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Community and Q&A

Trim bottom of rigid foam on exterior of house with slab on grade

ktwidwell | Posted in GBA Pro Help on

Hello, I live in Maine, I have a stick built with osb sheathing with drain wrap over that. I plan to do 2″ rigid foam on the exterior walls. The house sits on an on grade 6″ cement slab with a 4.5″ pressure treated sill. There will also be 3/4″ strapping with clapboard siding. I am lost as to how I trim out the bottom of the whole ordeal.

I plan on keeping my siding 8″ from the ground, to meet code, so the foam will be exposed. Also, bugs could come in from the underside. I also have not seen a prefab drip edge of some sort to put along the bottom prior to the foam.

Then there is the thought of how do I cover the seam of the sill to the cement slab.

Would it be wise also add more foam that goes into the ground to insulate the sides of my cement slab?

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  1. GBA Editor
    Martin Holladay | | #1

    In your climate zone, the building code requires at least R-10 insulation at the perimeter of a slab on grade. The insulation is installed vertically. So, yes -- you need to install at least 2 inches of rigid foam insulation -- more is better -- at your slab perimeter.

    This foam insulation can be in the same plane as the rigid foam that you plan to install on the exterior of your OSB wall sheathing, as long as your OSB is co-planar with your slab. (Otherwise, you may need to add 1/4-inch fanfold insulation to one of the foam layers to make them co-planar.)

    The rigid foam that insulates the edge of your slab needs to be protected from physical abuse and UV rays. This is done with one of the following products:

    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.

    If for some reason you need to trim the bottom of a foam-sheathed wall on a home without foundation foam, you would probably protect the bottom of the foam with metal flashing bent on a brake. To learn more about this detail, see this GBA video: How to Install Rigid Foam Insulation Outside a House.

  2. user-1115477 | | #2

    Just a few comments:

    First, if you want rigid foam over stick build it Maine, you probably want more like 2 layers of 2" rigid foam over assumed 2x4 or 2x6 insulation -filled walls, instead of just 2 inches, total. Read up on it.

    Second, Karen, you, and a lot of other people need to read and research your design problems, put in a little work, instead of taking the easy way and crying out for answers served on a platter to basic questions , when all your questions have been answered on this site before. Do a search.

  3. GBA Editor
    Martin Holladay | | #3

    It's OK to ask questions. Sonny was trying to point out that exterior rigid foam on above-grade walls needs to meet certain minimum R-value requirements to avoid moisture accumulation in the wall during the winter. Everything you need to know about that topic is explained here: Calculating the Minimum Thickness of Rigid Foam Sheathing.

  4. ktwidwell | | #4

    Thank you Martin for your response. I have researched this topic for about one year very extensively and understand the more rigid foam the better, but in climate zone 6 I only require R7.5 to ward off condensation. I actually only require an R21 total to meet minimum code up in Maine, just between the studs. Since this is just a converted garage, I don't feel it necessary to have 4" of foam on the outside at this time.

    As for waterproofing the bottom, I guess I will use a butylene flashing to keep water from getting into the drainwrap at the bottom and then as for ants nesting in the foam, I am still not sure.

    There doesn't seem to be dummy how-to info for this detail of exterior foam. The foam is going to cost about $1,500 so I want to make sure I did it correctly for my application.

  5. ktwidwell | | #5

    The only solution I have come up with is to buy door and window screen and wrap it around the foam underneath the ground.

  6. GBA Editor
    Martin Holladay | | #6

    You wrote, "I guess I will use a butylene flashing to keep water from getting into the drainwrap at the bottom and then as for ants nesting in the foam, I am still not sure."

    You also wrote, "The only solution I have come up with is to buy door and window screen and wrap it around the foam underneath the ground."

    If the "drainwrap" you are talking about is a wrinkled housewrap on top of your wall sheathing, you don't want to seal up the bottom of the drainwrap. The housewrap should lap over the top of a metal flashing, and the flashing should extend past the bottom of the rigid foam and terminate at a drip leg at the exterior of your house. This flashing is bent on a brake. It will protect the underside of your rigid foam.

    I wouldn't expect buried window screen to keep insects out of buried foam. First of all, most types of window screen with corrode or rot when buried. Second, insects can find their way through tiny cracks. If you are really worried about insects, you should order some borate-treated EPS.

  7. ktwidwell | | #7

    Wow, so should I tape at the bottom of the drainwrap to the flashing? I always assumed the drainwrap should be tight to the sheathing and not be part of the equation for letting water drain to the bottom between the wrap and the sheathing.

    I have a major ant infestation on the sand fill my garage sits on. I guess I could chance it and see what happens. I don't think I would win the war against the ants and don't approve of pesticide extermination (borax or borate might be acceptable depending).

    Thank you for all of the info, it is so appreciated to clear up the little details I could not figure out with reading.

  8. GBA Editor
    Martin Holladay | | #8

    Every wall needs a water-resistive barrier (WRB). It is required by code, and it is a good idea. I assumed that your wrinkled housewrap was your WRB -- but it doesn't have to be. To learn about all the various options, see All About Water-Resistive Barriers.

    Most people who choose a wrinkled housewrap hope that if any liquid water ever reaches the housewrap, it will be able to drain downward along the wrinkles. (Whether this ever happens or not is a different story; however, it doesn't hurt to allow for drainage.) That's why the wrinkled housewrap should lap over the flashing at the base of your wall.

    A simple lap of about 4 inches is all you need. There is no need to tape the housewrap to the metal flashing.

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