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Exterior foam installation details

Kacey Zach | Posted in Green Building Techniques on

Our house is 2×6 construction built in 1984 near Milwaukee, WI. Going from the inside out, drywall, vapor barrier, fiberglass insulation, EPS foam sheathing, and Cedar board and batten siding.

I recently had some of the cedar off to reveal huge gaps where the foam has shrunk, I guess that explains last winter’s (our first in the house) high heating bills also, none of the joints are taped either. The EPS seems like a pretty bad installation, lots of holes which I assume the mice were coming in from plus no sheathing. Also, there is EPS on the foundation, but it is mostly degraded above ground.

Here is my plan (please comment on any issues you may see):

Tear off the cedar siding and scrap the EPS sheathing. Inspect the cavity, pull out any poly vapor barrier I can get to. Fix and/or replace any of the fiberglass insulation that needs attention. as for putting it back together; 1st layer of rigid foam: Dig down 1′ around the foundation and install 4′ of XPS, this will come up on the walls also. Then use PolyIso for the remainder of the 1st layer on the house. Then 2nd layer 2′ of XPS with remainder PolyIso (this will give me staggered horizontal joints) If I do a 3rd layer, it would be fully PolyIso on the above ground portion only. Then 1×4 horizontal strapping followed by the cedar board and batten siding (My wife is pretty hung up on keeping the siding look). All joints in the foam will be taped. The cost difference from the PolyIso to XPS seems small enough that I’d like to use as much PolyIso as I can for the increased R value.

How do I treat the bottom edge? How do I keep critters from getting in? What about flashing? Do I need any? I plan to leave the windows (casement) alone and treat them as an “innie” system much like many of the drawings available here on GBA. Should I use an adhesive on the studs and foundation to adhere the foam?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Kacey,
    You may be interested in reading this article: How to Install Rigid Foam Sheathing.

    Q. "How do I treat the bottom edge? How do I keep critters from getting in?"

    A. Check out the details in this video: https://www.greenbuildingadvisor.com/video-how-install-rigid-foam-insulation-outside-house

    Q. "What about flashing? Do I need any?"

    A. All window openings, door openings, and other penetrations need to be properly flashed. Flashing should be integrated with your water-resistive barrier (WRB).

    Q. "Should I use an adhesive on the studs and foundation to adhere the foam?"

    A. On the foundation -- yes. Adhesive on the studs is possible but not necessary.

  2. Kacey Zach | | #2

    I've been non-stop reading everything I can get ahold of. I'm now leaning towards removing the windows to ensure proper drainage. Once I tear off the siding around the windows, I will determine whether or not the windows come out, but they will be going back in as I cannot afford new windows. Right now I am curious as to how they are flashed and where the WRB is located. Right now I don't really see a true WRB as the sheathing is a foil backed EPS that is not taped and has huge gaps from shrinkage.

    Any tips on where to get flashing? That stuff in the video looks pretty deep. I did not see anything in the video regarding an insect screen. I assume one should go at the bottom edge of the cedar, do they make some sort of foam block that I could sandwich in there? Are mice going to eat thru my PolyIso?

  3. GBA Editor
    Martin Holladay | | #3

    Kacey,
    Q. "Any tips on where to get flashing?"

    A. There are many types of flashing, ranging from flexible flashing made of rubberized asphalt or butyl (usually but not always a peel-and-stick product), to painted steel, to aluminum, to copper. Flashing can be purchased at a building supply house (lumberyard).

    Q. "I did not see anything in the video regarding an insect screen. I assume one should go at the bottom edge of the cedar. Do they make some sort of foam block that I could sandwich in there?"

    A. You can use ordinary insect screening -- see a photo here: http://www.100khouse.com/2010/05/14/the-vented-rain-screen-via-furring-master/ -- or you can use a commercial product like one from Cor-a-Vent (http://www.cor-a-vent.com/siding-vent-sv-3.cfm).

    Q. Are mice going to eat thru my PolyIso?

    A. Probably not.

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