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Attaching Roxul (Comfortboard 80) to exterior foundation wall

User avatar
Eric Whetzel | Posted in Green Building Techniques on

My wife and I are building a “green” home (mix of Passive House, Pretty Good House, and Net Zero). For the basement foundation (9′ basement) we intend to use spray-on waterproofing then two layers of Roxul Comfortboard 80 mineral wool (3″ + 2″ = 5″ total) on the exterior side.

Initially we thought we would use the Ramset system of fasteners to attach the Roxul to the foundation, but it’s pretty clear that waiting at least 28 days after the foundation pour would be necessary (per manufacturer’s recommendations and opinions found in online searches) to avoid potential damage to the concrete wall. Obviously we would like to avoid this delay if at all possible.

What are sensible alternatives? Pre-drill through Roxul and then use Tapcon fasteners?

In addition, in a belt-and-suspenders approach, does it make sense to then add a dimple membrane layer to the outside of the Roxul? Presumably this is an insurance policy against water making its way through the Roxul and finding space around the fasteners to enter the concrete wall? In a Hammer and Hand video it looked like they incorporated this approach.

Any other cost-effective way to achieve this? Instead of dimple membrane, what about using 10 mil Stego wrap?

Thanks ahead of time for any suggestions or answers. GBA is a resource we go back to again and again to find effective answers to our endless questions.

Thanks so much,

Eric

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Eric,
    I look forward to hearing suggestions from GBA readers on you question about fastening. You might want to phone Roxul and ask to speak to someone in their technical help department -- they may have fastening suggestions.

    Before giving you advice on the dimple membrane question, it would be good to know if you have a plan to protect the above-grade portion of the foundation insulation. What materials will you use to protect the insulation above grade?

  2. User avatar
    Eric Whetzel | | #2

    Hi Martin,

    We do have this question in to Roxul. The technical rep from Roxul that we've been dealing with, Fiona Schofield, has been fantastic --- she always responds quickly and supplies us with great information and answers.

    Based on what she's already supplied us with, I'm assuming she will suggest either waiting the 28 days in order to use the Ramset fasteners, or else pre-drilling and using concrete screws. We're just curious, since the use of Roxul seems to be growing, whether anyone has come up with alternative ways to successfully attach it to the foundation, or if they have any comments (positive or negative) regarding either the use of Ramset or concrete fasteners in this kind of application.

    Awhile ago I found a YouTube video in which someone used a roofing torch to heat up a waterproofing membrane, making it sticky enough in order to attach/embed a layer of Roxul up against the foundation. Unfortunately, I can't seem to find it now. This could work for the first layer, but then we'd still have to come up with a solution for the second layer of Roxul.

    I just did another search for attaching Roxul to concrete and saw a video for Rodenhouse fasteners. Here's the link:

    https://youtu.be/IAu9fffQvVg

    The plastic fastener, shown in the video, inserted after pre-drilling through the Roxul and concrete, looks like a straightforward solution. Anyone have experience using this fastener? Is the fastener sitting in the concrete tightly enough to avoid water penetration issues?

    I also noticed a video for a UK product, Sto mineral fibre board, in which they applied a mortar to the back of the insulation before applying it to what appears to be a cinder block wall. Here's the link:

    https://youtu.be/YM8ml86M-Q4

    The video says the substrate must be smooth, so not sure if a new foundation wall would be smooth enough to even consider this as an option. Nowhere in the Roxul technical info we've received is this presented as an option, so it's doubtful whether Roxul would approve this installation method, especially below-grade.

    Nevertheless, if the goal is just to get the Roxul seated securely enough against the foundation before backfill goes in, could this be a viable option? Even if the mortar/adhesive failed over time, would it be safe to assume the Roxul has nowhere to go with backfill/soil applying significant pressure against it and the foundation?

    As far as protecting the insulation above-grade, we're currently planning to use a cementitious board-lathe-parge coating system. Based on what I've read here on GBA and elsewhere, this seems like the best (but still not great?) way to proceed. We're still a month or two away from commencing the build, so we're still open to alternatives if anyone has suggestions.

    Thanks,
    Eric

  3. Dave De C | | #3

    Have you asked your foundation contractor about the possibility of installing furring strips directly into the concrete forms?

  4. User avatar
    Eric Whetzel | | #4

    Dave,

    Thank you for the suggestion.

    No, we didn't think about that, but we definitely will now.

