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Musings of an Energy Nerd

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

You don't need squash blocks. Researchers at a Building Science Corporation test facility have shown that vertical furring strips can be screwed through Roxul mineral wool insulation without the use of squash blocks.
Image Credit: Building Science Corporation

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 [no-glossary]sheathing[/no-glossary] 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 polyiso.

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 bridging, 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…

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  1. dankolbert | | #1

    I have some Drainboard
    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

  2. Mike Eliason | | #2

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

    Any word on pending ComfortBoard IS MSDS data?

  3. KHWillets | | #3

    Fireproof sheathing
    This seems like a good exterior insulation for wildfire areas.

  4. user-988403 | | #4

    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)?

  5. cascadian76 | | #5

    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?

  6. cascadian76 | | #6

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

  7. GBA Editor
    Martin Holladay | | #7

    Response to Philipp Gross
    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.

  8. GBA Editor
    Martin Holladay | | #8

    Response to Ian Brown
    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).

  9. GBA Editor
    Martin Holladay | | #9

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

  10. RPWKoiPGMk | | #10

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

  11. user-939142 | | #11

    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

  12. GBA Editor
    Martin Holladay | | #12

    Resonse to Brett Welch
    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.

  13. GBA Editor
    Martin Holladay | | #13

    Response to Bob Coleman
    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.

  14. user-869687 | | #14

    Thermafiber VersaBoard 80
    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.

  15. Gregory La Vardera | | #15

    I like this much better than foam or fiberglass
    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.

  16. GBA Editor
    Martin Holladay | | #16

    Response to Thomas Jefferson
    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.

  17. albertrooks | | #17

    For Phillip Gross
    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!

  18. user-869687 | | #18

    Response to Martin
    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.

  19. Mike Eliason | | #19

    difficulty of


    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:

  20. albertrooks | | #20

    Reply to Mike
    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. :)

  21. albertrooks | | #21

    Change up the fastener!
    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 [email protected]. 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!

  22. homedesign | | #22

    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.

  23. albertrooks | | #23

    Reply to John
    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!

  24. homedesign | | #24

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

  25. albertrooks | | #25

    John, No worries.
    They had me laughing.

  26. propeller | | #26

    Question and Report
    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

  27. JFink | | #27

    A few questions
    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

  28. user-720070 | | #28

    R value performance when wool insulation is wet?
    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?

  29. Elden Lindamood | | #29

    This might be the solution
    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!

  30. GBA Editor
    Martin Holladay | | #30

    Response to Justin Fink
    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.

  31. albertrooks | | #31

    Reply to Marc

    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?

  32. albertrooks | | #32

    Double response to Justin
    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.

  33. CKHyjroWpG | | #33

    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.

  34. propeller | | #34

    Thanks Albert
    Much appreciated info!

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

  35. GBA Editor
    Martin Holladay | | #35

    Response to Katherine Nichols
    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.

  36. Hammerpost | | #36

    Stresskin Roxul Panels
    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).

  37. GBA Editor
    Martin Holladay | | #37

    Response to Paul Freeman
    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.

  38. user-444644 | | #38

    Questions about MW and the "Rooks Wall" prototype
    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.

  39. albertrooks | | #39

    Reply to Greg Winger

    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.

  40. albertrooks | | #40

    Reply to Sherrie VandePutte

    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.

  41. Mike Eliason | | #41

    you might be


    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...

  42. albertrooks | | #42

    Mitre vs. butt joint.

    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.

  43. joe1871uk | | #43

    Rockwool Durock in the UK
    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?

  44. albertrooks | | #44

    Reply to 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."

    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?


  45. joe1871uk | | #45

    Reply to Albert Rooks
    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.

  46. GBA Editor
    Martin Holladay | | #46

    Response to Joe Ayres
    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.

  47. albertrooks | | #47

    WRB Location
    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!

  48. Tekpan | | #48

    rock wool boards for roof insulation?
    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?

  49. GBA Editor
    Martin Holladay | | #49

    Response to Gregory Caplan
    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.

