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Community and Q&A

Below-grade insulation

Mark Bartosik | Posted in Green Building Techniques on

I’m going to be using Roxul Comfortboard IS to build 4″ (R16) on the exterior of the north wall of our home. I didn’t want to use XPS (like I have in the past) because of the embedded green house gases.
I’m going to continue 2″ (R8) of this below grade down to the footing.

Roxul have already confirmed that Comfortboard is okay for below grade and it is easier to source than ordering Comfortboard and Drainboard.

I would like to use Grace Bituthene against the foundation wall. But then I don’t see how I can fit the Comfortboard over it, as I’m not aware of a glue that’s allow on the Grace membrane and I don’t want to put nails or other mechanical penetrations. So I might have to water proof with a liquid Asphalt coating which will double as a sealer for the wall and a glue for the Comfortboard.

Any suggestions about how I could use both Grace membrane and the Roxul Comfortboard.

– Mark

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  1. Expert Member
    Dana Dorsett | | #1

    Type-II EPS is fine for below grade use, and runs R4.2/inch, comparable to Comfort Board in insulation value, but with a much higher compressive strength. It's blown with pentane (7x CO2), which is pretty much gone by the time it reaches the distributors yard, which is FAR less climate damaging than the HFC134a (~1400x CO2) used in XPS.

  2. Mark Bartosik | | #2

    Thanks Dana, but I think that still leaves me with the same question but applied to EPS.
    That is to say, if I wanted to use a membrane on the basement wall below grade like Grace Bituthene then how would I attach the EPS ?
    Alternatively I could use a liquid Asphalt coating on the wall and use that as 'glue' for the EPS.
    But the point is that I would prefer to use a membrane like Grace Bituthene and some below grade insulation (either EPS or Comfortboard), but I cannot see how to attach the EPS or Comfortboard if the wall has a membrane on it too.

  3. GBA Editor
    Martin Holladay | | #3

    If you are installing exterior insulation on a foundation wall, the insulation is usually held against the wall by the backfill. That is sufficient.

    Above grade, the insulation can be mechanically fastened to the mudsill with cap nails. If you need to attach the insulation to the concrete, you can use TapCons and washers, or a a specialty fastener like Hilti IDP insulation anchors or Rodenhouse Plastic-Grip PMF fasteners.

    While these fasteners require you to penetrate the peel-and-stick membrane, I wouldn't worry too much about these penetrations, because (a) these penetrations are above grade, where water entry isn't usually an issue, and (b) the peel-and-stick membrane tends to seal around fastener penetrations to some extent.

  4. Mark Bartosik | | #4

    Thanks Martin,
    some more research and I found some solutions, for while the trench is open and I don't want the insulation to move so easily. 1) Use Roofing Seam Tape (it's double sided) -- as the membrane is pretty similar to Grace Ice and Water Shield so it's compatible. 2) Grace also said that their PAK Adhesive can be used (but it is not easily available).

    So to help during work, before the trench is filled in, I'll try some roofing seam tape. But I'll only use a little, enough for a temporary hold while working until the trench is filled in. Then the backfill should hold it in place.

    Since I'm using Roxul Comfortboard for the full height of the wall which is a rock wool and provides a capillary break, so a membrane is providing a "belt and braces" approach. Thus I might save a little $ and only use the membrane on the lower 3 feet of the basement wall (total is about 6 feet below grade).

  5. Sean Wiens | | #5

    Mark, I have just finished this exact task on my owner build using Roxul. You can find all the details on the journal located on my site Soprema make a torch on membrane called Colphene Torch'N Stick that is designed just for this application. The 'top' surface (one away from foundation wall) also has a torch off plastic film that allows you to apply a light torch to the already installed membrane and then 'stick' a panel product to it. It worked with the Roxul ComfortboardIS long enough to allow for the backfill. I also used a Foamed Spray Adhesive (from GreatStuff - not spray foam) to stick a dimple sheet to the Roxul. I have pictures and comments in my journal. Please feel free to contact me should you have any questions. Cheers Sean

  6. Mark Bartosik | | #6

    Here's what I used in the end... Some mason's line -- yes string. Basically tied a knot in one end and used a 2" x 2" square of Grace Ice and Water to stick this string to the wall. Then I looped it over the Roxul and stuck the other end of the string in place with another 2" x 2" piece of Grace Ice and Water. The knot stopped the string from pulling out from under the square of Grace Ice and Water. Two pieces of string for each 3'x4' Roxul was plenty good enough to hold in place. My prior idea to use seam tape didn't work as it only sticks to the Roxul's surface and pulls off the top layer of Roxul.

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