    If the furring strips are acceptable (to the building dept., concrete contractor, and Roxul), is it safe to assume they will eventually degrade over time? If so, is there much concern in this regard, or will there be enough soil/backfill pressure against the Roxul and foundation so even when the holding power of the fasteners in the furring strips begins to fail, the Roxul will not be able to move?

  5. Charlie Sullivan | | #5

    Yes, I'd expect that wood embedded in the concrete would eventually rot out. That's OK structurally, because the backfill does much more to hold the Roxul in place, but I would worry about the integrity of the waterproofing layer.

    I suspect Tapcons would be your best bet. I'd be tempted to rig up some temporary clamping system that would attach above the foundation wall, just to hold the top of the Roxul during the backfilling process, and then just rely on the backfill holding the cement board, and the cement board holding the top of the Roxul. But rigging that temporary clamp might be complicated enough that it's not worth it.

  6. Malcolm Taylor | | #6

    Eric,
    I've never seen them on the exterior but I have seen embedded strips on the interior of a foundation wall. Use ground contact rated PT and they will never degrade.

    Whether that's the best solution I don't know. The Ramset advice sound a bit like they are just being cautious. Most concrete mixes do take around 28 days to fully cure, but the rate of curing is like (half) a bell curve. At 24 hours they may be about 50% and at seven days should hit 80%. If a fastener won't take in week old concrete something is off.

    Now whether you want to be the one seeing if they will take is another matter.

  7. User avatar
    Eric Whetzel | | #7

    Based on the response from Roxul, the three best ways to attach the Comfortboard 80 to a concrete foundation are:

    *Wait 28 days and use the Ramset fasteners.

    *Pre-drill through the insulation and into the concrete, and then use Tapcon fasteners.

    *Pre-drill through the insulation and into the concrete, and then use the Rodenhouse Plasti-Grip PMF fasteners.

    It looks like we're going to use the Rodenhouse fastener, mainly for ease of use. It's not currently shown on their website, but they do have a 6.25" fastener, which should work for our application.

    In terms of moisture below-grade, Roxul believes the Comfortboard will "act as a capillary break to relieve the foundation of hydrostatic pressure", so no dimple board or other membrane is necessary between the Roxul and the backfill. They believe any moisture present in the soil will only produce "surface wetting of the board". They forwarded a study that appears to confirm this (based on a structure that had rock wool installed below-grade in 1976).

  8. Mike M | | #8

    I am using 4" of roxul and have had a hell of a time with 6" Tapcons. I have gone through at least three bits in less than a complete box (cured concrete, avoiding form pins/rebar).

    I have also had an issue driving them as some of them seam to drive about 1/2" shy of depth no matter how well I clean the holes, air or vacuum. I have read others have similar issues as me, and others who never had a problem.

  9. John Charlesworth | | #9

    Considering using comfortboard on the exterior of our foundation.

    I've had a test patch of comfortboard attached to my foundation above-grade for 2 years now and it's still attached rock-solid.

    I found that orange great stuff spray foam not only is fireproof, but holds comfortboard to concrete better than anything other adhesive I've found. The foam soaks into the comfortboard enough to form a very strong grip.

    Hope this helps,
    .../j

  10. User avatar
    Eric Whetzel | | #10

    Mike M. ---
    Your experience with the Tapcon fasteners is why we're leaning towards using the Rodenhouse Plasti-Grip fasteners. If their video is accurate, then simply tapping home the Plasti-Grip fastener should be no problem. If we end up using the Rodenhouse fastener, I'll come back here and let others know how it went.

    John Charlesworth ---
    So maybe it's possible to attach the first layer of Roxul to the foundation with a spray foam or other adhesive, as you suggest, and then use the Rodenhouse or Tapcon fastener to attach the second layer of Roxul to the first layer and the foundation? This seems like it would definitely save time by not having to pre-drill a set of holes in the concrete for each layer of Roxul.

    I'm curious if anyone has experience with the Rodenhouse Plasti-Grip PMF fasteners?

  11. User avatar
    Eric Whetzel | | #11

    We ended up going with the Rodenhouse PMF fasteners. They worked out really well. Everyone on the crew found them easy to use and install (apart from the unavoidable hammer drilling beforehand).

    You can check out how we used them on our project here:

    http://www.kimchiandkraut.net/2017/01/02/foundation-details

    We would definitely recommend using them.

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