Wall to foundation detail with exterior insulation
I’m building a home for myself and my partner in Central Maine and planning on using 3″ of Rockwool Comfortboard on the exterior. We’ll be building over a frost wall due to the slope of our building site and we’d like to insulate it to ~R-20 using 4″ of recycled XPS foam. The flooring on the first floor will be polished concrete.
The only way I could think of to insulate the frost wall to that level without having a chunk of styrofoam poking out around the edge of our floors was to put the insulation on the outside of the foundation (important caveat: I’m very inexperienced). The attached diagram shows the assembly I’m thinking of with the 2×6 stud walls cantilevered 1/2″ over the frost wall, sheathed with 1/2″ ZIP, followed by 3″ rockwool, then diagonal 1×4 strapping, then 1×8 siding. What’s not shown in this diagram, but still part of the plan, is some kind of protective layer over the foam where it comes above grade.
A couple of questions:
1.) Considering that the foam on the exterior of the frost wall will be placed inside the form, what’s the best way to air seal the sill and sheathing to it? Am I making it unnecessarily difficult to air seal the wall-to-foundation connection with this assembly?
2.) I would prefer to be able to insulate the frost wall on the interior side. Am I missing an effective way to do that to the R-value we’re looking for while using a polished concrete slab as the flooring?
3.) Are there any other concerns with this assembly? We’re going to be installing our septic in a couple of weeks and will be having the excavators prepare the site for our frost walls and slab, so I’d super appreciate any feedback about how this whole thing looks from folks who know better than me (…which I’m sure you all do).
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