  50. HKdcsSa6PR | | #50

    Historic Brick
    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.]

  51. GBA Editor
    Martin Holladay | | #51

    Response to Jim Chu
    If you are talking about an old building with multi-wythe brick exterior walls, you can't just insulate one room at a time. You have to insulate the whole building, because of thermal transmission through the thick brick wall.

    Ideally you need to insulate the wall on the exterior. Your approach won't work, because it's possible for liquid water to run down the cold brick wall during the winter.

    Here's an article that will give you more information on how to insulate a building like yours: Insulating Old Brick Buildings.

  52. HKdcsSa6PR | | #52

    Hi Martin, I saw that article which is what lead me to the mineral wool solution. Insulating the entire building at this time is not an option as I'm not demoing all the plaster walls. The water issue you raise would, potentially be handled by the mineral wool pressed up against the brick, would it not? The water would simply sit in the mineral wool until it evaporated one direction or the other.

    Furthermore, we can't be talking about a ton of moisture, because the original plaster was in fantastic shape.

  53. GBA Editor
    Martin Holladay | | #53

    Response to Jim Chu
    Frankly, the idea of trying to insulate single room makes little sense, since the heat will continue to flow from the bricks to the outdoors on all sides of the bathroom.

    I don't know how much water would accumulate, but the amount of water will be different from the amount of moisture you see on the plaster. In the center of your bathroom wall, the new mineral wool will make the bricks colder than they used to be. That's why condensation is a concern. If the wall is uninsulated, all the bricks are warm. If some areas are insulated, the insulated bricks are colder.

  54. HKdcsSa6PR | | #54

    Re: martin
    I see your point about the heat gradient in the wall. I was more concerned about the level of comfort in the bathroom, with a cold wall. Won't a lack of any insulation at all make the drywall the new hot/cold boundary? It seems like the only option is to go back up with plaster or do spray foam.

  55. GBA Editor
    Martin Holladay | | #55

    Response to Jim Chu
    If you want to insulate the wall, I think that closed-cell spray foam is the best product to use.

    If you are wondering what makes more sense to cover an uninsulated wall -- plaster or drywall -- either one will work. If the bathroom is heated, either product will stay warm during the winter and will therefore stay dry. However, my personal opinion is that plaster is always nicer than drywall.

  56. user-869687 | | #56

    Compressive strength
    Where the blog gives the compressive strength of 8# Roxul, it should say between 4 and 5 pounds per square inch, not per square foot.

    Also, this might be of interest to some:
    It's a lengthy technical document discussing strength limitations when installing cladding over furring over rigid insulation. The insulation in the tests was mostly EPS, but the principle would be the same for other types: bearing strength depends on insulation thickness and the type and number of fasteners used. Scroll down to p. 52 for a chart that boils it down.

  57. GBA Editor
    Martin Holladay | | #57

    Response to Thomas Jefferson
    Thanks very much for the compressive strength correction. I have corrected the article.

  58. user-1030561 | | #58

    How about this wall with vertical siding?
    I've been looking at how to detail a wall with exulation behind vertical siding, and concluded that it would require a cross-batten approach, i.e. use horiz. battens as nailers for the siding, but then hold these away from the foam with vertical battens to allow drainage. But with the Roxul I wonder if I could omit the vertical battens and allow excess water to drain through the wool. Horizontal battens would still crate an air channel / capillary break.

  59. GBA Editor
    Martin Holladay | | #59

    Response to Jack Barnes
    I'm glad that you are building a house with "exultation." You sound enthusiastic.

    When it comes to attaching furring strips for vertical board siding over mineral wool, drainage is not an issue. There is no need for drainage. The only issue is one of attachment. If you can get an engineer to agree that your attachment plan for horizontal furring strips is adequate, there is no need for vertical furring strips.

