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Community and Q&A

Leave a gap at bottom of exterior rigid foam?

Clay Whitenack | Posted in Green Building Techniques on

I could have sworn I saw/read a “howto” on installing exterior rigid foam that recommended leaving a 1/4″ gap at the bottom of your exterior rigid foam. I assume it is for allowing moisture to drain out and/or keep the foam from wicking moisture up that may find its way to the foundation. I have tried to go back and find that recommendation and absolutely can’t find it anywhere.

Did I dream that up?

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  1. Paul Kuenn | | #1

    Clay, I believe most of us use a "Z" flashing in between upper and lower exterior foam. A top flange of the flashing sits under the weather barrier and overlaps the lower foam by about a half inch with a drip flange. The upper can't wick then and any moisture moving downwards will drain to the outside. There should be a slight downward slope to the flashing and the foam can be cut at an angle to accommodate that if you want the insulation kept tight. As I don't have the time to cut angles, I just leave the 90 degrees touch the flashing so there is about a 1/4" gap. You can see it at the bottom of my photo in the 12/31/15 blog of my house retrofit.

    Hope this helps. PK


  2. Paul Kuenn | | #2

    Forgot to mention there is a fine example of bottom flashing and insect screening in the Greenbuildingadvisor video series on exterior insulation.

  3. GBA Editor
    Martin Holladay | | #3

    I don't really understand your question. Paul assumes that you are installing exterior rigid foam on an above-grade wall, and that this foam is co-planar with a similar layer of rigid foam installed on the exterior side of your concrete foundation. Is that the case? You didn't really describe your wall that way, so I'm unsure of what you are talking about.

  4. Clay Whitenack | | #4

    Thanks for the reply. It's not identical, but similar.

    I have a framed 2x6 wall, with OSB sheathing, sitting on a 9" thick poured concrete foundation wall. The extra 3" of concrete is for my two layers of 1-1/2" fiberglass faced pollyiso insulation. I have Z flashing that is sealed to the bottom of the OSB, turns out at the concrete foundation, runs under the foam, and then down the concrete wall. My WRB is sitting outside the 2nd layer of foam.

    In theory, the WRB and 2 layers of polyiso should stop any water before it gets back to the OSB, making the Z flashing unneeded, but we are including it anyway just in case. In my mind, if we are including the Z flashing and leaving a gap between the flashing and the polyiso, it would defeat the purpose to seal that 1/4" gap.

  5. GBA Editor
    Martin Holladay | | #5

    If I were you, I would make sure that the WRB runs longer than the rigid foam, and laps over the Z-flashing. Leaving a 1/4-inch gap between the bottom of your rigid foam and the Z-flashing is fine, although if you didn't have the gap, it wouldn't make much of a difference.

    As I'm sure you know, your foundation detail is unusual. On most homes, the exterior rigid foam on the above-grade wall would be proud of the foundation. But your way works.

  6. Clay Whitenack | | #6

    Thanks Martin. Yes, I'm sure it is unusual. We are going with a brick veneer exterior, so we also have a brick drop a few feet below the top of the foundation wall. I guess the builder wanted the top of the foundation and foam to be on the same plane for a consistent air gap behind the brick.

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