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Board-and-Batten Siding Over CMU Block Wall

Timothy Godshall | Posted in General Questions on

I am located in Virginia in climate zone 4. Clients have asked me to put vertical board and batten cypress wood siding on the north side of a house built with hollow 8″ CMU blocks. The house currently has no wall insulation, so this seems like a good opportunity to get some insulation at least on that one side. Is there any moisture-related danger to putting rigid foam on the outside of a block wall?

I would want to use at least 2″ of foam and fasten it to the wall with horizontal 2×4 strapping.  I realize that every fastener I put into the block wall to hold the strapping will be a major thermal bridge so I want to minimize fasteners but have enough to hold well. Any suggestions on a reasonable number of Tapcons or some other fastener to use?


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  1. Expert Member
    Akos | | #1

    Since you don't have any fluffy insulation on the inside, you can use pretty much any amount of exterior rigid. Zone 4 is not too cold, but it will still make a pretty big difference how cold the wall feels in the winter time.

    There is some thermal bridging from the screws but not enough to worry about. If you are not in a high wind area and since you are using light weight cladding, you don't need to go overboard, just enough to get the 2x nice and flat. 16" OC is pretty common for this.

    Masonry screws into block wall can be a pain, I would check first how well they hold. Hammer set concrete pins are much easier if you can find long enough ones (usually special order).

    1. Timothy Godshall | | #4

      Thanks for the response - I'll see what I can find for fasteners.

    2. C L | | #5

      The above response makes sense, but should you also consider the following:
      - Is the 8" CMU really hollow?? Doesn't code require at least 25% of the cells have resteel and be filled with grout to hold the wall?
      - If all the cells really are hollow, perhaps it is worth assessing if the wall is sustainable before putting more money into it, and if monies spent should first be spent on stabilizing the wall with resteel and filled cells.
      - If the CMU does have resteel and filled cells as needed for structural stability
      a. Is the frequency of filled cells sufficient to use those for your fastening points?
      b. Is it worth considering foam filling the remaining cells for added insulation before you adhere the siding?

      Also, you mentioned installing vertical board/batten, fastened to horizontal 2x4 strapping that also holds the rigid foam under the strapping. The strapping will set the board and batten 1.5" in front of the foam to create an airspace behind the board and batten for handling of bulk water. But the horizontal strapping would trap that bulk water. I'm not sure how this issue is typically handled. I would be inclined to use treated lumber for the strapping, install the horizontal strapping at a slight angle, and stagger the horizontals such that wherever water enters, it has a way out and is not trapped by the strapping.

      1. Timothy Godshall | | #6

        Thank you for these thoughts. I don't know if the blocks are core filled and reinforced at all. I'm not sure what the code says about that and I haven't investigated. I will be demolishing one wall so I guess I'll find out at that point and proceed accordingly.

        Good points on the handling of bulk water. I'll definitely cut the horizontal strapping at an angle and I will probably put some thin vertical furring strips behind the strapping so there is some air flow through the assembly.

  2. DCContrarian | | #2

    If you're worried about thermal bridging, I'd look at the top and bottom of that wall. Are they inside the building envelope?

    1. Timothy Godshall | | #3

      Good point about the bigger areas of thermal bridging. The top has attic insulation covering it, but the bottom goes right down to the ground so that will remain a major thermal bridge.

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