  60. rwittman | | #60

    flammability of rainscreened walls
    just thought i would share that a fireman buddy of mine has now seen a half dozen rainscreened homes go up in flames, and he reports that they go "like a rocket" compared to their non-rainscreened counterparts. turns out that rainscreen cavity is a great little chase which all too often has a nice clear shot into your tinder-dry soffitt. So, in light of this, cavityrock is looking better and better, not only as an exterior thermal break, but as firestop between the siding (or strapping) and the structure. I see everyone gravitating towards exterior foam- but the flammability, particularly in conjunction with rainscreen, scares the crap out of me.

    Having said all that, anyone got a good idea how to get a blueskin-type exterior air barrier to stick to the house without using a bunch of contact cement that forever off-gasses to the inside?

  61. user-869687 | | #61

    Response to Jack
    This would be a good question for the Q&A. One thought: if the screws that hold the battens need to be 16" on center and the studs are at 24", then it might make sense to install vertical battens (with screws every 16") and then horizontal counter-battens. Or with horizontal battens only, drive two screws at each stud/batten intersection. As for water draining through the wool underneath horizontal battens, I don't think that would happen because the wool is hydrophobic. But it's also quite vapor permeable and it seems likely that moisture would distribute safely behind the siding even in the absence of a clear vertical cavity. In fact I tend to think using rockwool for out-insulation makes ventilation behind cladding unnecessary, and you could for example install 2x2 battens with 1.5" rockwool batts fitted between, and have no problems.

  62. user-869687 | | #62

    Response to Merrill
    This discussion is getting buried several pages down in the blogs list, so it would be better to put questions in the Q&A where more people will see them and respond.

    I would say if you want a fully adhered air barrier you could look into spray applied latex systems, for example Henry Air Bloc. It's also becoming popular to think of plywood/OSB sheathing as an ideal air barrier with the addition of high performance tape over all the panel joints. I haven't heard any complaints about off-gassing from these tapes.

  63. GBA Editor
    Martin Holladay | | #63

    Response to Merrill
    Q. "Anyone got a good idea how to get a blueskin-type exterior air barrier to stick to the house?"

    A. I'm not sure what type of air barrier you are talking about. Hopefully it's not a peel-and-stick vapor barrier. If you want an air barrier that sticks to the exterior of your house, I agree with Thomas: install a liquid-applied air barrier. Here is more information: Housewrap in a Can: Liquid-Applied WRBs.

  64. rwittman | | #64

    Thomas and Martin
    Thanks very

    Thomas and Martin
    Thanks very much for your replies. I like the liquid wrbs but prefer the peel& stick for it's self-sealing ability around nails. To clarify the envelope i'm talking about, we have no vapor barrier on the inside, gwb on 2x4 studs with roxul, we have plywood sheeting covered with peel and stick (or equivalent), and then we have this lovely cavityrock stuff for a thermal break and a firestop, then strapping (rainscreen) and cladding. I am trying to get the details right here and I am going on this ashrae article by joseph lstiburek as my guide:

    in such a wall, the vapor profile of the wall is that it dries out from the air/vapor barrier and it also dries inwards from the air/vapor barrier, ie the outside dies out and the inside dries in. so we want a class 1 vapor retarder yes? and i got the idea for peel and stick from my years in commercial, where it is common as dirt, and also from your nice article on PERSIST. If I have misunderstood the basic physics here please let me know before I go and build it- we have so many leaky condos and houses up here that it's actually a fairly major crisis, and people are eager to get something that is absolutely bulletproof. peel and stick and exterior foam with rainscreen is actually a fairly common thing in these parts. are we nuts? please advise.

  65. GBA Editor
    Martin Holladay | | #65

    Response to Merrill Wittman
    Q. "Are we nuts?"

    A. No, you are not nuts -- as long as you have most of your R-value (at least 2/3 of the wall assembly's R-value) on the exterior side of the vapor barrier. If your exterior insulation is too thin, and your cavity insulation keeps the plywood cold, you've built a mold-and-rot machine.

