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Roxul Comfortboard – disadvantages?

mfredericks | Posted in Green Building Techniques on

I’m intending on insulating my 1950’s Cape style house with exterior insulation. I’ve been planning on following the CCHRC’s REMOTE wall assembly with an air tight membrane over the sheathing and 6″ of EPS foam (2, 3″ layers) then furring strips and siding.

My concern with this approach has always been limiting the drying potential outwards and having an existing foil vapor barrier in some parts of the house and poly in another part of the house. To remain living in the house, I’d like all the work to be done from the outside. Worried about creating a double vapor barrier, I’ve accepted the idea of using a vapor open insulation on the outside of the house and I could simply substitute the 3″ layers of EPS with 3″ layers of Roxul Comfortboard IS. I like this because it doesn’t change many of the details I have planned, and Roxul doesn’t burn or shrink and its a safer bet for potential moisture risks to allow drying to the outside.

The only disadvantage I can see is that the Roxul could be susceptible to some wind washing. Where the seams of the EPS or other foam board can be spray foamed or taped together, Roxul would just be butted together. If the membrane over the sheathing is the dedicated air barrier, this wind washing probably isn’t so bad, but I’m curious what others see as disadvantages of using Roxul in the place of foam board for exterior insulation?

Located in Nova Scotia, Canada – Climate Zone 6.

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  1. Expert Member
    Dana Dorsett | | #1

    The vapor permeance of the "air tight membrane over the sheathing" matters, if you have foil or poly on the interior side of the stackup. It should have vapor permeance of 3 US perms or higher if you are planning to put 6" of EPS on the exterior.

    The permeance of 6" of Type-II EPS is ~0.5 perms max, which isn't much drying capacity. If you did it with Type-I EPS it would be between 0.8-1 perm, which is more reasonable.

    Comfort board is a full order of magnitude more permeable, AND it's fireproof- if anything it is a superior way to go in your stackup. But unless your air-barrier membrane is also fairly vapor permeable it won't actually allow the sheathing to dry. Most standard housewraps are at least 10 perms, many are over 25. with ~2/3 of the total R on the exterior you could skip the interior vapor barriers that are currently installed, but there is no harm in an NS climate to making the exterior side very high permeance. (In hotter humid climates where you would be air-conditioning the interior to a temp below the outdoor dew point for weeks on end it might become an issue, but not in the Canadiang maritimes.)

  2. GBA Editor
    Martin Holladay | | #2

    I know of one other job like the one you are describing -- an energy retrofit job that specified 6 inches of mineral wool to be installed on the exterior side of the wall sheathing. Here is a link to the article about that project: Wrapping an Older House with Rock Wool Insulation.

    The main challenge is dealing with the fact that mineral wool is a little bit squishy. That can make it difficult to get the furring strips plumb and co-planar. It's not impossible, but it's trickier than a job with rigid foam.

  3. user-1041981 | | #3

    Martin, can you give some tips on getting the furring strips plumb and co-planar over ComfortBoard? My builder will be dealing with this next week (he's installing 2-3/4" of ComfortBoard this week).

  4. GBA Editor
    Martin Holladay | | #4

    I've never done it. Here is a link to another article on the topic: Installing Roxul Mineral Wool on Exterior Walls.

    In that article, Shannon Cowan and Patrick Walshe write, "When adding the 1x4 strapping over the Comfortboard, Dubenski and crew placed one screw at the top and and once screw at the bottom of each furring strip, and then checked it with a long level to be sure it was plumb before adding the remaining screws. “Once we got the hang of it, we didn’t need to fiddle around much backing screws out,” Dubenski said. “We also took care when installing the fiber-cement siding to make a few adjustments.”"

  5. mfredericks | | #5

    CB, in the article that Martin linked to, they used these dual threaded screws which I've looked at before and are really well suited for installing battens over mineral wool. Your builder might be using standard screws, so a really long level will come in handy!

    See the install video in this link too.

  6. Kopper37 | | #6


    We've used Roxul rigid boards as exterior insulation on two houses now. We're in a milder climate (4A), so we only used a single 2" layer on both projects. Some comments from this experience:

    We used the Rockboard 80 product.

    The most common board size is 2' x 4'. Although this seemed a like a disadvantage at first, the size is just right for material handling.

    Sheet goods are never perfectly square or uniform (plywood, insulation, etc.); however, we've found that Roxul boards are very uniform in size. The material strikes a good balance between its rigidity and flexibility. Because it can be slightly compressed it's not too difficult to mate the boards together to create a seamless field of insulation.

    If you are butting between two surfaces simply cut the board 1/16" larger than the actual measurement and pressure fit the board.

    We fasten 1x4 furring strips to the frame with 5" x No. 10 screws. It's very easy to compress the insulation, especially near the end of a furring strip. We try to compress it as little as possible---usually just enough to press the raised dimples flush with the main surface. Our technique is to overdrive the screw slightly in order to get the head flush with the furring strip, back it out, then drive it back in enough to firmly set the furring strip against the insulation.

