Tricky footing, foundation and wall interface for a walk-out basement
I am building a continuously externally insulated ranch on a walkout basement in Michigan. The lot is sloped and slanted, so two walls are below grade and two are at grade. The below grade walls will be ICF poured on a standard 10″ x 20″ footing. The at grade walls will be framed 2X6 with 2″ of exterior rigid foam sitting on a 4′ trench footing below the frost line. I will insulate around the slab as well. In my research, I have not see a lot of discussion on the best design and build practices on the intersection of the two different footings (standard and trench) and the two different (ICF and framed) wall sections. Appreciate any advice or guidance.
Just to complicate this design, I have radiant pex tubings in my slab and an external retaining wall outside of the foundation (at the corners of the walkout). Subjects for another thread.
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Danny, I'm not sure what you mean by a 4' trench footing. Where I am we always form our footings, but I know that in other places they are sometimes poured directly into excavated soil. But in those cases there is a concrete wall placed on top. Any chance you have images you could share that would help explain your situation?
I might be using wrong terminology. what I mean by trench footing is more like a monolithic footing and slab floor, poured all at one time. here is a sketch of what I am talking about. Appreciate your comments.
Ah, got it. That's best called a monolithic slab with turned-down footing. (Rolls off the tongue, eh?)
I'm working on the same problem, hoping I can do a frost protected shallow foundation style slab, fully insulated underneath, and up the walls to the rim. Then, using edge and wing insulation when it get above frost depth.
I am also treating the walk out edge of my slab as a shallow foundation by insulating under and outside. I want to insulate the outside of the poured walls to keep the wall on the warm side. I can get a nearly continuous external thermal envelop. Good luck on your project.
When you have a walkout with at least 1/2 of the basement as traditional footings and stem wall (I realize they are full walls), in my opinion it is far easier to step the footings down and continue with traditional stem walls 4' down, around the entire perimeter. Then pour a normal slab. Same cost, much easier.
The retaining wall is a separate issue. You can't tie it into your house wall. Pour it later. And btw, you can't just pour a 6' high concrete retaining wall. It will turn over in a couple of years. Retaining walls over 3' high are a separate structural design issue, that need to be built properly if you want them to stay where you put them.
When you say “step the footings down”, do you mean step the wall down? The traditional footing that underpins the poured wall is all at the same plane on my house. The footing for the walkout edge is another 4’ below the traditional footing bc I am treating that edge as a shallow foundation. I think I understand your comment to say to only pour the concrete wall to get above grade, in 4’ sections, which I agree with. I will do this.
I also agree with your retaining wall comment. Its a whole separate design/build issue.
Thanks for your reply.
Usually you would keep the footings at the FRONT end of both side walls 4' into the ground as well. Then in a couple of different increments as you head up the hill, you would "step up" the level of those side wall footings until the footings are just under the slab (same height as the footings for the back wall). This is assuming your finished ground cover grade against the concrete wall rises along with the hill.
This eliminates any need to carefully insulate the transition, or under the edge of the slab with insulation "wings". And if this is a seasonal home (i.e. maybe vacant in the winter with no heat) you need to be very careful about slab on grade designs in cold weather climates. I personally wouldn't do one unless the building is going to be heated all year.