  66. rwittman | | #66

    "you've built a


    "you've built a mold-and-rot machine" is probably the most terrifying sentence anyone has ever sent me, context notwithstanding.
    If we only get r10 out of the 2 3/8" cavity rock then the 2/3 rule is going to be a problem, unless we put rigid foam behind it, or switch to the much thicker roxul products instead (maybe that's a good idea?) Or does the 2/3 rule apply only when you have a vapor barrier at the sheeting and moisture cannot expirate to the outside? We live in a pretty mild climate here and the local building science geniuses are telling us that 1" of exterior rigid foam (over a 2x6 batted wall) is enough to ensure that the dew point (almost)never falls withing the wall cavity. Not saying they're right, just saying that's what we get told by the guys with good teeth and clean shoes when we go to their seminars.

    i favor roxul in the cavities for fire resistance. i favor roxul on the exterior for fire resistance and thermal break. i need a rainscreen, and it rains a lot, i mean a lot, out here. how do i put these together in the most robust envelope so that nobody ever uses the phrase "you've built a mold-and-rot machine" at me again...

    Also, I'm wary of endorsing any wall system that will fail when the first renovator gets into it and treats it like a typical wall (ie installs poly and batt insulation while muttering under his breath about shoddy builders).

    If we do exterior insulation, do we need to adhere to 2/3 rule if we have a vapor permeable air barrier at the sheeting?

    For that matter, do you agree with Lstiburek that the ideal vapor profile is outside to out, inside to in, with exterior thermal break and an air/vapor barrier at the sheeting?

    Thanks very much for your responses and of course your awesome blog

  67. GBA Editor
    Martin Holladay | | #67

    Another response to Merrill Wittman
    Here's my article about the minimum required R-values for exterior foam: Calculating the Minimum Thickness of Rigid Foam Sheathing.

    You're in climate zone 4, right? So, for a 2x6 wall, you need at least R-3.75 on the outside of a 2x6 wall.

    However, this chart assumes that you are covering your OSB or plywood with rigid foam, not with peel-and-stick. Using peel-and-stick over wall sheathing is a very aggressive and risky strategy for an ordinary wall, because it doesn't breathe at all. Once you install the peel-and-stick, you're entering another category of construction -- the realm of PERSIST. And most PERSIST builders advise that it's wise to keep your stud bays empty of insulation, so that your sheathing stays warm.

    Remember, any cavity insulation you install has the effect of keeping your plywood or OSB cold. Cold is bad -- warm is good.

    Which brings us back to the original question: Why are you installing peel-and-stick?

  68. rwittman | | #68

    Good news is we

    Good news is we haven't done it yet. But to answer your question, it's appealing because you get air/vapor and wrb in one, and it's self sealing around nails. I was impressed with listiburek's logic and have been trying to tweak his "perfect wall" to suit local needs. He puts an airbarrier/vapor retarder (not barrier) at the sheeting. Out dries out and in dries in. So i am beginning to get that, even though the vapor profile of the wall is out to out and in to in, we still want vapor permeability at the sheeting, but i don't know why.

    Why do we need vapor permeability at the sheeting if the sheeting is always above the dew point? Is it just a matter of not begging for trouble if you don't have to?

  69. user-1080227 | | #69

    Roxul rockboard 105
    I'm currently working on a project (40,000 SF) that utilizes a "vapor open" air barrier, 4" Roxul Rockboard 105, G-90 Perforated 18 Gauge hat channel, with a pure zinc shingle facade.
    This assembly is a "vapor open" rain screen design. The stud cavity is 8" (steel studs)

  70. GBA Editor
    Martin Holladay | | #70

    Response to Peter Reed
    I hope that you aren't installing the insulation between the steel studs. If you are, the thermal bridging through the steel will defeat the insulation, and your R-value will be very low.

    Even if you install the Roxul as a continuous layer over the studs to stop thermal bridging, you're only getting an R-value of R-16.4. That's not much. What's your climate?

    Finally, I hope you are building your wall with attention to airtightness. You'll need to seal all your air leaks to get good thermal performance.

  71. user-1080227 | | #71

    Roxul Rockboard 105
    8" Semi Rigid between studs, 4" monolithic high density (105) exterior. The Vaproshield SA is a self adhering air barrier. The exterior 105 breaks the thermal bridging.

