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‘Finishing’ above-grade Roxul Comfortboard: What are the best practices?

Ryan Lazuka | Posted in General Questions on

I’m building a new home. The exterior of the home, from the bottom of the basement to the edge of the roofline, is covered in 2″ Roxul Comfortboard.

90% of the Roxul will be covered by Hardi lap siding, however, there’s a gap where the Roxul is not covered. That gap is between the ground level and the top of the foundation wall. Now there are solutions to this problem. For instance, Roxul recommends using cement board to cover the exposed Roxul. But Roxul makes no recommendations on exactly which cement board to install, makes no recommendations on how to fasten the cement board through the Roxul and to the CMUS, and make no recommendations on how to seal the cement board.

It’s easy for Roxul to say, which they have, that all of the above problems can be answered by the manufacturers of the cement board. But when I call Hardi or Durock, both of them have issues with this particular application. Hardi says to absolutely not do what Roxul recommends. And Durock, only warranties their product when attached to furring strips.

With all that said, my questions are below:

– What type of cement board is appropritae to cover Roxul Comfortboard?
– How should the cement board be attached, through the Roxul, and to the Foundation wall?
– How should the cement board be finished and/or water sealed?

Thanks in advance!

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  1. GBA Editor
    Martin Holladay | | #1

    It's up to you to decide whether a warranty from Durock matters to you. If the warranty matters, you could first install pressure-treated plywood or pressure-treated 1x4 furring strips, followed by Durock.

    However, it probably makes more sense to just install the Durock and skip the Durock warranty. Or to just install the pressure treated plywood, and skip the Durock.

    For a list of 11 materials that have been used to protect rigid foam installed in this location, see How to Insulate a Basement Wall. Many of these materials can also be used to protect mineral wool insulation. What you decide to use depends in part on what is aesthetically acceptable.

  2. Ryan Lazuka | | #2

    Thanks for the reply Martin!

    Yes, I am willing to void the Durock warranty, however I still ha e some questions below.

    - How should the cement board be attached, through the Roxul, and to the Foundation wall? What fasteners specifically?
    - How should the cement board be finished and/or water sealed?

    Thanks in advance!

  3. GBA Editor
    Martin Holladay | | #3

    I've never done it, but imagine that you would use TapCon fasteners. Most builders would probably finish the Durock with a coat of stucco (cement-based plaster).

  4. Expert Member
    Malcolm Taylor | | #4

    It's worth looking at the actual terms of warranties before deciding whether they matter or not. Very often they cover the replacement of the material if defective, but not labour or damage to other components of the structure.
    Shingle warranties are a good example. A claim is typically pro-rated for normal wear and only the defective portions included, so you are lucky to get a couple of thousand back on a re-roof costing ten times that amount.

  5. Hobbit _ | | #5

    I'm tentatively planning to cover about 18 - 24 inches of exposed
    CMU foundation wall in 2" Comfortboard CIS [which is slightly denser
    and more rigid than generic Comfortboard]. The plan is to simply PL
    glue it to the block tucked up against the sill flashing, burying the
    lower edge maybe 6 inches into the dirt, and call it a day. The wall
    is well protected from direct water by structures that overhang it,
    and Roxul isn't really damp-sensitive anyway. It and the wall will
    dry to the exterior [the only choice now since the interior is
    covered with foil-face and CC sprayfoam]. Point is that I'm not
    planning to cover it with anything.

    It's an experiment, and will look kind of funny given the natural
    color of the comfortboard, but the wall is already kind of greenish
    in places so if anything it would look more uniform. The only mild
    concern voiced so far is critters attacking it for material [e.g.
    birds wanting pieces of it for nesting]. This doesn't seem like
    a huge concern, as I wouldn't think fiberglass makes a very nice
    material to your coddle offspring in. If it gets a couple of dings in
    it over time it's not a big deal, but I can't see how any circumstance
    would trash the installation wholesale.

    I've got a test patch on one wall, and after seeing how it fares as
    spring segues into summer I'll make the call to go ahead or not.
    It did make a big difference in the temp of the blocks behind it
    on cold nights; I've got one removable section I can pull and shoot
    with the IR cam.

    So I would consider covering strategies to be of cosmetic value only,
    but think about material permeability and drying paths as you review


  6. John Charlesworth | | #6

    I've run the same experiment. What I've learned so far:
    - Exposed ComfortBoard IS will increase the moisture load on the concrete foundation. I ignored Roxul's advice to damp-proof the foundation wall before applying ComfortBoard. I should've listened to them. Roll-on tar or peel-and-stick membrane would've helped a lot.
    - Yes, it made a significant difference in the indoor temperature of the concrete wall (verified with IR camera).
    - PL adhesive does not bond ComfortBoard to concrete very well. Over time, it peels off. I've found that orange Great Stuff spray foam works *really* well as an adhesive in this situation. Plus it's fire-retardant.

