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Musings of an Energy Nerd

Attaching Corner Trim on Walls With Rigid Foam

At outside corners, you’ll need to plan ahead by installing wide furring strips or 2x4s embedded in channels carved into the rigid foam

Ordinary furring strips (1x3s or 1x4s) don't provide a nailing surface at outside corners on walls with a thick layer of exterior rigid foam. If you intend to install vertical trim on your exterior corners, you have to plan ahead by installing wide furring strips made out of 3/4" plywood or OSB.
Image Credit: Image #1: Martin Holladay

Many GBA readers have built homes with 4 inches or 6 inches of rigid foam on the exterior side of their walls. Typically, these walls include vertical 1×4 furring strips, 16 inches or 24 inches on center, on the exterior side of the rigid foam. The furring strips perform at least three functions: they hold the foam in place, they create a rainscreen gap, and they provide something for the siding to be fastened to.

If you look at the illustration at right, you’ll realize that there is no easy way to install a 1×4 furring strip at the exterior corner of a wall with thick rigid foam. Sure, you can attach a furring strip to the stud nearest the corner — but the rigid foam extends further, beyond the last furring strip.

When it’s time to install vertical corner board trim, you realize that there isn’t anything at the corners to nail to. Oops.

OSB or plywood to the rescue

The solution is to plan ahead by installing wide strips of 3/4-inch-thick material (usually OSB or plywood).

[Photo credit: Adam Emter]

If your house has 4 inches of exterior rigid foam, then these strips should be 6 or 7 inches wide; if you have 6 inches of exterior rigid foam, these strips should be 8 or 9 inches wide. It’s also possible to use a 1×8 or a 1×10 board instead of OSB or plywood for this purpose.

[Photo credit: Joe Suhrada]

One of these strips (or a wide board) is screwed through the foam to the last stud on the wall, so that the strip extends all the way to the exterior corner. After the first one is installed, the corresponding strip on the intersecting wall is installed to overlap the first, and the two strips are fastened together at the corner with staples,…

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  1. dvaut | | #1

    Integrated corner post with rain screen.
    I am currently building a home with thick foam and using fern strips as a rainscreen/nailer. My father pre-assembled the corner posts on saw horses using 1"X 6" trim boards screwed together to form a typical corner post. Then he nailed the corner to a 3/4" nailer before attaching it to the wall. The nailer which could be any width to suit your foam thickness, was cheated out from the inside corner of the post which left a vent space on the outside corner of the foam. The corner assembly was screwed through the nailer to the framing studs in the corner. Siding was also nailed to this same nailer where it met the corner post. Of course I only am installing a measly 3" of foam to my exterior, but I can assure you it is a pain in the butt, Ah yes the crazy things we will go through to say I have super insulated walls, do you?

  2. GBA Editor
    Martin Holladay | | #2

    Response to Dillon Vautrin
    Thanks for sharing your method.

  3. dvaut | | #3

    This is an undershot before I slipped in some cobra vent. I never took a picture before it was put on the wall but this might help explain what we did. You can see the corner posts with the air space and where eventually the corner and freeze board are overlaid on the furring strip. And yes I could have painted the back side of my trim boards, but I didn't, and yes my LP smart boards warranty is void because I do not have 2"X furring strips. Call me lazy, cheap and half hazard.

  4. dvaut | | #4

    Try again

  5. user-7527443 | | #5

    How does someone meet code when using 6 inches of foam and furring strips? My inspector has informed me that i must get an engineer involved because his chart only going to 4 inchs of foam.

    Thanks in advance for any guidance

    1. GBA Editor
      Brian Pontolilo | | #6

      Hi user-7527443 (it would be great to get your real name).

      You can check out this article: Fastening Furring Strips to a Foam-Sheathed Wall, but it is only going to agree with your inspector. I think it is pretty common that you'll need an engineer's stamp when using exterior foam over 4 inches; at least, I've heard this before. And, if that's what your building official said, then, well, meeting code is getting an engineer to spec your fasteners and details.

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