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Community and Q&A

Proper way to detail bottom edge of rigid foam next to sill plate

CTSNicholas | Posted in Green Products and Materials on

I am going to be installing 2″ of rigid foam over my OSB sheathing soon and I have two concerns. The current one is what is the most DIY friendly method to protect that bottom edge of foam that is under the siding, but next to the ground near the sill plate? I want to prevent bugs of course, but also keep moisture out too. I see videos of pros using aluminum flashing but they have fancy breaks and coils of it. Is there a DIY alternative? Local supply store would be Menards if there is a certain product to suggest. Thanks.

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  1. Expert Member

    Are you installing your siding on furring, creating a rain-screen?
    If you are I'd suggest you forgo the DIY preference and a sheet metal fabricator to make up appropriately shaped perforated metal flashing.
    If your siding is being applied directly to the foam, you can use one of the stock pre-finished L or drip flash that any lumberyard sells.

  2. CTSNicholas | | #2

    There will be furring strips (1x3 or 1x4) if I use vinyl siding or similar product. If I use stucco, I wouldn't have the furring strips. I was thinking maybe applying an acrylic based stucco to that bottom edge of foam would be sufficient? We are talking 2" thick to cover all around the perimeter.

  3. GBA Editor
    Martin Holladay | | #3

    Every trade uses specialized tools. Roofers and siding contractors use a brake to form flashing.

    If you don't want to buy a brake, you can go to any sheet-metal shop with a sketch of the flashing profile you want made, and the shop can make the flashing for you.

  4. CTSNicholas | | #4

    So is the proper method to use aluminum with a 90 degree bend that extends out from the OSB the thickness of the foam, or the thickness of the foam + 3/4" furring gap?

    Would using a stucco over the bottom not be okay?

  5. Expert Member

    Nicholas, It depends on a couple of things.
    If you are placing your WRB (house wrap or building paper) behind your foam, you need some way for moisture to be able to drain if it gets that far into your wall. Covering the bottom of the foam with something solid like stucco isn't a good idea.
    If the WRB is on top of your foam then you can cover the bottom with whatever you like.
    If you are providing a drainage plane of furring you should use something, like perforated flashing, that allows air and water to move through the bottom of the cavity.
    What would I do given that my lumberyard has a metal shop? I'd buy U-shaped perforated flashing deep enough to cover the foam and the furring strips.

  6. DIYJester | | #6

    Short of renting a brake or finding pre-bend pieces the only DIY route I can think of would be to try to make a set of dies to pull the coil through. I've worked in sheet metal factories and thought I could replicate a die set with a few wooden cut outs progressively making the shape for cheap, but it turned out like crap. I was much happier with renting an aluminum brake from a big box and spent a day bending and slitting all of the coil I'll use on my entire house.

    I didn't want to use pre-bent pieces as I was able to exactly match my original flashing by searching for the color and bending it myself. The brake ended up costing $60 for the entire day, which ended up being the cost of the contractor's quoted price for about 5-6 feet of flashing. I made over 150 feet of flashing for about $130 after rental and materials, the lowest quote I got was close to $1000.

  7. CTSNicholas | | #7


    I will be using GreenGuard RainDrop 3D *over* my 2" rigid foam. I will have fiberglass or mineral wool insulation on interior and no poly or intentional vapor barrier.

    There will be a 3/4" gap between house wrap and siding unless I apply stucco finish. The U-shaped flashing you describe...not sure how that looks. But it sounds like it would/could be applied after the foam is installed?

    That is good to know. I don't know about renting one as there isn't any rental place nearby. I will check around though. I just know the crews that do it for a living will charge me a lot for labor when I can do it myself. I think I can, at least. :)

  8. Expert Member

    The flashing used at the bottom of rain screens, whether it covers just the strapping or the foam too, is usually formed as a U with the back leg a bit higher so it can be nailed to the sheathing. It acts as a level base you can set the foam or furring into.

  9. CTSNicholas | | #9

    I was able to find 90 degree bend flashing and J channel but not really anything perforated or specific u channel flashing. This was the closest thing I can find -


  10. LucyF | | #10


    In this article on exterior insulation with Roxul, there is a very good photo of the kind of flashing Malcolm and Martin are telling you about.

    If I were to redo the foil-covered polyiso foam exterior insulation on my house (well, first I would use Roxul instead), I would tape the bottom of the foam sheets to prevent carpenter ants and termites from having easy access. We did do an insect screen, but I think some small ants and termites can get around that. I would also install some of the material they use on ridge vents between the insect screen to help it hold it's shape better. I think that would be more effective and preventing insect intrusions.

  11. CTSNicholas | | #11

    So it seems it is quite the concern to have that foam bottom edge covered, but still allow ventilation, yet try to minimize termites and ants from entering.... I am not sure all three can be achieved. I think I would feel more comfortable doing a layer of stucco up to the edge of the foam and putting some screen material for the rain screen gap. Otherwise, I will try to find a local supply for metal flashing that is either bent like a 90 or u shaped. If the flashing only covers the foam edge I don't see the need for the perforations and feel that would allow bugs to get into the foam compared to my other two ideas. I understand the rain screen will need to have holes for drainage and air flow.

  12. Expert Member
    MALCOLM TAYLOR | | #12

    Nicholas, that sounds like a good plan. As long as no moisture can get as far into the wall as the foam, you don't need the perforated flashing underneath it.
    The reason they used it in the house Lucy linked to was that the Roxul allows water to move through it fairly easily. Also, as our code mandates rain screens, suppliers all stock the necessary accessories. If it becomes more common where you are they will probably eventually do the same.

  13. CTSNicholas | | #13

    I talked to the lumber yard and they only sell custom in steel which would rust and need painted so I found someone who will custom make the piece with aluminum...I didn't ask on price, because I am tired of trying to find the right material. I requested 3" backing to fasten to the OSB then 2" after a 90 degree bend, and then another 90 degree bend that goes up 1" and preferable makes a little crease for support and to aid water out like drip edge on roofs. This will be what I slide the foam into as I install it. I will then drill a hole every foot or so in the material to be protection if any water gets to or behind the foam.

  14. CTSNicholas | | #14

    I read through more articles on this and this one really bothered me because I don't understand the point.

    "1. Use bug screens and flashing below the foam
    Many builders protect foam sheathing from insects by wrapping the bottom edge with strips of window screen. We take a belt-and-suspenders approach to air-sealing by backing up the screen with a sheet-metal drip edge. This directs water away from the foundation surface and adds extra protection from bugs and wind.
    My crew and I bend aluminum or galvanized steel sheeting so that it extends up behind the foam and out past the bottom edge far enough to nail to the bottom of the wood strapping. Then we nail a strip of window screen several inches wider than the flashing to the vertical part of the drip edge, leaving enough length to wrap under the foam and over the bottom of the strapping that holds the foam in place.

    So that means that aluminum goes past the thickness of the foam and does one of two things a.) attaches to literally the bottom end of the furring strip or b.) wraps up the front of the furring strip to be nailed. Either way that goes against everything mentioned here about being sure to get perforated flashing or to be sure that moisture can escape. Is my plan to only cover the foam bottom edge the proper way or improper way?

  15. Expert Member
    MALCOLM TAYLOR | | #15

    The only protection the foam needs is against insects and rodents. It is protected from mechanical and UV damage by being behind the siding. Why you would protect if from wind is beyond me. Window screening is a poor way to protect the cavity against insects or rodents and no one here in BC uses it anymore. Your plan sounds fine.

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