  72. uXsVjCwrDJ | | #72

    Another Metal Stud Question
    Hi Martin - Similar to Peter, I am working on a project that requires metal studs (in NYC). I came across some research from NAHB showing that 2" of exterior foam dramatically increases the actual R-value of a wall with steel studs. In that vein I am thinking of using 3" of exterior rock wool with 5.5" interior rock wool in a 2x6 bay. I would also be making a well air-sealed and vapor open wall using some fancy European products like ProClima's Intello on interior and Solitex on exterior (over OSB or plywood) or SIGA's equivalent products. Any advice on this approach? Would you rather see a thicker layer of rock wool on the exterior or the same thickness on the exterior but a double stud wall with a 1" gap in the middle? Would you worry about any areas where careless siders screw the furring strip directly into the metal stud and the resulting thermal bridge, especially at tricky details around windows? Last question is would you see a wall that used Certainteeds MemBrain material on the interior with Zip-system for sheathing as being a materially inferior wall to one using those fancy and expensive European materials?

  73. GBA Editor
    Martin Holladay | | #73

    Response to Mark Hutchinson
    Q. "Any advice on this approach?"

    A. I think that insulating between steel studs is a waste of insulation. All of your insulation should be on the exterior of your stud wall. Your current design has only 3 inches of rock wool on the exterior of the wall. That's just R-11 or so. Not much.

    Q. " Would you rather see a thicker layer of rock wool on the exterior or the same thickness on the exterior but a double stud wall with a 1" gap in the middle?"

    A. Either approach would be an improvement -- especially if the second stud wall had wood studs instead of steel studs, and especially if you could install 1 inch of rigid foam or spray foam in that gap between the two walls.

    Q. "Would you worry about any areas where careless siders screw the furring strip directly into the metal stud and the resulting thermal bridge?"

    A. I'm not sure what you mean. Aren't you planning to have 3 inches of rock wool between the metal stud and the furring strips?

    Q. "Would you see a wall that used Certainteed's MemBrain material on the interior with Zip-system for sheathing as being a materially inferior wall to one using those fancy and expensive European materials?"

    A. Hard to say. MemBrain is a perfectly adequate vapor retarder, but I'm not sure if it would make as good an air barrier as some of the European membranes. However, you didn't tell me whether you intended to use the MemBrain as an air barrier.

  74. uXsVjCwrDJ | | #74

    Thanks Martin.
    - I will look

    Thanks Martin.
    - I will look into using wood on the interior stud that's a great idea if code will let us.
    - I was planning to put insulation in the studs based on a Building Science research report (RR 0901 from 2009 by John Straube, page 9) that reviews steel stud walls with R10-15 of exterior insulation and says they reduce thermal bridging enough that cavity insulation would have about 80% of the R-value as it would in a typical wall. I don't care either way, I just want a well insulated wall, but can you address my confusion? Did I misinterpret the paper or is that paper wrong for some reason?
    - My mistake, I assumed that you could just drill into the sheathing and didn't need to drill into the metal stud's to support the exterior insulation and cladding.
    - I was intending to use MemBrain as an air barrier. I will likely go with one of the European membranes in this case now though.

  75. user-1102578 | | #75

    I have a simple squash block
    For when you want to go big depths with ComfortBoardIS or RockBoard80 and need squash blocks I came up with a solution; just drill pvc drain pipe into it using an oil filter wrench on a drill. See this video:

    * one-step process and very fast
    * the middle of the pvc gets filled with rockwool
    * it goes in very tight; no air gaps
    * PVC is R2.4/in

  76. user-954155 | | #76

    preventing wavy siding with full thread screws?
    I was wondering if anyone has field tested Albert Rooks suggestion of using full thread (or broken thread) screws to get consistent screw compression on the strapping and prevent wavy siding. That sounds way simpler than cutting holes in roxul and inserting sections of plastic pipe. One builder did not know about these options and ripped all the roxul off a side due to this issue. His Comfort Board was 1.5 inches thick I believe. I would like to put untreated strapping over comfort board then fiber cement siding.

