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Rigid foam over zip osb

dennis_vab | Posted in General Questions on

My wall assembly will consist of vertical vinyl siding, 2” of rigid foam, 7/16” zip osb, 2×6 wall with faced fiberglass, and gypsum board. 

I’ve been planning to line the 2×6 wall plane with my 8” concrete foundation wall. That would result in about 2-1/2” of material protruding past the foundation. I’m not a fan of this but feel like this makes most sense since my zip osb will be the water and air control layer. This would also require me to protect the exposed rigid foam from below.

Another option would be to make the foundation 2” wider in all directions and use flashing to get any water off the mud sill. 

Any thoughts of either of these options? A couple other things I am thinking about are whether I need a better drainage plane between the zip and rigid foam. I will probably tape the rigid foam, but prefer not to make that a water control layer unless someone can point out something that I am overlooking. 

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  1. Expert Member

    There could be another option, which is somewhat a hybrid version of the two you mentioned.

    If you make your foundation 2+7/16" wider all around, you can rip a wedge of wood at say a 45 degree angle, and screw it to the inside of the top of the outer form. That will create a sloped plane that you can tape your sheathing to at the base. You could then rip the bottom edge of the first row of insulation at the same angle, and hold it up maybe 1/8". That will create a more flush look, if that's what you're after, while protecting the bottom, and allowing any water to freely drain away.

    It adds a bit of concrete, but probably saves as much as some flashing configurations I've seen.

    1. dennis_vab | | #2

      That’s a good idea! I was actually thinking in my head about ripping the same 45 degree wedge and nailing it into the corner of the sill plate and sheathing, and then taping or flashing it.

      1. Expert Member

        Another thought that just came to mind -

        A few brick sizes have a thickness in the 2.25 - 2.5" range. If you could find a solid core brick on a style you liked, you might be able to put them in a shiner running bond for a finished appearance, adds a bit of work though.

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