comment to the un-natural beam orientation has no point of reference. Uugh!

Aug 28, 2011 7:54 PM ET

Albert.....sorry for editing
by John Brooks

I had second thoughts about the I-beam comment and was trying to edit before you saw it.

Aug 28, 2011 8:14 PM ET

John, No worries.
by albert rooks

They had me laughing.

Aug 29, 2011 9:24 PM ET

Question and Report
by Marc Labrie

Albert, in post 17 you refer to 10" of Fab Rock HT. Meanwhile Roxul website mention 1" to 4" in 1/2" increment,+ht . Is it a new product not yet on the market? I love the idea and envision using it on our future house.

On the subject, here's a document from Roxul on "Cavity Rock" excellent water repellency

Aug 31, 2011 4:55 PM ET

A few questions
by Justin Fink

Great post, and like the others, I'm excited to see mineral wool put to use in applications beyond residential draft-stopping and commercial assemblies. I had a few questions, though. Maybe you have some answers, or maybe others do:

1 - What is the surface of the 8pcf panels like? I understand that it has good compressive strength, but is it smooth and tight? The only rigid mineral wool I've used is nearer to the drainboard variety, and that was more like a really thick wool carpet pad.

2 - Relating to my first question, I'm wondering if seam tape and flashing tape are possibilities with this type of board? I know you and others are saying that having the air barrier and wrb behind the mineral wool is the way to go, but I'm curious if that's because of problems surrounding the integration of this material with common seam tapes, spot-filling with expanding foam, and tying in self-adhered flashings, or because of it's other characteristics?

3 - You mentioned the R-value of drainboard being 10 (@2 3/8" thickness), but unless I missed it, you didn't mention the thermal resistance of this much denser variety coming to market. If the price is "in the ballpark" of rigid foam (which rigid foam, by the way...because they are very different in pricing), is the R-value as well?

4 - Do you have any information from Roxul on other characteristics of the panels, such as edges being square vs. T&G, for instance?

Justin Fink

Aug 31, 2011 5:13 PM ET

R value performance when wool insulation is wet?
by katherine nichols

I'm wondering if the same behavior that applies to fiberglass insulation, also applies to mineral wool:

When water vapor or moisture is present does mineral wool's 'effective' R value go down?

Am I correct in assuming the building science experts would additionally reccommend insulating the stud cavity?

Aug 31, 2011 5:31 PM ET

This might be the solution
by Elden Lindamood

I have long considered adding insulating sheathing to my house, but it is only about 3 feet from my neighbor's house, and I was squeemish about having to cover foam board with gypsum sheathing as an ignition barrier to make my building code official happy. A 3" thick, non-combustable insulating sheathing product might just be the ace in the hole. Thanks!

Aug 31, 2011 5:47 PM ET

Response to Justin Fink
by Martin Holladay

Q. "What is the surface of the 8 pcf panels like?"

A. About the same as Roxul's other 8 pcf products, including Drainboard and RockBoard 80. It's not really a new product -- just a new thickness, a new marketing campaign, and a new distribution network.

Q. " I understand that it has good compressive strength, but is it smooth and tight?"

A. No. Don't expect it to look like XPS.

Q. "The only rigid mineral wool I've used is nearer to the drainboard variety, and that was more like a really thick wool carpet pad."

A. Yup -- you got it.

Q. "Relating to my first question, I'm wondering if seam tape and flashing tape are possibilities with this type of board?"

A. No.

Q. " I know you and others are saying that having the air barrier and WRB behind the mineral wool is the way to go, but I'm curious if that's because of problems surrounding the integration of this material with common seam tapes, spot-filling with expanding foam, and tying in self-adhered flashings, or because of its other characteristics?"

A. It's not because of problems -- just because of the characteristics of the material. It's not the same as XPS or polyiso.

Q. "You mentioned the R-value of Drainboard being 10 (@2 3/8" thickness), but unless I missed it, you didn't mention the thermal resistance of this much denser variety coming to market."

A. It's the same stuff -- it isn't different. The R-value is about R-4.1 or R-4.2 per inch.

Q. "If the price is 'in the ballpark' of rigid foam (which rigid foam, by the way...because they are very different in pricing), is the R-value as well?"