  77. GBA Editor
    Martin Holladay | | #77

    Response to Patrick Walshe
    If you want to receive more answers to your question, I suggest that you post it on GBA's Q&A page:

  78. 415irwin | | #78

    In the same vein as applying
    In the same vein as applying an adequate amount of foam to exterior sheathing to keep it warm (and dry), should the same guidelines be followed for roxul board? i.e. for zone 7 2x6 wall R-15 is recommended.

  79. GBA Editor
    Martin Holladay | | #79

    Response to Jessie Pratt
    Q. "In the same vein as applying an adequate amount of foam to exterior sheathing to keep it warm (and dry), should the same guidelines be followed for Roxul board?"

    A. Yes.

  80. user-857644 | | #80

    Mineral Wool & Portland Cement Based Stucco
    Is anyone aware of any further research/follow-up reports or field studies regarding the use of rigid Mineral Wool Insulation behind Portland-cement-based Stucco? We use a lot of stucco out here in California and I would prefer to use rigid Mineral Wool to other types of rigid insulation.

    As the article mentions, “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.'

    I know that Siegel and Strain Architects (here in CA) have done some research on the use of mineral wool but I have yet to see any of their findings.


  81. GBA Editor
    Martin Holladay | | #81

    Response to Tyson Dirksen
    Q. "Is anyone aware of any further research/follow-up reports or field studies regarding the use of rigid Mineral Wool Insulation behind Portland-cement-based Stucco?"

    A. I have not yet heard of any new reports or research on this matter.

  82. J_James | | #82

    Maximum Insulation in Thinnest Wall

    I live in downtown Toronto (Zone 6), Canada. I'm building a brand new addition to an existing 1888 brick house, on a narrow 17' wide lot. I'm interested in getting the best thermal performance (R40) within the thinnest assembly (11" wide) so here is my idea:

    From inside to outside -
    5/8" GWB
    2x4 wood stud framing with
    3.25" of high density spray foam (R21)
    1/2" plywood
    air barrier
    5" Roxul (R20)
    1" wood battens or alternately 6" thermally broken z-girts
    metal cladding (I am hoping to use a flat seam metal panel of some sort.)

    Any thoughts would be much appreciated. I want to make certain that I haven't missed any major holes with the thinking. I've been reading this forum for a little while now and am impressed by all the curiosity and wisdom.


  83. GBA Editor
    Martin Holladay | | #83

    Response to Jeanne Ng
    Sounds good to me. You might want to re-post your question on GBA's Q&A forum. That way more GBA readers can see your question and post their opinions.

  84. J_James | | #84

    Maximum Insulation in Thinnest Wall
    Thanks Martin!

    I appreciate it and I will re-post it.

  85. 415irwin | | #85

    The BSC report discusses the
    The BSC report discusses the deflection due to cladding. Do you think that the weight of Roxul comfortboard at 5" thick would be a concern? Our tallest wall is 23 feet high and will have Roxul panels from top to bottom. Any thoughts on needing to support the weight of all this Roxul or do you think that the furring strips would be adequate?

  86. GBA Editor
    Martin Holladay | | #86

    Response to Jessie Pratt
    My instincts tell me that you'll be fine. But my common sense tells me that you should consult an engineer to be sure.

  87. edkrause | | #87

    Detailing bottom of Roxul Comfort board IS
    How are those of you using this product detailing the bottom of the wall where the Roxul is exposed?

    I'll be using 3" Comfort board over 1/2 CDX sheathing, then 3/4" x 3" furring strips, then cedar shingles.

    I noticed on one post aboard a project in BC, they used a perforated galvanized channel to encase the bottom of the IS. Does anyone know where to source a channel like that?

  88. GBA Editor
    Martin Holladay | | #88

    Response to Edward Krause
    I suggest that you post your question on Green Building Advisor's Q&A page. That way, more GBA readers will see your question, and you are more likely to get a response. Here is the link:
    GBA's Q&A page.

