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Foundation flashing detail

climb_on | Posted in General Questions on

We are in Minnesota (Zone 6a). 2200 sq/ft single level, slab on grade, with a 1700 sq/ft/ shop.

My 10″ double stud wall will cantilever 4″ to sit flush with the 4″ of perimeter foundation foam. I can flash the connection between the bottom plate and protect the foundation insulation all in one shot.

I attached a quick sketched detail of what I plan to do. I’ll just buy rolls of painted aluminum and nail it through the plywood sheathing into the studs and backfill. Then the WRB will lap over the top of the flashing above grade, rain screen battens and then siding.

No costly custom bent flashing, I can choose a nice color, it’s suitable for a below grade application, and it’s durable enough to protect the foam pretty much forever. Actually it seems too easy – what am I missing?

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  1. GBA Editor
    Martin Holladay | | #1

    You'll need some type of screening between the furring strips to prevent insects from entering your rainscreen gap. But except for that, your plan will work.

    Choose a heavy gauge of flashing for durability. The possible disadvantages are that the metal flashing can get dinged up over the years, and can look like a metal entry door that someone has tried to kick in. The thicker the flashing, the less likely it will look dinged up.

    There is no perfect solution to the question of how to protect above-grade exterior foam. Metal flashing is a good solution. Other options include pressure-treated plywood or some type of stucco (with or without metal lath for reinforcement).

  2. user-2310254 | | #2

    I used Cor-a-Vent on my build. It includes a fine mesh to prevent insects from crawling up into the rainscreen gap.

  3. Expert Member

    I'd be uncomfortable cantilevering the whole outer-wall. Even if the inner-wall is load bearing, the 1 1/2" deep bottom plate ends up taking all the dead-loads of the outer framing, sheathing, siding and trim. it's something that should be run by an engineer.

  4. climb_on | | #4

    Thanks Malcom. I did consult an engineer and got his stamp. As you noted the interior wall is load bearing and it's only a single story - not sure if it would work in a multi story application. It just seems to make sense to do it this way, for easy flashing, using less concrete and maintaining a nice vertical plane. But, we'll see how it goes when I build it....

  5. climb_on | | #5

    Thanks Steve. I hadn't selected a screen for the rainscreen gap yet. I'll look at the coravent. Was thinking I might just cut some ridge vent screen to use there, but haven't priced any of it yet.

  6. climb_on | | #6

    Thanks Martin. I was going to look at roll aluminum valley. It seems to be much heavier than most of the roll flashing I am finding. Pressure treated ply is also a consideration, just won't look as good and harder to attach. Wish there was slick solution to this.

  7. Expert Member

    it looks like a pretty buildable wall assembly, especially if it's only one storey.

    When rain screens were first mandated in our code, builders used insect screen at the bottom, but soon found it didn't last very long, and a replacement was hard to retrofit. Now almost everyone used a perforated flashing. It's available in all our lumberyards, but there is no reason you couldn't bend your own from any perforated stock you can find.

    If you are in carpenter ant country, like we are, it's probably worth spending the time to really seal the edges of the flashing protecting your rigid insulation. They love it.

  8. RobInNorCal | | #8

    I know it's major $'s but I used sheet copper on my last frost-free slab on grade. I used heavy aluminum roll flashing when I built my shop 15 years ago and didn't like the oxidation that showed up after 8-10 years. YMMV; we have a lot of iron in our soil and acid in our rain here..

  9. user-6184358 | | #9

    Malcom - you comment that the rain screen in a code requirement? I see in IRC R703 about exterior coverings on wall surfaces. I am ready to install Hardie Plank Lap Siding on a exterior wall. The WRB is the Grace Vycor enV-S peel & stick product. Are you installing rain screens on projects like this in California?

  10. Expert Member
    MALCOLM TAYLOR | | #10

    I'm up in British Columbia. It was brought in about six years ago in response to widespread building envelope failures here. i grumbled at first, but wouldn't build without one now even if it wasn't required.

    Edit: Sorry I didn't really respond to your question. All the walls I've built have been designed to dry to the outside. I've never done a project with an impermeable membrane as a WRB.

  11. climb_on | | #11

    Rob - was the oxidation on bare aluminum or painted? I haven't priced copper, but I doubt I can justify it. I see a local building supply company had pvc coated aluminum. That would eliminate the oxidation issue I imagine. Not sure about fading though.

  12. GBA Editor
    Martin Holladay | | #12

    The copper you used must look nice. Another pricey option is stainless steel -- used by Steve Giroux in 1984. The stainless steel on Giroux's house is still in good condition, of course.


  13. Expert Member
    Michael Maines | | #13

    Covering the exposed foam with thin metal is effective and can be affordable. However, even thick stainless steel flashing never lays perfectly flat, so you see ripples that draw attention to the flimsy materials used at what intuitively should feel like a very solid part of the house.

    Exposed concrete is a perfect material for the location, so I always try to design foundations with that in mind. When I have to use exterior foam I prefer a parge coating of some sort, so it has the appearance of masonry, even if it needs to be touched up every ten years. I know of one builder who was using pressure treated plywood to cover the foam, instead of metal flashing, but to me that also has a temporary-looking appearance. So that's what you're missing--somewhat subjective aesthetics.

  14. RobInNorCal | | #14

    Scott, it was bare. I thought about painting it but decided the paint wouldn't last given the impact of weedwhcking etc. Also bought some of the cheap painted roll stock from HD but it was so flimsy I threw it out.

    Copper is definitely good locking, definitely expensive. Michael Maines' parge coating also looks good but, as he points out, needs replenishment after some years. I have also had rodent damage to one cement-coated install; they chewed a nice hole through the top of the foam, then found a way to tunnel through the gap between a door and the framing to make a perfect runway into the barn.

  15. Jon_R | | #15

    I have had good experience with QUIKRETE® Foam Coating.

  16. GBA Editor
    Martin Holladay | | #16

    My article, “How to Insulate a Basement Wall,” includes the following list of materials that can be used to protect the above-grade portion of the exterior rigid foam used to insulate a foundation wall:

    A cementitious coating or cementitious stucco (for example, Styro Industries Brush On ST), with or without metal lath
    A cementitious coating that includes chopped fiberglass (for example, Quikrete #1219 foam coating or surface-bonding cement)
    An acrylic coating like Styro Industries FlexCoat or Styro Industries Tuff II
    EIFS (synthetic stucco)
    Cement backerboard, with or without a layer of stucco
    Pressure-treated plywood
    Metal flashing
    A fiberglass panel like Ground Breaker from Nudo Products
    Styro Industries FP Ultra Lite panels (XPS coated with mineral granules adhered to one side)
    Protecto Wrap Protecto Bond (a flexible peel-and-stick membrane with a textured, gritty coating)
    ProGuard Cement Faced Insulated Sheathing.

  17. Robert Opaluch | | #17

    I used aluminum flashing with a darker brown baked enamel finish to cover 2" exterior XPS foam, similar to your design. However I had to bend the flashing and didn't do a very nice job of it. Not very visible anyway. After decades, the flashing looked like it was originally installed, except with a layer of dust. It was protected from traffic and weed-wackers by shrubs and gravel along most of the foundation. I'd use the same materials again.

    The only problem I'd worry about is termites or carpenter ants if they are a problem in your area. I've heard of ways of sealing copper or copper mesh flashing against the foundation wall, to keep insects from being able to get to wood walls without being visible outdoors.

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