A. Good point about the price ... but I couldn't get Roxul to be any more specific. As I said, the R-value is about R-4.1 or R-4.2 per inch.

Q. "Do you have any information from Roxul on other characteristics of the panels, such as edges being square vs. T&G, for instance?"

A. As far as I know, this is a square-edge product.

Aug 31, 2011 5:49 PM ET

Reply to Marc
by albert rooks


Sorry for the delay. I've been traveling for a couple of days. The FAB ROCK HT or LT in 10" is a stock item for the dealers that I worked with. It what is used for making MW insulation layers around high temp plumbing in both commercial buildings and shipping. They buy it in 10" thick and cut it down into odd shapes just like you see for insulating galvanized or copper (or any) plumbing in foam tubes. You want to start with a rep who can steer you to the right distribution channel.

The max density of the 10" material is 6.5lbs. I think it's fine.

Enough info?

Aug 31, 2011 5:54 PM ET

Double response to Justin
by albert rooks

Q. "Relating to my first question, I'm wondering if seam tape and flashing tape are possibilities with this type of board?"
A. No.

Justin, Tape will not stick to MW. ( I can attest to that because I'm a Tape importer). You wont need it. It's really dense and you can just stuff fall material into cracks.

Btw... After 3 inches, I believe the WRB is redundant. That said, I would not omit one. I believe it's cheap insurance. That comment is not science, just an opinion.

Aug 31, 2011 7:45 PM ET

by Sherrie VandePutte

What about insects? “Insects don’t eat rocks,” says Straube, “so I’m not too concerned about insects.”

I didn't think insects ate EPS either, but that didn't stop them from hollowing out areas in the foam to nest in.

Aug 31, 2011 8:15 PM ET

Thanks Albert
by Marc Labrie

Much appreciated info!

That is why I follow this site. Way better information than what's available from manufacturers website,,,.

Aug 31, 2011 9:01 PM ET

Edited Aug 31, 2011 9:02 PM ET.

Response to Katherine Nichols
by Martin Holladay

Q. "I'm wondering if the same behavior that applies to fiberglass insulation, also applies to mineral wool: When water vapor or moisture is present does mineral wool's 'effective' R value go down?"

A. Here is information from a Roxul technical document: "The R-value of all insulation diminishes when moisture or water is present within the thermal insulation body. With the moisture data provided above (ASTM, Fraunhofer Institute and Modeling), the water content within the mineral wool insulation is minimal. Based on testing from the Fraunhofer Institute, the water content based on the second scenario (worst case) would have to be increased by 25 times its maximum level to affect the mineral wool’s R value. Even up to elevated relative humidity levels of 100%, the Fraunhofer institute has shown that Roxul CavityRock will only store 0.5kg/m3 of water."
Here's the link to the document:

Q. "Am I correct in assuming the building science experts would additionally recommend insulating the stud cavity?"

A. Yes.

Sep 1, 2011 11:20 AM ET

Stresskin Roxul Panels
by Paul Freeman

Has anyone tried building a stresskin panel with Roxul? I suspect it may not make a good structural panel but it might be a good curtainwall application. Or used like a "nailbase" panel (one layer of osb attached to foam sheets).

Sep 1, 2011 11:22 AM ET

Response to Paul Freeman
by Martin Holladay

I don't know how you'd solve the thermal bridging problem -- such panels would end up with a lot of studs. You can't just glue the OSB to mineral wool like you can with EPS.

Sep 1, 2011 1:26 PM ET

Questions about MW and the "Rooks Wall" prototype
by Greg Winger

Can anyone comment on the approx cost of rockwool 'board' products?

Albert- Thanks for sharing your prototype idea. Very interesting. What does the FabRock cost per square foot, and as you are using 10" thick panels which are not listed on their website, can you confirm what the panel dimensions are? how about i-beam spacing?
Too bad I didn't read this posting a few days ago, I was just in Tumwater working for a few days (from Seattle) and would have liked to stop by and see the prototype in person.

Sep 3, 2011 2:18 AM ET

Edited Sep 3, 2011 2:33 AM ET.

Reply to Greg Winger
by albert rooks


You'd be welcome to stop by anytime. The "stockwool" prototype wall is in the shop and it's fun stuff to talk about. I think that developing numerous methods of building super insulated envelops that are based on, or start with our basic American wall assemblies is the challenge of the next decade. It's both interesting and entertaining to work on. Sadly we can only do it as we find the time.

