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Community and Q&A

What kind of fastener should I use to attach 3/8″ cement board over rigid insulation on our foundation at grade?

ERIC WHETZEL | Posted in Green Building Techniques on

When using rigid insulation on the exterior of a foundation, drawings usually show some kind of cement board covering the rigid insulation at grade.

I’m having trouble finding out what kind of fasteners we should be using to attach it. I can’t find any videos, diagrams, or other resources that show this detail.

Does anyone have experience doing this set-up, and if so, what kind of fastener did you use, and were you happy with the results? Does anyone know of online resources I could consult?

Our set-up is 5″ of Roxul Comfortboard 80 on the exterior of our foundation.

We were planning to use 3/8″ HardieBacker board for the cement board at grade (cut pieces approximately 12″ wide around the perimeter of the foundation).

And we think we found the parge coating we want to use: Tuff II. It’s sold in the big box stores in our area (Chicago suburbs). I did a sample board with it, and it seems like it will be easy to use and durable.

It’s just the fasteners for the cement board that are the remaining sticking point.

Any ideas or suggestions are welcome.

Thank you!

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, you should now that James Hardie will not provide a warranty on the use of Hardiebacker for exterior use in Illinois. Your state is excluded for their warranty. See the map here:

    Durock would probably be a better choice here, but I'm not sure whether Durock will warrant their product if it is in contact with grade. That doesn't mean that your plan won't work -- just that you may need to proceed with your eyes open, aware that warranty coverage may not apply.

    The usual fasteners for this type of application are TapCon fasteners.

    An alternative approach would use metal lath (also applied with TapCons, with washers if necessary) rather than cement board.

  2. ERIC WHETZEL | | #2

    Martin --- Thanks for the info regarding the Hardie board, I didn't even think to look that up. We'll definitely use the Durock instead.

    I was expecting people to suggest TapCon fasteners. I was just hoping someone had tried something else with success. We were able to avoid TapCon's for installing the Roxul on the foundation and the crew appreciated it, but it looks like TapCon's are the best and only option.


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