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Community and Q&A

Installing vertical cedar siding over external foam and housewrap

scattleberry | Posted in Energy Efficiency and Durability on

I’m installing vertical siding (stained cedar boards, in the reverse board and batten pattern) over external foam (1.5 inch polyiso). My building department requests that I use a WRB over the foam. I was planning on installing the vertical siding directly against the housewrap, but I’m concerned that the cedar will eat up the tyvek housewrap. Is a reasonable solution to this problem to install strapping on top on the housewrap, so the cedar boards don’t sit against the housewrap? Another option could be to flash the windows to the external foam, so when the housewrap goes away, I still have a working WRB (that being the external foil faced polyiso).

Are there other ideas? Obviously, I’m looking for the easiest solution that will ensure an effective WRB down the line.


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  1. Expert Member

    There are a couple of things you can do. One is seal the cedar on all six sides, which as good practice anyway, or change to a WRB that isn;'t affected by cedar tannins such as Typar.

  2. GBA Editor
    Martin Holladay | | #2

    Tyvek claims to have reformulated their housewrap so that it is not affected by cedar tannins. It's hard to evaluate whether that's true, of course.

    You can always choose asphalt felt instead of housewrap. Asphalt felt performs well as a WRB.

    If you are installing reverse board-and-batten, that means that the battens (usually 1x3s or 1x4s) get installed first, and that the boards (usually 1x10s) are on the exterior. If that's the case, you won't be able to see much of the battens. You can use any species of wood you want for the battens -- they don't have to be cedar if using cedar makes you nervous.

  3. codypup | | #3

    I would not use any building wrap with cedar or redwood, but instead suggest 15#felt with a rain plane between the felt and the siding. I used corrugated plastic battens manufactured specifically for the application. Vertical siding will allow rain to get behind the siding and I feel more comfortable knowing it has a place to go before it has a chance to get inside the wall cavity.
    It does require some thought to get windows properly flashed though-I ended up with a system utilizing 2x head trim with head flashing that fit flush with the back of the siding and 1x side and bottom trim that was installed over the siding.

  4. scattleberry | | #4

    I appreciate everyone's responses. I was under the impression that all wrap materials were susceptible to the surfactants in cedar, so I'm glad to stand corrected. Since I already have the boards in hand (and it is all cedar) I think I'll just go with the 15lb felt idea. Obviously, with the foam up, I can't staple the felt on, so I think I'll just use cap nails to get it in place until the siding is up. I'm using the reverse board and batten pattern so there's a raindrain behind the boards, and I'm counting on Martin's opinion that this will provide sufficient drying for the siding assembly. I'm glad I won't have to use horizontal nailers on top of the foam, since the building department is also insisting on 2X blocking in the walls behind the sheathing.

    Anyone see any problems with my plan?

  5. user-1072251 | | #5

    strap the wall horizontally, and use a drainwrap behind the strapping to allow trapped water to run down the wall. Inexpensive, relatively fast and best of all, allows the cedar to remain dry on both sides, rather than pushing it against some surface which will be damp for long periods of time. the wood will stay straighter, last longer and will hold the finish.

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