    We come back later (during siding) and verify that we don't have any problem areas. We use the level to check the plumb on individual strips, then spread it across several strips to verify that we don't have any horizontal waves. I should note here that we're also using fiber cement siding. On our first project we built next to another (conventionally framed) house that had fiber cement siding. The house was only 12' away, so it provided a good comparison. Nobody could tell the difference. Point is . . . you can get the furring strips coplanar.

    If the material is installed behind siding then wind washing is not an issue. The 8 lb. product is very dense.

    The boards repel water initially (it beads on the surface). As the surface tension breaks then it soaks into the board. We've found that it drains and dries very quickly.

    For a 2" thick board our cost is slightly higher than equivalent Type IX EPS foam board ($0.80 / sq. ft. compared to $0.65 sq. ft.).

    It doesn't burn. It's insect resistant. It provides drying potential.

  7. GBA Editor
    Martin Holladay | | #7

    Thanks for the excellent post. If you are so inclined -- and if you have a few photos of your work -- please write a guest blog on this topic. Feel free to contact me: martin [at] greenbuildingadvisor [dot] com.

  8. mfredericks | | #8

    Thanks for the feedback guys, I'm feeling much more comfortable about using this product now.
    Daniel I'm also curious to see some photos or a guest blog of your installation. Thanks for sharing your experience!

  9. KeithH | | #9


    Do you know where you are sourcing the product? And where are you located? I'm in the Denver area and have had a hard time sourcing mineral wool board. If you are in the northeast, I'm guessing you have sourcing options I don't have but if you are anywhere else in the country, I'd be eager to hear how you sourced it.

  10. mfredericks | | #10

    Keith H, I'm in eastern Canada, and several local retailers carry Roxul products including the 1.5" comfortboard. I'm in contact with Roxul/ the retailers about ordering the 3" version. I'm hoping this won't be an issue, although its probably no help to you in Denver anyway. Good luck!

  11. itserich | | #11

    I was able to order Comfortboard through Lowe's. Originally was told minimum order size of a pallet, eventually told multiples of 3 packs. In Iowa. Delivery takes a month. Have not received it yet.

  12. JC72 | | #12

    Question for Dana (Resurrecting an old thread).

    Your initial comments "..but there is no harm in an NS (Nova Scotia) climate to making the exterior side very high permeance. (In hotter humid climates where you would be air-conditioning the interior to a temp below the outdoor dew point for weeks on end it might become an issue, but not in the Canadiang maritimes.).."

    Mineral wool has been mentioned as an alternative to exterior foam board with regards to termites and carpenter ants. Being that these creatures are very prominent in warm/humid climate zones what sort of exterior insulation would you suggest in lieu of mineral wool?


  13. GBA Editor
    Martin Holladay | | #13

    I'm interested in what Dana has to say -- but if we are talking about continuous insulation for the exterior side of your wall sheathing in a hot climate, and you don't want to use mineral wool insulation, then I recommend that you select either polyisocyanurate or EPS.

  14. Expert Member
    Dana Dorsett | | #14

    In warm humid climates rigid foam board works well. Foil facers are class-I vapor retardrs, and also provide some insect resistance.

    If you want to use rigid rock wool for insect resistance (and I'm not totally convinced that it's as critter-repellent as some seem to think), installing a class-I vapor retarder between the sheathing and rock wool MAY be necessary in the extreme cases, but otherwise class-II vapor retarders should be fine.

    In the less extreme warm humid climate cases, OSB or plywood are already "smart" vapor retarders, about 1 perm (minimal class-II) or less when dry, and if detailed as the primary air barrier there would be little risk of mold-inducing moisture levels or or condensation inside the wall cavities with standard latex paint as the interior finish. Using #15 felt between the rock wool & sheathing rather than a high-perm housewrap would be the right choice.

    If it's behind brick or stucco cladding, you'll want a lower-perm vapor retarder between the rock wool and sheathing in warm humid climates.

  15. JC72 | | #15

    Thanks Dana,

    I guess ZIP Sheathing with its 12-16 perms rules out MW in zone 3A. Perhaps ZIP-R actually is the better route being that the foam layer is on the interior of the OSB and somewhat protected from termites/ants.

    Darn. I was really thinking MW with its shrinkage resistance was the way to go.

  16. christopherw | | #16

    The 12-16 perms for Zip is specifically for the WRB coating on the zip panels. The OSB part of it is something like 2-3 perms. The 12-16 perms you see mentioned with Zip can be a bit misleading.

  17. Expert Member
    Dana Dorsett | | #17

    Christopher has it right about ZIP- the OSB itself is variable (and much lower) permeance, but the factory applied spray on WRB allows it to dry toward the exterior.

    That said, ZIP-R is a very reasonable way to go in zone 3A. A 2x4/R15 wall with 1.5" ZIP-R beats the IRC 2015 code minimum (= 2x4 / R13 + R5) by a bit, and a 2 x 6 / R23 wall with 1.5" ZIP-R beats code minimum (= 2x6 / R20) by a good margin. If you put foil tape over the exposed top & bottom edges of the ZIP-R would even further improve it's insect resistance.

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