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Stucco over 4-inch rigid foam?

Goldenskulldrums | Posted in General Questions on

I am trying to figure out if I am crazy for wanting to Stucco over 4 inch rigid insulation…
I am building an earthship-inspired house in a county with no building codes. As you can see in the picture, the southern wall of the home is a row of glass windows that sit on top of a concrete “footer” that is on top of 3 rows of tires.
The glass will be a huge loss of heat and we want to insulate the tires on the out side of the home.We have an abundance of 4×8 sheets of 4″ insulation and want to use it to insulate the tires and concrete. Tires are 30″ tall and concrete is another 14″. Tires a packed with sand.
From the research I have done it seems like the problem I would run into is being able to fasten the insulation and lath well enough to the tires so that when the weight of the stucco is applied it doesn’t give. Are there other issues? Would 5″screws that go through the tires be sturdy enough? I am also wondering about how a weep Edge wood look on this design. Any advice or suggestions are greatly appreciated. Thank you in advance

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Golden,
    That's a tough wall to insulate.

    The best solution would be to insulate the exterior of the tires with closed-cell spray foam.

    If you want to install rigid foam, you'll be faced with the daunting task of preventing hidden convection loops and air movement in the spaces behind the rigid foam. Perhaps you could try to pack this space with sand, but my experience is that the sand would settle and the air spaces would return.

    If you install rigid foam, you have at least four challenges:

    1. Securing the foam. Probably long screws with big insulation washers would work -- but frankly, I've never tried to drive a screw into a tire. (I've pulled a few out -- that happens when my car finds a screw on my driveway.)

    2. Sealing the perimeter of the rigid foam to limit air currents.

    3. Installing metal Z-flashing to integrate the wall's water-resistive barrier (WRB) with the rigid foam.

    4. Choosing a protective covering for the rigid foam. I think that metal flashing would make more sense than stucco.

    This list of challenges helps explain why very few foundation contractors use old tires to build foundations.

  2. Jon_R | | #2

    Do some reading about what usually happens with sealed/insulated crawlspaces that have no plastic ground barrier and no mechanism to dehumidify the air.

    Interior side insulation might be the better choice here.

  3. Aedi | | #3

    If that's all glass, it probably is not even worth it to insulate the tires. Insulating the walls will not prevent heat from coming in/leaving through the glass. Usually more windows means extra insulation is less worthwhile, since it will just be bypassed. What climate zone are you in? You seem to be in a hot desert. What direction is that glass facing?

    Is your entire rear wall also made of tires? Are they insulated?

  4. Expert Member
    BILL WICHERS | | #4

    I’d double think using the tires the way the earthship guys do. Besides a MASSIVE amount of labor packing those, Martin is right about the difficulty of insulating.

    If you do choose to go with the tire walls, and spray foam as Martin suggested, I’d get some large-aperture wire mesh (like is used for cattle fencing), and attach that to the tires with about 1-1/2” spacing out from the tire surface. Spray foam over the mesh. The mesh will act like a reinforcing mesh to hold the foam together, and with the mesh secure to the tires is numerous places, you have a way to keep the foam attached to the tire wall.

    Bill

  5. Goldenskulldrums | | #5

    Thanks everyone for the pointers. I still haven't decided...
    The house is already built. We packed the 550 tires and it wasn't too bad. The other 3 sides of the home are tires that are insulated to an R-40 with rigid and a vapor Barrier that is buried in an earth burm behind the house. The south wall is where the windows are. Inside the home, 12 feet to the north of the south wall is another wall of glass that separates the living quarters from the greenhouse. So the greenhouse acts as a buffer zone for the climate of the living space. We have insulated curtains for the massive window wall at night. We are in the high desert at 8000 feet.
    Now my question has changed, can I use the 4-inch foam to insulate the sidewalls above our tires on the exterior and stucco over that? The walls are conventionally framed with 2x6 and sheathed with 5/8 OSB. I just really want to use up the 30 sheets of 4" foam we have.

    Thanks again.

  6. Expert Member
    Peter Yost | | #6

    I know that you live in the desert but I still recommend that you monitor your interior relative humidity in your living space, given that you have a greenhouse attached. See this blog for some rough numbers on the moisture contribution of plants to interior moisture.

    https://www.greenbuildingadvisor.com/article/moisture-sources-relative-humidity-and-mold

    Peter

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