The Fab Rock HT is available in the standard sizes of size of 24" x 48" or 1M x 48". Order enough and that can change to suit your purpose. In the Pacific Northwest, I like to work with Paragon Pacific. I've received costing for Fab Rock HT at $0.50 per board foot. That is just the panel. no joinery.

I placed the I Beam spacing at 24" OC. 24" just appeared to be an easy number to work with.

I ran a pallet of this design (about 10 "bricks" 24"x48") and it's ready to build around a corner to see how we do. Part of the question is how you transition into corners and odd angles. I'm going with miters. To cut them, Imaging a really big old fashioned mitre box. The old hand saw type, but scaled up for this material. All you need is the small set tooth like you cut metal with. I was thinking of just cutting down a large band saw blade and putting a hard back on it. The above are for site work. In the shop one of the best tools I've seen is a rope saw. It's a really big band saw that uses an abrasive wire rather than a blade.

The mitre approach also allows the I beams to go around corners with a simple site cut joint. That way the joinery on the brick that "fits" the I beam just follows the mires... A simple solution.

I knew that this beam was not going to be the final design. It needs a little finessing. The updated design will have a smaller beam face on both the front and back. The current beam requires a 1.5" x 1.75" deep dado on the Fab Rock. That front 2" of remaining material is too unsupported and could damage in handling.

Sep 3, 2011 2:41 AM ET

Reply to Sherrie VandePutte
by albert rooks


My impression is that the insects just don't like the environment of being inside of high density MW. It literally is rock. While they can cut (or chew) their way through the foam boards, I think they would have a hard time doing the same with MW. It would be nice to hear from some of the commercial builders on this. They are the ones with long term MW experience.

Sep 3, 2011 11:39 AM ET

albert, you might be
by mike eliason


you might be overthinking the TJI/mineral wool passivhaus wall. the assemblies i've seen (prefab panels) continue the air barrier of one panel to the full length of the panel, so there is no additional work for mitering. this is a roof/wall connection that is similar (diagrammatically) but i can draw up what i'm talking about next time i see you if this is too confusing...

Sep 4, 2011 1:42 AM ET

Mitre vs. butt joint.
by albert rooks


Perhaps over complicating. Hopefully at some point reducing it down to be effective and simple...

The drawing you posted shows a butt joint. That would be simplest. On review this design will butt join just fine. Anything other than a 90 degree corner will be easier if the MW "brick" and therefore, the TJI is mitered. That's really not too big of an issue.

When it comes to the air barrier, I don't think that we are on the same page. The TJI's are all applied to the face of a standard 2x6 code wall. That 2x6 wall (Ply or OSB) is taped (with SIGA) and is both the air barrier and vapor control layer. All of the MW is outboard of that taped wall. The mitering is only regarding the MW and TJI's going around any corners. The air and vapor is continuos and intact below the MW.

Sep 4, 2011 6:38 PM ET

Rockwool Durock in the UK
by Joe Ayres

Similar to Roxul I am looking at Rockwool Durock 50mm thick in 140kg m3 density size 1200mm x1000mm to fix onto exterior OSB with a housewrap over. Then 25mm counter batterns and horizontal wood cladding as the exterior finish.
145mm x 50mm studs at 600centres with mineral wool between, and an internal vapour barrier with 25mm service cavity covered with 12.5mm plasterboard and plaster skim finish is the recommendation here for a wall that can dry to the exterior,and still help reduce thermal bridges.
Could this wall build up be improved or made more efficient?

Sep 5, 2011 4:51 AM ET

Edited Sep 5, 2011 4:53 AM ET.

Reply to Joe Ayres
by albert rooks

"Similar to Roxul I am looking at Rockwool Durock 50mm thick in 140kg m3 density size 1200mm x1000mm to fix onto exterior OSB with a housewrap over. Then 25mm counter batterns and horizontal wood cladding as the exterior finish."

If I have my numbers right, the 140kg m3 density is near the US 8lb ft3.