  89. user-6867435 | | #89

    Ted Cullen - Mineral Wool
    I agree, when continuous Rockwool insulation takes on moisture it dries out faster than foam. That said, logically; if it dries faster, it must take on moisture faster (from both sides). Therefore, in moist climates Rockwool ci is somewhat damp most of the time. My question is; how much R-value is lost when Rockwool ci is wet? Thank you

  90. GBA Editor
    Martin Holladay | | #90

    Response to Ted Cullen
    First of all, it isn't true that "in moist climates, rockwool is somewhat damp most of the time." Rockwool is a mineral fiber that isn't absorbent. It drains readily. As an analogy, think of rockwool like a stainless-steel scrubbing pad used to wash pots and pans.

    In the presence of moist air, the R-value of mineral wool insulation is not degraded.

  91. Smc1900 | | #91

    Density of MW
    Hi Martin

    In the section titled Mineral Wool What's That? In the last paragraph you talk about density of the board as about R4 per inch. Comfortbatt has the same r4 per inch. Intuition, which in this case is probably wrong, tells me 2 products with the same r value per inch, made of the same material, should have the same qualities, thus the same ridgid structure. Why could I not use r10 comfortbatt as continuous insulation like the board? What makes one different than the other?

  92. GBA Editor
    Martin Holladay | | #92

    Response to Steve M
    Q. "In the last paragraph, you talk about density of the board as about R-4 per inch."

    A. Actually, R-4 is the R-value, not the density. The density of the products mentioned in this article ranges widely, from 4 pounds per cubic foot (pcf) to 8 pcf. The 8 pcf product (ComfortBoard IS, also known as RockBoard 80) was easier to handle for people used to the stiffness of foam and it also worked the best.

    Q. "Why could I not use R-10 Comfortbatt as continuous insulation like the board?"

    A. As far as I can tell, the density of R-10 ComfortBatts is less than 2 pcf. (Source: "ComfortBatt Thermal Batt Insulation Product Data.") That's a much lower density than ComfortBoard IS or Rockboard 80. You wouldn't want to use a product with such a low density for the purposes discussed in this article, because it is too fluffy and squishy. John Straube doesn't recommend anything with a density under 4 pcf -- and 6 pcf or 8 pcf is better.

    Q. "What makes one different than the other?"

    A. The density (which affects stiffness and resistance to pressure). Denser products have more fibers (more material) per cubic foot.

    Q. "Intuition, which in this case is probably wrong, tells me 2 products with the same R value per inch, made of the same material, should have the same qualities, thus the same rigid structure."

    A. You're wrong. There is no linear correlation between R-value and density for fiberglass or mineral wool batts. Up to a point, increasing the density of the product also (slightly) increases R-value per inch. Once that optimum point is reached, increasing the density further will result in a decrease in R-value per inch. Two products that both have an R-value of R-3.8 per inch might have very different densities.

  93. erikdavitt | | #93

    I have always thought it would be nice to add exterior insulation on my house which is in climate zone 5a. Only problem is there were areas that were renovated with a flash and batt technique. This makes exterior rigid foamboard not an ideal option from what I understand as it would not allow the sheathing to dry.

    I am curious about thoughts on using rock wool on the exterior instead as this would allow the sheathing to dry more readily then foamboard. Any thoughts? or would this just be a risky practice?

    1. GBA Editor
      Martin Holladay | | #94

      Mineral wool insulation is, indeed, vapor-permeable. You can add a layer of continuous exterior mineral wool to your walls, of any thickness, without worries about moisture accumulation. Of course, as with any wall, you need to pay attention to flashing details to prevent water entry from rain.

  94. Haverlock2022 | | #95

    If not, can you update this article to be in-sync with the times? We’ve even used the fluffy R-23 roxul on outside of a home with longer screws through battens and it’s held up fine after 7 years...

  95. Haverlock2022 | | #96

    Ok, I just read the bottom remarks m didn’t see a minute ago, sorry but still..

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