    Hope this helps,

  7. Expert Member
    Malcolm Taylor | | #7

    John, a bit late now but... another alternative is to get your concrete batch plant to use an admixture like Xypex, which dramatically improves the walls resistance to moisture. Xypex also has products you can apply to the interior, if it is exposed, which act in a similar manner.

  8. John Charlesworth | | #8

    It's a 1942-vintage house so, you're right, it's a bit late! :-)

    I noticed it because I did this in a stairwell. From the inside during the rainy season, you can see the moisture outline of the steps/ComfortBoard...

  9. Ryan Lazuka | | #9

    The last answers are scaring the hell out me.

    All of my basement foundation wall are exterior clad with 2" Roxul Comfortboard IS. The other day, after a significant rainfall, I noticed the interior basement walls looked a little dark. They were not wet to the touch, but they were darker than bone dry CMUS. Is this Normal?

    The foundation walls have drain tile on both the exterior and interior of the walls. There is stone backfill from the footers till 1 foot above grade. And the entire exferior foundation wall was damproofed.

    Any feedback is appreciated.

    Thank You!

  10. GBA Editor
    Martin Holladay | | #10

    John Charlesworth,
    The need for dampproofing (or waterproofing) and the need for insulation are two separate concerns. Just because you decided to install mineral wool insulation on the exterior side of your basement wall, was no reason to omit dampproofing. It always makes sense to limit the amount of moisture that is transferred from the soil to your basement wall. Mineral wool is not designed to do that.

    You evidently realize these facts now, but I'm clarifying them so that Ryan Lazuka, who claims that information on this page is "scaring the hell" out of him, understands what is going on.

  11. Ryan Lazuka | | #11

    Martin, thanks for the clarification! It's appreciated as always.

    Can you address my question though? In your opinion, are CMUS, ones which are visibly darker than bone dry CMUS, normal? Especially with the way my wall is engineered.

    Thank you!

  12. Hobbit _ | | #12

    So ... y'all are saying that rather than the whole CMU + Roxul
    assembly easily drying toward the outside, that it will tend to
    concentrate more moisture on the block? Even above grade? Why?
    Does the roxul wick it up out of the ground or something? I'm
    only intending to go 6 or 8 inches down, would that still
    produce adverse effects with a generous free-air area above?

    I haven't noticed anything untoward at my test patch, but I'll
    take a closer look.

    Thanks for the reminder about the sprayfoam. I've used the yellow
    stuff as adhesive before, and it does work well. The thought with
    the PL was actually to soak lines of wood plasticizer into the Roxul
    first and then glue along there -- I did some test pieces with and
    without for comparison, and the wood-hardener increases the bonding
    area -- or really, volume within the Roxul piece -- quite a bit.
    The Loctite PL "5x fast grab" stuff seems well suited for this, and
    sets up faster than generic caulk.


  13. GBA Editor
    Martin Holladay | | #13

    Dampproofing is not waterproofing. It sounds as if the dark-colored CMUs are damp. The CMUs may be getting damp because the dampproofing is imperfect, or the moisture may be wicking up from a damp footing because there is no capillary break between the footing and the CMU wall.

    Is this "normal"? I suppose it is, especially in April. Many concrete and CMU foundations have no capillary break above the footing, or have no dampproofing, or have dampproofing that allows basement walls to get damp in April.

    Whether or not this is a problem depends on conditions in your basement. If there are no bad smells, liquid water, or mold in your basement, I wouldn't worry.

    That said, if your footing drains lead to daylight or are equipped with cleanout fittings, it never hurts to inspect your footing drain line to make sure that it seems clear and seems to be working.

  14. John Charlesworth | | #14

    My test area was on an exposed foundation wall *above grade* (in a stairwell).

    It had been exposed for 70+ years without dampproofing and I wanted to test whether just adding mineral wool would be sufficient.

    Turns out that adding the mineral wool *increased* the moisture/vapour flow through that part of the wall, even though it was all above-grade.

    Something to consider...

  15. GBA Editor
    Martin Holladay | | #15

    It's possible that your concrete basement wall gets damp because it is wicking water up from a damp footing, and because there is no capillary break between the footing and the wall.

    If the wall used to be uninsulated, the above-grade portion of the wall would have allowed the damp wall to dry to the exterior, especially on sunny, windy days. Covering up the above-grade concrete with insulation would reduce the rate of drying to the exterior.

  16. Mike M | | #16

    How did you end up finishing the above grade Roxul? Also did you use Comfortboard below grade too? The densities are the same and the compressive strength actually seems slightly higher with the Comfortboard.

    Roxul wants me to order half a truck load at a time of Drainboard but I have no need for $17,000 worth of the stuff when the whole house need less than $10,000 worth.

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