The layer build up that I read and convert to Imperial is, from the inside out:

12.5mm (1/2") platserboard + 25mm (1") service cavity + Vapor barrier + 145mm x 50mm (5.7" x 2") studs at 600mm (24") centers with stud bays filled with mineral wool + 12mm (1/2") ?? OSB + housewrap + 50mm (2") 140kg m3 (8.5lbs ft3) Rockwool Durock + 25mm (1") strapping + Siding.

Joe... To me that is a really great wall. The attributes that I think are important to protect are: 1, the internal 25mm service cavity over the vapor barrier. 2, the house wrap over the OSB if you stay with 50mm Durock on the exterior.

A word about the vapor barrier: I would check this with local building experts, but my suggestion is to stay away from poiy sheeting. In the UK you have access to great vapor control layers from both SIGA, Pro Clima, and Isover. The vapor control membrane should have some low level of permeability (less than 1 US perm or sd-5m). This membrane is where you carefully make the air barrier.

It looks like a really great wall. if you wanted to improve it, consider 100mm of durock instead of 50mm. I also really like your 25mm exterior strapping. Over here in the US, the strapping ranges from 7mm to 19mm (1/4" to 3/4") which I think is a great start but the air circulation in a 25mm (1") cavity is far superior and should keep the siding in really good shape for a very long time.

just out of curiosity, where in the UK are you?


Sep 5, 2011 5:17 PM ET

Reply to Albert Rooks
by Joe Ayres

South east england 3 miles from the sea. I intended on using the 50mm as considered relatively easier to fix with batterns and 120mm screws. Dupont Tyvek airguard was recommended for VCL though this has a high SD value of 200, but good heat reflection.
In place of cladding on some of the building elevations I would like use a render board carrier something like aquapanel exterior then a mesh and render finish , do you think the 25mm batterns would carry this board OK?
Can you tell also me why it is better to put the house wrap on the osb and not to cover the Rockwool?
Many Thanks Joe.

Sep 6, 2011 4:11 AM ET

Edited Sep 6, 2011 4:12 AM ET.

Response to Joe Ayres
by Martin Holladay

Q. "Why it is better to put the housewrap on the OSB and not to cover the Rockwool?"

A. For me, the most compelling argument is ease of installation. I wouldn't want to try to fasten the housewrap to the wall through the mineral wool with long nails or screws before the furring strips are up; that would be awkward and would require a lot of long fasteners.

Sep 11, 2011 11:33 PM ET

Edited Sep 11, 2011 11:35 PM ET.

WRB Location
by albert rooks

I'd place the WRB over the OSB because it needs the protection. The MW does not need any protection at all. At 50mm, it will be as long-lasting as the rocks on the shore. We can't say the same for the wood. So close to the sea... Buy the best WRB that you can get and seal all the seams with really good tape.

Sounds like a great area, enjoy the project!

Sep 12, 2011 12:41 PM ET

rock wool boards for roof insulation?
by Gregory Caplan

I've only heard people discuss application of these products for exterior wall applications. Is there a non-poly rigid board option for exterior roof insulation?

Sep 12, 2011 1:00 PM ET

Response to Gregory Caplan
by Martin Holladay

Commercial roofers have been using mineral wool panels to insulate low-slope roofs (above the roof sheathing) for years. However, I'm not aware of anyone who has used mineral wool panels on steep-slope roofs.

Sep 15, 2011 1:11 PM ET

Historic Brick
by Jim Chu

My project is insulating a 2nd story, 12" thick, west facing exterior brick wall for a bathroom in the mid Atlantic climate zone. Originally the wall finish was plaster on brick. I also have floor joists in air pockets in the brick wall. I'm moving toward 2x4 wood stud and gypsum walls. I would like to insulate these walls, but the concern is vapor transmission. With historic brick, I need them to dry adequately and I'd prefer not to use spray foam.

It appears that I could use rockwool mounted against the interior face of the brick wall with no moisture concerns other than the wood studs themselves. Any moisture that makes it through the wall will be harmless in the rockwool and can permeate through the drywall or back out through the brick as needed. Similarly, I can pack the joist cavities with rockwool and they should be able to dry out as needed without leaking cold air in the winter.

Any holes in this approach?

[Editor's note: the answer to this question is posted on Page 2.]

Register for a free account and join the conversation

Get a free account and join the conversation!
Become a GBA PRO!